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Speaking of AM modulation,, we all know that the carrier amplitude
does not change with modulation. Or does it? Here is a question that has plagued many for years: If you have a plate modulated transmitter, the plate voltage will swing down to zero and up to two times the plate voltage with 100% modulation. At 100% negative modulation the plate voltage is cutoff for the instant of the modulation negative peak. How is the carrier still transmitted during the time there is zero plate voltage? If we lower the modulation frequency to say 1 cps or even lower, 1 cycle per minute, then wouldn't the transmitter final be completely off for half that time and unable to produce any carrier output?? Question is, at what point does the carrier start to be effected? 73 Gary K4FMX On Wed, 22 Oct 2003 20:39:20 -0700, Roy Lewallen wrote: The amplitudes of the sideband components are symmetrical (at least for modulation by a single sine wave), but the phases aren't. The phases of all the upper sideband components are in phase with the carrier; in the lower sideband, the odd order components (and only the odd order ones) are reversed in phase. With multitone modulation, things get a whole lot more complex. Unlike AM, FM is nonlinear, so there are sideband components from each tone, plus components from their sum, difference, and harmonics. The inability to use superposition makes analysis of frequency modulation with complex waveforms a great deal more difficult than AM. Note also that unlike AM, whatever fraction of the carrier that's left when transmitting FM also contains part of the modulation information. Of course, at certain modulation indices with pure sine wave modulation, the carrier goes to zero, meaning that all the modulation information is in the sidebands. But this happens only under specific modulation conditions, so you'd certainly have an information-carrying carrier component present when modulating with a voice, for example. Roy Lewallen, W7EL Joel Kolstad wrote: Avery Fineman wrote: There isn't any corresponding similarity of FM and PM to AM for the repetition of sidebands' information when looking at the spectral content. Umm... last I looked the spectrum of FM and PM was symmetrical about the carrier frequency? (Well, the lower sideband is 180 degrees out of phase with the upper, but that's true of AM as well.) Looking at a single sine wave input to an FM or phase modulator, this comes about from the Bessel function expansion of the sidetones and J-n(x)=-Jn(x)? I know you're far more experienced in this area than I am, however, so I'll let you explain what I'm misinterpreting here! ---Joel Kolstad |
Speaking of AM modulation,, we all know that the carrier amplitude
does not change with modulation. Or does it? Here is a question that has plagued many for years: If you have a plate modulated transmitter, the plate voltage will swing down to zero and up to two times the plate voltage with 100% modulation. At 100% negative modulation the plate voltage is cutoff for the instant of the modulation negative peak. How is the carrier still transmitted during the time there is zero plate voltage? If we lower the modulation frequency to say 1 cps or even lower, 1 cycle per minute, then wouldn't the transmitter final be completely off for half that time and unable to produce any carrier output?? Question is, at what point does the carrier start to be effected? 73 Gary K4FMX On Wed, 22 Oct 2003 20:39:20 -0700, Roy Lewallen wrote: The amplitudes of the sideband components are symmetrical (at least for modulation by a single sine wave), but the phases aren't. The phases of all the upper sideband components are in phase with the carrier; in the lower sideband, the odd order components (and only the odd order ones) are reversed in phase. With multitone modulation, things get a whole lot more complex. Unlike AM, FM is nonlinear, so there are sideband components from each tone, plus components from their sum, difference, and harmonics. The inability to use superposition makes analysis of frequency modulation with complex waveforms a great deal more difficult than AM. Note also that unlike AM, whatever fraction of the carrier that's left when transmitting FM also contains part of the modulation information. Of course, at certain modulation indices with pure sine wave modulation, the carrier goes to zero, meaning that all the modulation information is in the sidebands. But this happens only under specific modulation conditions, so you'd certainly have an information-carrying carrier component present when modulating with a voice, for example. Roy Lewallen, W7EL Joel Kolstad wrote: Avery Fineman wrote: There isn't any corresponding similarity of FM and PM to AM for the repetition of sidebands' information when looking at the spectral content. Umm... last I looked the spectrum of FM and PM was symmetrical about the carrier frequency? (Well, the lower sideband is 180 degrees out of phase with the upper, but that's true of AM as well.) Looking at a single sine wave input to an FM or phase modulator, this comes about from the Bessel function expansion of the sidetones and J-n(x)=-Jn(x)? I know you're far more experienced in this area than I am, however, so I'll let you explain what I'm misinterpreting here! ---Joel Kolstad |
You have to be careful in what you call the "carrier". As soon as you
start modulating the "carrier", you have more than one frequency component. At that time, only the component at the frequency of the original unmodulated signal is called the "carrier". So you have a modulated RF signal, part of which is the "carrier", and part of which is sidebands. General frequency domain analysis makes the assumption that each frequency component has existed forever and will exist forever. So under conditions of modulation with a periodic signal, you have three components: A "carrier", which is not modulated, but a steady, single frequency, constant amplitude signal; and two sidebands, each of which is a frequency shifted (and, for the LSB, reversed) replica of the modulating waveform. You can take each of these waveforms, add them together in the time domain, and get the familiar modulated envelope. So, the short answer is that the carrier, which is a frequency domain concept, is there even if you're modulating at 0.001 Hz. But to observe it, you've got to watch for much longer than 1000 seconds. You simply can't do a meaningful spectrum analysis of a signal in a time that's not a lot longer than the modulation period. Roy Lewallen, W7EL Gary Schafer wrote: Speaking of AM modulation,, we all know that the carrier amplitude does not change with modulation. Or does it? Here is a question that has plagued many for years: If you have a plate modulated transmitter, the plate voltage will swing down to zero and up to two times the plate voltage with 100% modulation. At 100% negative modulation the plate voltage is cutoff for the instant of the modulation negative peak. How is the carrier still transmitted during the time there is zero plate voltage? If we lower the modulation frequency to say 1 cps or even lower, 1 cycle per minute, then wouldn't the transmitter final be completely off for half that time and unable to produce any carrier output?? Question is, at what point does the carrier start to be effected? 73 Gary K4FMX |
You have to be careful in what you call the "carrier". As soon as you
start modulating the "carrier", you have more than one frequency component. At that time, only the component at the frequency of the original unmodulated signal is called the "carrier". So you have a modulated RF signal, part of which is the "carrier", and part of which is sidebands. General frequency domain analysis makes the assumption that each frequency component has existed forever and will exist forever. So under conditions of modulation with a periodic signal, you have three components: A "carrier", which is not modulated, but a steady, single frequency, constant amplitude signal; and two sidebands, each of which is a frequency shifted (and, for the LSB, reversed) replica of the modulating waveform. You can take each of these waveforms, add them together in the time domain, and get the familiar modulated envelope. So, the short answer is that the carrier, which is a frequency domain concept, is there even if you're modulating at 0.001 Hz. But to observe it, you've got to watch for much longer than 1000 seconds. You simply can't do a meaningful spectrum analysis of a signal in a time that's not a lot longer than the modulation period. Roy Lewallen, W7EL Gary Schafer wrote: Speaking of AM modulation,, we all know that the carrier amplitude does not change with modulation. Or does it? Here is a question that has plagued many for years: If you have a plate modulated transmitter, the plate voltage will swing down to zero and up to two times the plate voltage with 100% modulation. At 100% negative modulation the plate voltage is cutoff for the instant of the modulation negative peak. How is the carrier still transmitted during the time there is zero plate voltage? If we lower the modulation frequency to say 1 cps or even lower, 1 cycle per minute, then wouldn't the transmitter final be completely off for half that time and unable to produce any carrier output?? Question is, at what point does the carrier start to be effected? 73 Gary K4FMX |
You're probably thinking of AM vs. narrow band FM. Although the equations
look very similar on paper and the MAGNITUDE spectrum is identical, the phase spectrum is different Joel- Perhaps that is what I'm remembering. Now, if you use a filter to eliminate the other sideband, the higher frequency components and the carrier, don't you have a nearly identical remainder? See the message I posted earlier tonight for a discussion of whether or not you can recover NBFM with an envelope detector Somehow I missed that one. It seems that AOL does not post messages in the order in which they were originated! I think we are in agreement that you can't recover FM modulation with just an envelope detector, but there is another approach. Again, you need a filter, but maybe one that is not as sharp as above. If you tune the radio so the carrier is just outside the passband, an amplitude variation will occur as the signal slides up and down the shoulder of the filter. The result is a pseudo AM signal that is detected by the envelope detector. I recall that this approach is called "slope detection". 73, Fred, K4DII |
You're probably thinking of AM vs. narrow band FM. Although the equations
look very similar on paper and the MAGNITUDE spectrum is identical, the phase spectrum is different Joel- Perhaps that is what I'm remembering. Now, if you use a filter to eliminate the other sideband, the higher frequency components and the carrier, don't you have a nearly identical remainder? See the message I posted earlier tonight for a discussion of whether or not you can recover NBFM with an envelope detector Somehow I missed that one. It seems that AOL does not post messages in the order in which they were originated! I think we are in agreement that you can't recover FM modulation with just an envelope detector, but there is another approach. Again, you need a filter, but maybe one that is not as sharp as above. If you tune the radio so the carrier is just outside the passband, an amplitude variation will occur as the signal slides up and down the shoulder of the filter. The result is a pseudo AM signal that is detected by the envelope detector. I recall that this approach is called "slope detection". 73, Fred, K4DII |
Fred McKenzie wrote:
Perhaps that is what I'm remembering. Now, if you use a filter to eliminate the other sideband, the higher frequency components and the carrier, don't you have a nearly identical remainder? At that point I don't think you could tell the difference since there's no longer any local phase reference (i.e., the carrier) to compare with. I suppose this is why your SSB-AM rig is able to (somewhat) receive low frequency (and thereby presumably narrowband) FM broadcasts; this is what you were saying in your last post, correct? I think we are in agreement that you can't recover FM modulation with just an envelope detector Yes, at least you can't recover a signal that directly corresponds to what you transmitted. It does appear that you can recover the signal's square, however, so this approach might be useful for, e.g., remote command transmissions. (But probably just for the novelty of having said you did it... since it's probably not much harder to build the slope detector you describe!) ---Joel |
Fred McKenzie wrote:
Perhaps that is what I'm remembering. Now, if you use a filter to eliminate the other sideband, the higher frequency components and the carrier, don't you have a nearly identical remainder? At that point I don't think you could tell the difference since there's no longer any local phase reference (i.e., the carrier) to compare with. I suppose this is why your SSB-AM rig is able to (somewhat) receive low frequency (and thereby presumably narrowband) FM broadcasts; this is what you were saying in your last post, correct? I think we are in agreement that you can't recover FM modulation with just an envelope detector Yes, at least you can't recover a signal that directly corresponds to what you transmitted. It does appear that you can recover the signal's square, however, so this approach might be useful for, e.g., remote command transmissions. (But probably just for the novelty of having said you did it... since it's probably not much harder to build the slope detector you describe!) ---Joel |
Along the same line consider that the envelope of an SSB signal has no
direct relationship to the original modulation the way that an AM signal does. This is why you can not use RF derived ALC to control the audio stage of an SSB transmitter the way you can with an AM transmitter. Or audio clipping that works on AM but does not work the same on SSB. Transmit a square wave on an AM transmitter and you see a square wave in the AM envelope. Do the same with an SSB transmitter and you only see sharp spikes in the envelope. 73 Gary K4FMX On Thu, 23 Oct 2003 12:08:31 -0700, "Joel Kolstad" wrote: Fred McKenzie wrote: Perhaps that is what I'm remembering. Now, if you use a filter to eliminate the other sideband, the higher frequency components and the carrier, don't you have a nearly identical remainder? At that point I don't think you could tell the difference since there's no longer any local phase reference (i.e., the carrier) to compare with. I suppose this is why your SSB-AM rig is able to (somewhat) receive low frequency (and thereby presumably narrowband) FM broadcasts; this is what you were saying in your last post, correct? I think we are in agreement that you can't recover FM modulation with just an envelope detector Yes, at least you can't recover a signal that directly corresponds to what you transmitted. It does appear that you can recover the signal's square, however, so this approach might be useful for, e.g., remote command transmissions. (But probably just for the novelty of having said you did it... since it's probably not much harder to build the slope detector you describe!) ---Joel |
Along the same line consider that the envelope of an SSB signal has no
direct relationship to the original modulation the way that an AM signal does. This is why you can not use RF derived ALC to control the audio stage of an SSB transmitter the way you can with an AM transmitter. Or audio clipping that works on AM but does not work the same on SSB. Transmit a square wave on an AM transmitter and you see a square wave in the AM envelope. Do the same with an SSB transmitter and you only see sharp spikes in the envelope. 73 Gary K4FMX On Thu, 23 Oct 2003 12:08:31 -0700, "Joel Kolstad" wrote: Fred McKenzie wrote: Perhaps that is what I'm remembering. Now, if you use a filter to eliminate the other sideband, the higher frequency components and the carrier, don't you have a nearly identical remainder? At that point I don't think you could tell the difference since there's no longer any local phase reference (i.e., the carrier) to compare with. I suppose this is why your SSB-AM rig is able to (somewhat) receive low frequency (and thereby presumably narrowband) FM broadcasts; this is what you were saying in your last post, correct? I think we are in agreement that you can't recover FM modulation with just an envelope detector Yes, at least you can't recover a signal that directly corresponds to what you transmitted. It does appear that you can recover the signal's square, however, so this approach might be useful for, e.g., remote command transmissions. (But probably just for the novelty of having said you did it... since it's probably not much harder to build the slope detector you describe!) ---Joel |
So what you are saying is that the carrier of a modulated signal is
ONLY a frequency domain concept? That would mean that it really does turn on and off in the time domain at the modulation rate. 73 Gary K4FMX On Thu, 23 Oct 2003 10:49:46 -0700, Roy Lewallen wrote: You have to be careful in what you call the "carrier". As soon as you start modulating the "carrier", you have more than one frequency component. At that time, only the component at the frequency of the original unmodulated signal is called the "carrier". So you have a modulated RF signal, part of which is the "carrier", and part of which is sidebands. General frequency domain analysis makes the assumption that each frequency component has existed forever and will exist forever. So under conditions of modulation with a periodic signal, you have three components: A "carrier", which is not modulated, but a steady, single frequency, constant amplitude signal; and two sidebands, each of which is a frequency shifted (and, for the LSB, reversed) replica of the modulating waveform. You can take each of these waveforms, add them together in the time domain, and get the familiar modulated envelope. So, the short answer is that the carrier, which is a frequency domain concept, is there even if you're modulating at 0.001 Hz. But to observe it, you've got to watch for much longer than 1000 seconds. You simply can't do a meaningful spectrum analysis of a signal in a time that's not a lot longer than the modulation period. Roy Lewallen, W7EL Gary Schafer wrote: Speaking of AM modulation,, we all know that the carrier amplitude does not change with modulation. Or does it? Here is a question that has plagued many for years: If you have a plate modulated transmitter, the plate voltage will swing down to zero and up to two times the plate voltage with 100% modulation. At 100% negative modulation the plate voltage is cutoff for the instant of the modulation negative peak. How is the carrier still transmitted during the time there is zero plate voltage? If we lower the modulation frequency to say 1 cps or even lower, 1 cycle per minute, then wouldn't the transmitter final be completely off for half that time and unable to produce any carrier output?? Question is, at what point does the carrier start to be effected? 73 Gary K4FMX |
So what you are saying is that the carrier of a modulated signal is
ONLY a frequency domain concept? That would mean that it really does turn on and off in the time domain at the modulation rate. 73 Gary K4FMX On Thu, 23 Oct 2003 10:49:46 -0700, Roy Lewallen wrote: You have to be careful in what you call the "carrier". As soon as you start modulating the "carrier", you have more than one frequency component. At that time, only the component at the frequency of the original unmodulated signal is called the "carrier". So you have a modulated RF signal, part of which is the "carrier", and part of which is sidebands. General frequency domain analysis makes the assumption that each frequency component has existed forever and will exist forever. So under conditions of modulation with a periodic signal, you have three components: A "carrier", which is not modulated, but a steady, single frequency, constant amplitude signal; and two sidebands, each of which is a frequency shifted (and, for the LSB, reversed) replica of the modulating waveform. You can take each of these waveforms, add them together in the time domain, and get the familiar modulated envelope. So, the short answer is that the carrier, which is a frequency domain concept, is there even if you're modulating at 0.001 Hz. But to observe it, you've got to watch for much longer than 1000 seconds. You simply can't do a meaningful spectrum analysis of a signal in a time that's not a lot longer than the modulation period. Roy Lewallen, W7EL Gary Schafer wrote: Speaking of AM modulation,, we all know that the carrier amplitude does not change with modulation. Or does it? Here is a question that has plagued many for years: If you have a plate modulated transmitter, the plate voltage will swing down to zero and up to two times the plate voltage with 100% modulation. At 100% negative modulation the plate voltage is cutoff for the instant of the modulation negative peak. How is the carrier still transmitted during the time there is zero plate voltage? If we lower the modulation frequency to say 1 cps or even lower, 1 cycle per minute, then wouldn't the transmitter final be completely off for half that time and unable to produce any carrier output?? Question is, at what point does the carrier start to be effected? 73 Gary K4FMX |
In article , W7TI
writes: On 22 Oct 2003 20:21:16 GMT, (Avery Fineman) wrote: (there are still some long-timers who refuse to accept that the carrier RF energy doesn't change in AM at less than 100% modulation, heh heh) _________________________________________________ ________ I still remember the first time I got hold of a really narrowband receiver and tuned in only the carrier of an AM signal. I was astonished. That was the most enlightening two or three seconds in my whole radio career. :-) Roger that! :-) The first time I had to really calibrate an FM deviation indicator, I had to use the "carrier null" technique with a narrow-bandpass detector (actually a spectrum analyzer). Say WHAT?!? I thought. The carrier amplitude goes to ZERO?!? Impossible, I thought, "everyone knows" that an FM carrier "swings from side to side." :-) Not long after that I got deep into modulation theory and discovered the how and why of that. Mind-blowing at the time, but it explained what was going on. It's a situation where one has to look at either frequency domain or time domain very hard in order to realize how the two are related. Len Anderson retired (from regular hours) electronic engineer person |
In article , W7TI
writes: On 22 Oct 2003 20:21:16 GMT, (Avery Fineman) wrote: (there are still some long-timers who refuse to accept that the carrier RF energy doesn't change in AM at less than 100% modulation, heh heh) _________________________________________________ ________ I still remember the first time I got hold of a really narrowband receiver and tuned in only the carrier of an AM signal. I was astonished. That was the most enlightening two or three seconds in my whole radio career. :-) Roger that! :-) The first time I had to really calibrate an FM deviation indicator, I had to use the "carrier null" technique with a narrow-bandpass detector (actually a spectrum analyzer). Say WHAT?!? I thought. The carrier amplitude goes to ZERO?!? Impossible, I thought, "everyone knows" that an FM carrier "swings from side to side." :-) Not long after that I got deep into modulation theory and discovered the how and why of that. Mind-blowing at the time, but it explained what was going on. It's a situation where one has to look at either frequency domain or time domain very hard in order to realize how the two are related. Len Anderson retired (from regular hours) electronic engineer person |
In article , Gary Schafer
writes: Speaking of AM modulation,, we all know that the carrier amplitude does not change with modulation. Or does it? Yes and no. It's a situation of subjective understanding of the basic modulation formulas which define the amplitude of an "RF" waveform as a function of TIME [usually denoted as RF voltage "e (t)" meaning the voltage at any given point in time]. With a REPETITIVE modulation waveform of a single, pure audio sinewave, AND the modulation percentage LESS than 100%, the carrier frequency amplitude does indeed remain the same. I put some words into all-caps for emphasis...those are required definitions for proof of both the math AND a bench test set-up using a very narrow bandwidth selective detector. One example witnessed (outside of formal schooling labs) used an audio tone of 10 KHz for amplitude modulation of a 1.5 MHz RF stable continuous wave carrier. With a 100 Hz (approximate) bandwidth of the detector (multiple-down-conversion receiver), the carrier frequency amplitude remained constant despite the modulation percentage changed over 10 to 90 percent. Retuning the detector to 1.49 or 1.51 MHz center frequency, the amplitude of the sidebands varied in direct proportion to the modulation percentage. That setup was right according to theory for a REPETITIVE modulation signal as measured in the FREQUENCY domain. But, but, but...according to a high-Z scope probe of the modulated RF, the amplitude was varying! Why? The oscilloscope was just linearly combining ALL the RF products, the carrier and the two sidebands. The scope "saw" everything on a broadband basis and the display to humans was the very SAME RF but in the TIME domain. But...all the above is for a REPETITVE modulation signal condition. That's relatively easy to determine mathematically since all that is or has to be manipulated are the carrier frequency and modulation frequency and their relative amplitudes. What can get truly hairy is when the modulation signal is NOT repretitive...such as voice or music. Here is a question that has plagued many for years: If you have a plate modulated transmitter, the plate voltage will swing down to zero and up to two times the plate voltage with 100% modulation. At 100% negative modulation the plate voltage is cutoff for the instant of the modulation negative peak. How is the carrier still transmitted during the time there is zero plate voltage? If we lower the modulation frequency to say 1 cps or even lower, 1 cycle per minute, then wouldn't the transmitter final be completely off for half that time and unable to produce any carrier output?? I don't blame you for being puzzled...I used to be so for many years long ago, too. :-) Most new commercial AM transmitters of today combine the "modulator" with the power amplifier supply voltage, getting rid of the old (sometimes mammoth) AF power amplifier in series with the tube plate supply. Yes, in the TIME domain, the RF power output does indeed vary at any point in time according to the modulation. [that still follows the general math formula, "e(t)"] You can take the modulation frequency and run it as low as possible. With AM there is no change in total RF amplitude over frequency (with FM and PM there is). If you've got an instantaneous time window power meter you can measure it directly (but ain't no such animal quite yet). If you set up a FREQUENCY domain test as first described, you will, indeed, measure NO carrier amplitude change with a very narrow bandwidth selective detector at any modulation percentage less than 100% using a REPETITIVE modulation signal. Actual modulation isn't "repetitive" in the sense that a signal generator single audio tone is repetitive. What is a truly TERRIBLY COMPLEX task is both mathematical and practical PROOF of RF spectral components (frequency domain) versus RF time domain amplitudes when the modulation is not repetitive. Please don't go there unless you are a math genius...I wasn't and tried, got sent to a B. Ford Clinic for a long term. :-) In a receiver's conventional AM detector, the recovered audio is a combination of: (1). The diode, already non-linear, is a mixer that combines carrier and sidebands producing an output that is the difference of all of them; (2). The diode recovers the time domain amplitude of the RF, runs it through a low-pass filter to leave only the audio modulation...and also allows averaging of the RF signal amplitude over a longer time. Both (1) and (2) are technically correct. With a pure SSB signal there is a constant RF amplitude with a constant-amplitude repetitive modulation signal, exactly as it would be if the RF output was from a Class C stage. Single frequency if the modulation signal is a pure audio tone. With a non-repetitive modulation the total RF power output varies with the modulation amplitude. The common SSB demodulator ("product detector") is really a MIXER combining the SSB input with a constant, LOCAL RF carrier ("BFO") with the difference product output...which recovers the original modulation signal. Question is, at what point does the carrier start to be effected? Beyond 100% modulation. The most extreme is a common radar pulse, very short in time duration, very long (relative) in repetition time. There's a formula long derived for the amplitude of the spectra of that, commonly referred to as "Sine x over x" when spoken. That sets the receiver bandwidth needed to recover a target return. I've gotten waist-deep into "matched filter" signals, such as using a 1 MHz bandwidth filter to recover 1 microSecond RF pulses. (bandwidth is equivalent to the inverse of on-time of signal, hence the term "matched" for the filter) Most folks, me included, were utterly amazed at the filtered RF output envelope when a detector was tuned off to one side by 1, 2, or 3 MHz. Not at all intuitive. The math got a bit hairy on that and I just accepted the late Jack Breckman's explanation (of RCA Camden) since it worked on the bench as predicted. It's all a matter of how the observer is observing RF things, time domain versus frequency domain...and whether the modulation is repetitive single frequency or multiple, non-repetitive. The math for a repetitive modulation signal works out as the rule for practical hardware that has to handle non-repetitive, multi-frequency modulation signals. When combining two basic modulation forms, things get so hairy its got fur all over. So, like someone explain how an ordinary computer modem can send 56 Kilobits per second over a 3 KHz bandwidth circuit? :-) Len Anderson retired (from regular hours) electronic engineer person |
In article , Gary Schafer
writes: Speaking of AM modulation,, we all know that the carrier amplitude does not change with modulation. Or does it? Yes and no. It's a situation of subjective understanding of the basic modulation formulas which define the amplitude of an "RF" waveform as a function of TIME [usually denoted as RF voltage "e (t)" meaning the voltage at any given point in time]. With a REPETITIVE modulation waveform of a single, pure audio sinewave, AND the modulation percentage LESS than 100%, the carrier frequency amplitude does indeed remain the same. I put some words into all-caps for emphasis...those are required definitions for proof of both the math AND a bench test set-up using a very narrow bandwidth selective detector. One example witnessed (outside of formal schooling labs) used an audio tone of 10 KHz for amplitude modulation of a 1.5 MHz RF stable continuous wave carrier. With a 100 Hz (approximate) bandwidth of the detector (multiple-down-conversion receiver), the carrier frequency amplitude remained constant despite the modulation percentage changed over 10 to 90 percent. Retuning the detector to 1.49 or 1.51 MHz center frequency, the amplitude of the sidebands varied in direct proportion to the modulation percentage. That setup was right according to theory for a REPETITIVE modulation signal as measured in the FREQUENCY domain. But, but, but...according to a high-Z scope probe of the modulated RF, the amplitude was varying! Why? The oscilloscope was just linearly combining ALL the RF products, the carrier and the two sidebands. The scope "saw" everything on a broadband basis and the display to humans was the very SAME RF but in the TIME domain. But...all the above is for a REPETITVE modulation signal condition. That's relatively easy to determine mathematically since all that is or has to be manipulated are the carrier frequency and modulation frequency and their relative amplitudes. What can get truly hairy is when the modulation signal is NOT repretitive...such as voice or music. Here is a question that has plagued many for years: If you have a plate modulated transmitter, the plate voltage will swing down to zero and up to two times the plate voltage with 100% modulation. At 100% negative modulation the plate voltage is cutoff for the instant of the modulation negative peak. How is the carrier still transmitted during the time there is zero plate voltage? If we lower the modulation frequency to say 1 cps or even lower, 1 cycle per minute, then wouldn't the transmitter final be completely off for half that time and unable to produce any carrier output?? I don't blame you for being puzzled...I used to be so for many years long ago, too. :-) Most new commercial AM transmitters of today combine the "modulator" with the power amplifier supply voltage, getting rid of the old (sometimes mammoth) AF power amplifier in series with the tube plate supply. Yes, in the TIME domain, the RF power output does indeed vary at any point in time according to the modulation. [that still follows the general math formula, "e(t)"] You can take the modulation frequency and run it as low as possible. With AM there is no change in total RF amplitude over frequency (with FM and PM there is). If you've got an instantaneous time window power meter you can measure it directly (but ain't no such animal quite yet). If you set up a FREQUENCY domain test as first described, you will, indeed, measure NO carrier amplitude change with a very narrow bandwidth selective detector at any modulation percentage less than 100% using a REPETITIVE modulation signal. Actual modulation isn't "repetitive" in the sense that a signal generator single audio tone is repetitive. What is a truly TERRIBLY COMPLEX task is both mathematical and practical PROOF of RF spectral components (frequency domain) versus RF time domain amplitudes when the modulation is not repetitive. Please don't go there unless you are a math genius...I wasn't and tried, got sent to a B. Ford Clinic for a long term. :-) In a receiver's conventional AM detector, the recovered audio is a combination of: (1). The diode, already non-linear, is a mixer that combines carrier and sidebands producing an output that is the difference of all of them; (2). The diode recovers the time domain amplitude of the RF, runs it through a low-pass filter to leave only the audio modulation...and also allows averaging of the RF signal amplitude over a longer time. Both (1) and (2) are technically correct. With a pure SSB signal there is a constant RF amplitude with a constant-amplitude repetitive modulation signal, exactly as it would be if the RF output was from a Class C stage. Single frequency if the modulation signal is a pure audio tone. With a non-repetitive modulation the total RF power output varies with the modulation amplitude. The common SSB demodulator ("product detector") is really a MIXER combining the SSB input with a constant, LOCAL RF carrier ("BFO") with the difference product output...which recovers the original modulation signal. Question is, at what point does the carrier start to be effected? Beyond 100% modulation. The most extreme is a common radar pulse, very short in time duration, very long (relative) in repetition time. There's a formula long derived for the amplitude of the spectra of that, commonly referred to as "Sine x over x" when spoken. That sets the receiver bandwidth needed to recover a target return. I've gotten waist-deep into "matched filter" signals, such as using a 1 MHz bandwidth filter to recover 1 microSecond RF pulses. (bandwidth is equivalent to the inverse of on-time of signal, hence the term "matched" for the filter) Most folks, me included, were utterly amazed at the filtered RF output envelope when a detector was tuned off to one side by 1, 2, or 3 MHz. Not at all intuitive. The math got a bit hairy on that and I just accepted the late Jack Breckman's explanation (of RCA Camden) since it worked on the bench as predicted. It's all a matter of how the observer is observing RF things, time domain versus frequency domain...and whether the modulation is repetitive single frequency or multiple, non-repetitive. The math for a repetitive modulation signal works out as the rule for practical hardware that has to handle non-repetitive, multi-frequency modulation signals. When combining two basic modulation forms, things get so hairy its got fur all over. So, like someone explain how an ordinary computer modem can send 56 Kilobits per second over a 3 KHz bandwidth circuit? :-) Len Anderson retired (from regular hours) electronic engineer person |
In article , Paul Keinanen
writes: On 22 Oct 2003 20:21:16 GMT, (Avery Fineman) wrote: Bill, I just dug out the 1977 issues of HR from storage and looked the article over. Author Richard Slater (W3EJD) said almost the same thing at the end of the article on page 15 under "closing comments." The nomenclatures for different modulations were formalized by the ITU-R since then but the FCC still doesn't have anything covering this "single-sideband FM" modulation type for U. S. amateur radio. The ITU-R emission designations are quite outdated and many modern emissions use din commercial and military systems would be designated as XXX. In each case the X means "none above" in the corresponding column. Okay, I won't argue the ITU-R thing since I haven't had the ability (by working for a subscribing corporation) to access them. I was going by the "Red Book" information from the U. S. National Telecommunications and Information Agency (NTIA). Over here the NTIA regulates government radio use while the U. S. Federal Communications Commission regulates civil radio use. The method of specifying modulation type, bandwidth, etc., are all explained in there and the FCC follows the same nomenclature. Anyway, why should the amateur radio regulations contain these ITU-R designations ? Here in Finland, ITU-R emission designations were removed from amateur radio regulations and exam in 1997 and only band specific power and bandwidth limits are used. I haven't heard of any problems due to this decision. That's a whole other area that, for amateur radio use, can be and has been argued in rec.radio.amateur.policy. As far as I'm concerned, and no one has ever proved otherwise, electrons, fields, and waves all follow the Laws of physics...and they don't give a @#$%!! about human laws. :-) Len Anderson retired (from regular hours) electronic engineer person |
In article , Paul Keinanen
writes: On 22 Oct 2003 20:21:16 GMT, (Avery Fineman) wrote: Bill, I just dug out the 1977 issues of HR from storage and looked the article over. Author Richard Slater (W3EJD) said almost the same thing at the end of the article on page 15 under "closing comments." The nomenclatures for different modulations were formalized by the ITU-R since then but the FCC still doesn't have anything covering this "single-sideband FM" modulation type for U. S. amateur radio. The ITU-R emission designations are quite outdated and many modern emissions use din commercial and military systems would be designated as XXX. In each case the X means "none above" in the corresponding column. Okay, I won't argue the ITU-R thing since I haven't had the ability (by working for a subscribing corporation) to access them. I was going by the "Red Book" information from the U. S. National Telecommunications and Information Agency (NTIA). Over here the NTIA regulates government radio use while the U. S. Federal Communications Commission regulates civil radio use. The method of specifying modulation type, bandwidth, etc., are all explained in there and the FCC follows the same nomenclature. Anyway, why should the amateur radio regulations contain these ITU-R designations ? Here in Finland, ITU-R emission designations were removed from amateur radio regulations and exam in 1997 and only band specific power and bandwidth limits are used. I haven't heard of any problems due to this decision. That's a whole other area that, for amateur radio use, can be and has been argued in rec.radio.amateur.policy. As far as I'm concerned, and no one has ever proved otherwise, electrons, fields, and waves all follow the Laws of physics...and they don't give a @#$%!! about human laws. :-) Len Anderson retired (from regular hours) electronic engineer person |
Gary Schafer wrote:
So what you are saying is that the carrier of a modulated signal is ONLY a frequency domain concept? Yes. That would mean that it really does turn on and off in the time domain at the modulation rate. "It" only exists in the frequency domain. Talking about the carrier in the time domain makes no more sense than talking about the sidebands in the time domain, or the envelope in the frequency domain. Roy Lewallen, W7EL |
Gary Schafer wrote:
So what you are saying is that the carrier of a modulated signal is ONLY a frequency domain concept? Yes. That would mean that it really does turn on and off in the time domain at the modulation rate. "It" only exists in the frequency domain. Talking about the carrier in the time domain makes no more sense than talking about the sidebands in the time domain, or the envelope in the frequency domain. Roy Lewallen, W7EL |
In article , Gary Schafer
writes: Along the same line consider that the envelope of an SSB signal has no direct relationship to the original modulation the way that an AM signal does. This is why you can not use RF derived ALC to control the audio stage of an SSB transmitter the way you can with an AM transmitter. You can't use ENVELOPE detection on SSB the same way it is done on conventional AM. But, you CAN use RF-derived feedback - if mixed with a steady carrier to recover the modulation content - to do that very well. Or audio clipping that works on AM but does not work the same on SSB. ? Wrongly-done audio clipping on AM is just as bad as on SSB. RF clipping circuits are quite another thing from audio. Transmit a square wave on an AM transmitter and you see a square wave in the AM envelope. Do the same with an SSB transmitter and you only see sharp spikes in the envelope. That depends on the frequency of this square wave. That also depends on what is being used to view the RF envelope. A 50 MHz scope will show the RF envelope of any HF rig. Put an electronic keyer on the SSB transmitter and transmit only dots at a high speed setting. The SSB envelope will show the dots as dots. Conversely, if you put a high-purity sinewave audio into a SSB xmtr, a spectrum analyzer display will show only a single frequency signal. No one can interchange frequency and time domains directly and get an explanation. Envelope viewing is time domain. Spectral analysis is frequency domain. Len Anderson retired (from regular hours) electronic engineer person |
In article , Gary Schafer
writes: Along the same line consider that the envelope of an SSB signal has no direct relationship to the original modulation the way that an AM signal does. This is why you can not use RF derived ALC to control the audio stage of an SSB transmitter the way you can with an AM transmitter. You can't use ENVELOPE detection on SSB the same way it is done on conventional AM. But, you CAN use RF-derived feedback - if mixed with a steady carrier to recover the modulation content - to do that very well. Or audio clipping that works on AM but does not work the same on SSB. ? Wrongly-done audio clipping on AM is just as bad as on SSB. RF clipping circuits are quite another thing from audio. Transmit a square wave on an AM transmitter and you see a square wave in the AM envelope. Do the same with an SSB transmitter and you only see sharp spikes in the envelope. That depends on the frequency of this square wave. That also depends on what is being used to view the RF envelope. A 50 MHz scope will show the RF envelope of any HF rig. Put an electronic keyer on the SSB transmitter and transmit only dots at a high speed setting. The SSB envelope will show the dots as dots. Conversely, if you put a high-purity sinewave audio into a SSB xmtr, a spectrum analyzer display will show only a single frequency signal. No one can interchange frequency and time domains directly and get an explanation. Envelope viewing is time domain. Spectral analysis is frequency domain. Len Anderson retired (from regular hours) electronic engineer person |
Hi Len,
I understand all of the points that you have made and agree that looking at a spectrum analyzer with a modulated signal, less than 100% modulation, shows a constant carrier. I also agree that looking at the time domain with a scope shows the composite of the carrier and side bands. I understand that AM modulation and demodulation is a mixing process that takes place. My question of "at what point does the carrier start to be effected" I was referring to low frequency modulation. Meaning when would you start to notice the carrier change. I don't know how you would observe the carrier in the frequency domain with very low frequency modulation as the side bands would be so close to the carrier. In my scenario of plate modulating a transmitter with a very low modulation frequency (sine or square wave), on the negative part of the modulation cycle the plate voltage will be zero for a significant amount of time of the carrier frequency. The modulation frequency could be 1 cycle per day if we chose. In that case the plate voltage would be zero for 1/2 a day (square wave modulation) and twice the DC plate voltage for the other half day. During the time the plate voltage is zero there would be no RF out of the transmitter as there would be no plate voltage. This is where I get into trouble visualizing the "carrier staying constant with modulation". As the above scenario, there would be zero output so zero carrier for 1/2 a day. The other 1/2 day the plate voltage would be twice so we could say that the carrier power during that time would be twice what it would be with no modulation and that the average carrier power would be constant. (averaged over the entire day). But we know that the extra power supplied by the modulator appears in the side bands and not the carrier. What is happening? 73 Gary K4FMX On 23 Oct 2003 23:04:34 GMT, (Avery Fineman) wrote: In article , Gary Schafer writes: Speaking of AM modulation,, we all know that the carrier amplitude does not change with modulation. Or does it? Yes and no. It's a situation of subjective understanding of the basic modulation formulas which define the amplitude of an "RF" waveform as a function of TIME [usually denoted as RF voltage "e (t)" meaning the voltage at any given point in time]. With a REPETITIVE modulation waveform of a single, pure audio sinewave, AND the modulation percentage LESS than 100%, the carrier frequency amplitude does indeed remain the same. I put some words into all-caps for emphasis...those are required definitions for proof of both the math AND a bench test set-up using a very narrow bandwidth selective detector. One example witnessed (outside of formal schooling labs) used an audio tone of 10 KHz for amplitude modulation of a 1.5 MHz RF stable continuous wave carrier. With a 100 Hz (approximate) bandwidth of the detector (multiple-down-conversion receiver), the carrier frequency amplitude remained constant despite the modulation percentage changed over 10 to 90 percent. Retuning the detector to 1.49 or 1.51 MHz center frequency, the amplitude of the sidebands varied in direct proportion to the modulation percentage. That setup was right according to theory for a REPETITIVE modulation signal as measured in the FREQUENCY domain. But, but, but...according to a high-Z scope probe of the modulated RF, the amplitude was varying! Why? The oscilloscope was just linearly combining ALL the RF products, the carrier and the two sidebands. The scope "saw" everything on a broadband basis and the display to humans was the very SAME RF but in the TIME domain. But...all the above is for a REPETITVE modulation signal condition. That's relatively easy to determine mathematically since all that is or has to be manipulated are the carrier frequency and modulation frequency and their relative amplitudes. What can get truly hairy is when the modulation signal is NOT repretitive...such as voice or music. Here is a question that has plagued many for years: If you have a plate modulated transmitter, the plate voltage will swing down to zero and up to two times the plate voltage with 100% modulation. At 100% negative modulation the plate voltage is cutoff for the instant of the modulation negative peak. How is the carrier still transmitted during the time there is zero plate voltage? If we lower the modulation frequency to say 1 cps or even lower, 1 cycle per minute, then wouldn't the transmitter final be completely off for half that time and unable to produce any carrier output?? I don't blame you for being puzzled...I used to be so for many years long ago, too. :-) Most new commercial AM transmitters of today combine the "modulator" with the power amplifier supply voltage, getting rid of the old (sometimes mammoth) AF power amplifier in series with the tube plate supply. Yes, in the TIME domain, the RF power output does indeed vary at any point in time according to the modulation. [that still follows the general math formula, "e(t)"] You can take the modulation frequency and run it as low as possible. With AM there is no change in total RF amplitude over frequency (with FM and PM there is). If you've got an instantaneous time window power meter you can measure it directly (but ain't no such animal quite yet). If you set up a FREQUENCY domain test as first described, you will, indeed, measure NO carrier amplitude change with a very narrow bandwidth selective detector at any modulation percentage less than 100% using a REPETITIVE modulation signal. Actual modulation isn't "repetitive" in the sense that a signal generator single audio tone is repetitive. What is a truly TERRIBLY COMPLEX task is both mathematical and practical PROOF of RF spectral components (frequency domain) versus RF time domain amplitudes when the modulation is not repetitive. Please don't go there unless you are a math genius...I wasn't and tried, got sent to a B. Ford Clinic for a long term. :-) In a receiver's conventional AM detector, the recovered audio is a combination of: (1). The diode, already non-linear, is a mixer that combines carrier and sidebands producing an output that is the difference of all of them; (2). The diode recovers the time domain amplitude of the RF, runs it through a low-pass filter to leave only the audio modulation...and also allows averaging of the RF signal amplitude over a longer time. Both (1) and (2) are technically correct. With a pure SSB signal there is a constant RF amplitude with a constant-amplitude repetitive modulation signal, exactly as it would be if the RF output was from a Class C stage. Single frequency if the modulation signal is a pure audio tone. With a non-repetitive modulation the total RF power output varies with the modulation amplitude. The common SSB demodulator ("product detector") is really a MIXER combining the SSB input with a constant, LOCAL RF carrier ("BFO") with the difference product output...which recovers the original modulation signal. Question is, at what point does the carrier start to be effected? Beyond 100% modulation. The most extreme is a common radar pulse, very short in time duration, very long (relative) in repetition time. There's a formula long derived for the amplitude of the spectra of that, commonly referred to as "Sine x over x" when spoken. That sets the receiver bandwidth needed to recover a target return. I've gotten waist-deep into "matched filter" signals, such as using a 1 MHz bandwidth filter to recover 1 microSecond RF pulses. (bandwidth is equivalent to the inverse of on-time of signal, hence the term "matched" for the filter) Most folks, me included, were utterly amazed at the filtered RF output envelope when a detector was tuned off to one side by 1, 2, or 3 MHz. Not at all intuitive. The math got a bit hairy on that and I just accepted the late Jack Breckman's explanation (of RCA Camden) since it worked on the bench as predicted. It's all a matter of how the observer is observing RF things, time domain versus frequency domain...and whether the modulation is repetitive single frequency or multiple, non-repetitive. The math for a repetitive modulation signal works out as the rule for practical hardware that has to handle non-repetitive, multi-frequency modulation signals. When combining two basic modulation forms, things get so hairy its got fur all over. So, like someone explain how an ordinary computer modem can send 56 Kilobits per second over a 3 KHz bandwidth circuit? :-) Len Anderson retired (from regular hours) electronic engineer person |
Let me elaborate a little. Maybe the following example will help.
Suppose you've 100% modulated a 1 MHz carrier with a 0.1 Hz sine wave. Our knowledge of frequency domain analysis tells us the spectrum will be a 1 MHz "carrier", with two sidebands, one at 1,000,000.1 Hz and the other at 999,999,999.9 Hz. At 100% modulation, the power amplitude of each sideband will be 1/4 the amplitude of the carrier; the voltage amplitude of each will be 1/2 the amplitude of the carrier. Now, imagine that you can draw three sine waves on a long piece of paper. They would have the frequencies and amplitudes of the three spectral components above. These are the time domain representations of the three frequency domain components. (In that sense, you *can* speak of a carrier or a sideband in the time domain -- so I was perhaps unduly dogmatic about that point.) But here's the important thing to keep in mind -- all three of these components have constant amplitudes. They extend from the beginning of time to the end of time, and don't start, stop, or change at any time. That's what those spectral lines mean, and what we get when we transform them back to the time domain. At each instant of time, look at the values of all three components and add them. At some times, you'll find that the two sideband sine waves are both at their maxima at the same time that the carrier sine wave is at its maximum. At those times, the sum of the three will be twice the value of the carrier wave alone. At some other times, both sidebands are hitting their maxima just when the carrier is at its minimum value. At those instants, the sum will be zero. After you plot enough points, you'll find you've reconstructed the time waveform of the modulated signal. You'll also find you need at least ten seconds of these three waveforms to create one full cycle -- repetition -- of the modulated wave. During that ten second period, the carrier sine wave doesn't change amplitude, nor do the sideband sine waves change amplitude. Only the time waveform, which is not the carrier or the sidebands, but always the sum of the three, changes. When we speak of a carrier wave, we mean that sine wave of constant amplitude that never changes -- in other words, a single component in the frequency domain. Roy Lewallen, W7EL Roy Lewallen wrote: Gary Schafer wrote: So what you are saying is that the carrier of a modulated signal is ONLY a frequency domain concept? Yes. That would mean that it really does turn on and off in the time domain at the modulation rate. "It" only exists in the frequency domain. Talking about the carrier in the time domain makes no more sense than talking about the sidebands in the time domain, or the envelope in the frequency domain. Roy Lewallen, W7EL |
Let me elaborate a little. Maybe the following example will help.
Suppose you've 100% modulated a 1 MHz carrier with a 0.1 Hz sine wave. Our knowledge of frequency domain analysis tells us the spectrum will be a 1 MHz "carrier", with two sidebands, one at 1,000,000.1 Hz and the other at 999,999,999.9 Hz. At 100% modulation, the power amplitude of each sideband will be 1/4 the amplitude of the carrier; the voltage amplitude of each will be 1/2 the amplitude of the carrier. Now, imagine that you can draw three sine waves on a long piece of paper. They would have the frequencies and amplitudes of the three spectral components above. These are the time domain representations of the three frequency domain components. (In that sense, you *can* speak of a carrier or a sideband in the time domain -- so I was perhaps unduly dogmatic about that point.) But here's the important thing to keep in mind -- all three of these components have constant amplitudes. They extend from the beginning of time to the end of time, and don't start, stop, or change at any time. That's what those spectral lines mean, and what we get when we transform them back to the time domain. At each instant of time, look at the values of all three components and add them. At some times, you'll find that the two sideband sine waves are both at their maxima at the same time that the carrier sine wave is at its maximum. At those times, the sum of the three will be twice the value of the carrier wave alone. At some other times, both sidebands are hitting their maxima just when the carrier is at its minimum value. At those instants, the sum will be zero. After you plot enough points, you'll find you've reconstructed the time waveform of the modulated signal. You'll also find you need at least ten seconds of these three waveforms to create one full cycle -- repetition -- of the modulated wave. During that ten second period, the carrier sine wave doesn't change amplitude, nor do the sideband sine waves change amplitude. Only the time waveform, which is not the carrier or the sidebands, but always the sum of the three, changes. When we speak of a carrier wave, we mean that sine wave of constant amplitude that never changes -- in other words, a single component in the frequency domain. Roy Lewallen, W7EL Roy Lewallen wrote: Gary Schafer wrote: So what you are saying is that the carrier of a modulated signal is ONLY a frequency domain concept? Yes. That would mean that it really does turn on and off in the time domain at the modulation rate. "It" only exists in the frequency domain. Talking about the carrier in the time domain makes no more sense than talking about the sidebands in the time domain, or the envelope in the frequency domain. Roy Lewallen, W7EL |
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On Thu, 23 Oct 2003 19:51:49 -0700, Roy Lewallen
wrote: Now, imagine that you can draw three sine waves on a long piece of paper. They would have the frequencies and amplitudes of the three spectral components above. These are the time domain representations of the three frequency domain components. (In that sense, you *can* speak of a carrier or a sideband in the time domain -- so I was perhaps unduly dogmatic about that point.) But here's the important thing to keep in mind -- all three of these components have constant amplitudes. They extend from the beginning of time to the end of time, and don't start, stop, or change at any time. That's what those spectral lines mean, and what we get when we transform them back to the time domain. It is quite easy to visualise this using a spreadsheet program. However, it would be easier to use a much higher modulation frequency compared to the carrier frequency. Assuming a carrier frequency of 1000 Hz and a modulating frequency of 100 Hz, so the sidebands would be at 900 and 1100 Hz. In column A put the time t and for each line increment the value by 0.0001 s or 0.00005 s. In column B calculate 0.5*sin(2*pi*900*t). In column C calculate 1.0*sin(2*pi*1000*t). In column D calculate 0.5*sin(2*pi*1100*t). In column E calculate the sum of columns B, C and D. Duplicate these lines 500 to 1000 times and draw a graph, with column A or time as the X-axis and display columns B, C, D and E as separate graphs on the Y-axis. Paul OH3LWR |
On Thu, 23 Oct 2003 19:51:49 -0700, Roy Lewallen
wrote: Now, imagine that you can draw three sine waves on a long piece of paper. They would have the frequencies and amplitudes of the three spectral components above. These are the time domain representations of the three frequency domain components. (In that sense, you *can* speak of a carrier or a sideband in the time domain -- so I was perhaps unduly dogmatic about that point.) But here's the important thing to keep in mind -- all three of these components have constant amplitudes. They extend from the beginning of time to the end of time, and don't start, stop, or change at any time. That's what those spectral lines mean, and what we get when we transform them back to the time domain. It is quite easy to visualise this using a spreadsheet program. However, it would be easier to use a much higher modulation frequency compared to the carrier frequency. Assuming a carrier frequency of 1000 Hz and a modulating frequency of 100 Hz, so the sidebands would be at 900 and 1100 Hz. In column A put the time t and for each line increment the value by 0.0001 s or 0.00005 s. In column B calculate 0.5*sin(2*pi*900*t). In column C calculate 1.0*sin(2*pi*1000*t). In column D calculate 0.5*sin(2*pi*1100*t). In column E calculate the sum of columns B, C and D. Duplicate these lines 500 to 1000 times and draw a graph, with column A or time as the X-axis and display columns B, C, D and E as separate graphs on the Y-axis. Paul OH3LWR |
In article , Gary Schafer
writes: I understand all of the points that you have made and agree that looking at a spectrum analyzer with a modulated signal, less than 100% modulation, shows a constant carrier. I also agree that looking at the time domain with a scope shows the composite of the carrier and side bands. I understand that AM modulation and demodulation is a mixing process that takes place. My question of "at what point does the carrier start to be effected" I was referring to low frequency modulation. Meaning when would you start to notice the carrier change. As long as the AM is less than 100% there won't be any change. The qualifier there is the MEASURING INSTRUMENT that is looking at the carrier. With low and very low modulation frequencies, the sidebands created will be very close to the carrier frequency. If the measuring instrument cannot select just the carrier, then the instrument "sees" both the carrier and sidebands...and that gets into the time domain again which WILL show an APPARENT amplitude modulation of the carrier (instrument is looking at everything). I don't know how you would observe the carrier in the frequency domain with very low frequency modulation as the side bands would be so close to the carrier. DSP along with very narrow final IF filtering can do it, but that isn't absolutely necessary to prove the point. Using "ordinary" narrowband filtering like a very sharp skirt 500 Hz BW filter and variable frequency audio modulation from about 1 KHz on up to some higher, one can separately measure the carrier and sideband amplitudes. It will also show that the sidebands and carrier do not change amplitude for a change in modulation frequency, which is predicted by the general AM equations. Ergo, decreasing the modulation frequency will not change amplitude but one bumps into the problem of instrument/receiver selectivity. That problem is one of instrumentation, not theory. In my scenario of plate modulating a transmitter with a very low modulation frequency (sine or square wave), on the negative part of the modulation cycle the plate voltage will be zero for a significant amount of time of the carrier frequency. The modulation frequency could be 1 cycle per day if we chose. In that case the plate voltage would be zero for 1/2 a day (square wave modulation) and twice the DC plate voltage for the other half day. During the time the plate voltage is zero there would be no RF out of the transmitter as there would be no plate voltage. It's a problem of observation again. Even with a rate of 1 cycle per day, the sidebands are still going to be there and the observing instrument is going to be looking at carrier AND sidebands at the same time. That would be right at 100% modulation, has to be if the carrier envelope is observed to go to zero. At 99.999% (or however close one wants to get to 100 but not reach it) modulation, the theory for frequency domain still holds. Above that 100% modulation, another theory has to be there. For greater-than-100% modulation, an extreme case would be on- off keying "CW." Sidebands are still generated, but those are due to the very fast transition from off to on and on to off. Those sidebands definitely exist and can be heard as "clicks" away from the carrier. In designs of on-off keyed carrier transmitters, the good rule is to limit the transition rate, to keep it slower rather than faster. [that's in the ARRL Handbook, BTW] Slowing the transition rate reduces the sidebands caused by transient effects (the on-off thing). Modulation indexes greater than 100% fall under different theory. For on-off keyed "CW" transmitters, the transient effect sideband generation is much farther away from the carrier than low-frequency audio at less than 100% modulation. It can be observed (heard) readily with a strong signal. This is where I get into trouble visualizing the "carrier staying constant with modulation". As the above scenario, there would be zero output so zero carrier for 1/2 a day. The other 1/2 day the plate voltage would be twice so we could say that the carrier power during that time would be twice what it would be with no modulation and that the average carrier power would be constant. (averaged over the entire day). But we know that the extra power supplied by the modulator appears in the side bands and not the carrier. What is happening? A lack of a definitive terribly-selective observation instrument is what is happening. Theory predicts no change in sideband amplitude with AM's modulating frequency and practical testing with instruments proves that, right down to the limit of the instruments. So, lowering the modulation frequency to very low, even sub-audio, doesn't change anything. The instruments run out of selectivity and start measuring the combination of all products at the same time. Instrumentation will observe time domain (the envelope) instead of frequency domain (individual sidebands). There's really nothing wrong with theory or the practicality of it all. The general equations for modulated RF use a single frequency for modulation in the textbooks because that is the easiest to show to a student. A few will show the equations with two, possibly three frequencies...but those quickly become VERY cumbersome to handle, are avoided when starting in on teaching of modulation theory. The simple examples are good enough to figure out necessary communications bandwidth...which is what counts in the practical situation of making hardware that works for AM or FM or PM. In the real world, everyone is really working in time domain. But, the frequency domain theory tells what the bandwidth has to be for all to get time domain information. In SSB with very attenuated carrier level, that single sideband is carrying ALL the information needed. We can't "hear" RF so the very amplitude stable receiver carrier frequency resupply allows recovery of the original audio. With very very stable propagation and a constant circuit strength, the original audio could go way down in frequency to DC. The SSB receiver could theoretically recover everything all the way down to DC...except the practicality of minimizing the total SSB bandwidth and suppressing the carrier puts the low frequency cutoff around 300 to 200 Hz. The carrier isn't transmitted, and it is substituted in the receiver at a stable amplitude in a SSB total circuit. Yet, theoretically it would be possible to get a very low modulation rate but nobody cares to do so. There ARE remote telemetering FM systems that DO go all the way down to DC...but most communications applications have a practical low-frequency cutoff. Theory allows it but practicality dictates other- wise. The same in instrumentation recording/observing what is happening...that also has practical limitations. If most folks stop at the "traditional" AM modulation envelope scope photos, fine. One can go fairly far just on those. To go farther, one has to delve into the theory just as deeply, perhaps moreso. Staying with the simplistic AM envelope-only view is what made a lot of hams angry in the 1950s when SSB was being adopted very quickly in amateur radio. They couldn't grasp phasing well; it didn't have any relation to the "traditional" AM modulation envelope concept. They couldn't grasp the frequency domain well, either, but that was a bit simpler than phasing vectors and caught on better than phasing explanations. :-) Basic theory is still good, still useable. Nothing has been violated for the three basic modulation types. Practical hardware by the ton has shown that theory is indeed correct in radio and on landline (the first "SSB" was in long-distance wired telephony). BLENDING two basic modulation types takes a LOT more skull sweat to grasp and nothing can be "proved" using simplistic statements or examples (like AM from just RF envelope scope shots) either for or against. I like to use the POTS modem example...getting (essentially equivalent) 56 K rate communications through a 3 KHz bandwidth circuit. That uses a combination of AM and PM. Blends two basic types of modulation, but in a certain way. Nearly all of us use one to communicate on the Internet and it works fine, is faster than some ISP computers, heh heh. So, the simplistic explanations of "one can't get that fast a communication rate through a narrow bandwidth!" falls flat on its 0 state when there are all these practical examples showing it does work. It isn't magic. It's just a clever way to blend two kinds of modulation for a specific purpose. It works. In the "single-sideband FM" examples, one cannot use the simplistic rules for FM in regards to bandwidth or rate. Those experiments were combining things in a non-traditional way. It isn't strictly single sideband, either, but many are off-put by the name given it. Len Anderson retired (from regular hours) electronic engineer person |
In article , Gary Schafer
writes: I understand all of the points that you have made and agree that looking at a spectrum analyzer with a modulated signal, less than 100% modulation, shows a constant carrier. I also agree that looking at the time domain with a scope shows the composite of the carrier and side bands. I understand that AM modulation and demodulation is a mixing process that takes place. My question of "at what point does the carrier start to be effected" I was referring to low frequency modulation. Meaning when would you start to notice the carrier change. As long as the AM is less than 100% there won't be any change. The qualifier there is the MEASURING INSTRUMENT that is looking at the carrier. With low and very low modulation frequencies, the sidebands created will be very close to the carrier frequency. If the measuring instrument cannot select just the carrier, then the instrument "sees" both the carrier and sidebands...and that gets into the time domain again which WILL show an APPARENT amplitude modulation of the carrier (instrument is looking at everything). I don't know how you would observe the carrier in the frequency domain with very low frequency modulation as the side bands would be so close to the carrier. DSP along with very narrow final IF filtering can do it, but that isn't absolutely necessary to prove the point. Using "ordinary" narrowband filtering like a very sharp skirt 500 Hz BW filter and variable frequency audio modulation from about 1 KHz on up to some higher, one can separately measure the carrier and sideband amplitudes. It will also show that the sidebands and carrier do not change amplitude for a change in modulation frequency, which is predicted by the general AM equations. Ergo, decreasing the modulation frequency will not change amplitude but one bumps into the problem of instrument/receiver selectivity. That problem is one of instrumentation, not theory. In my scenario of plate modulating a transmitter with a very low modulation frequency (sine or square wave), on the negative part of the modulation cycle the plate voltage will be zero for a significant amount of time of the carrier frequency. The modulation frequency could be 1 cycle per day if we chose. In that case the plate voltage would be zero for 1/2 a day (square wave modulation) and twice the DC plate voltage for the other half day. During the time the plate voltage is zero there would be no RF out of the transmitter as there would be no plate voltage. It's a problem of observation again. Even with a rate of 1 cycle per day, the sidebands are still going to be there and the observing instrument is going to be looking at carrier AND sidebands at the same time. That would be right at 100% modulation, has to be if the carrier envelope is observed to go to zero. At 99.999% (or however close one wants to get to 100 but not reach it) modulation, the theory for frequency domain still holds. Above that 100% modulation, another theory has to be there. For greater-than-100% modulation, an extreme case would be on- off keying "CW." Sidebands are still generated, but those are due to the very fast transition from off to on and on to off. Those sidebands definitely exist and can be heard as "clicks" away from the carrier. In designs of on-off keyed carrier transmitters, the good rule is to limit the transition rate, to keep it slower rather than faster. [that's in the ARRL Handbook, BTW] Slowing the transition rate reduces the sidebands caused by transient effects (the on-off thing). Modulation indexes greater than 100% fall under different theory. For on-off keyed "CW" transmitters, the transient effect sideband generation is much farther away from the carrier than low-frequency audio at less than 100% modulation. It can be observed (heard) readily with a strong signal. This is where I get into trouble visualizing the "carrier staying constant with modulation". As the above scenario, there would be zero output so zero carrier for 1/2 a day. The other 1/2 day the plate voltage would be twice so we could say that the carrier power during that time would be twice what it would be with no modulation and that the average carrier power would be constant. (averaged over the entire day). But we know that the extra power supplied by the modulator appears in the side bands and not the carrier. What is happening? A lack of a definitive terribly-selective observation instrument is what is happening. Theory predicts no change in sideband amplitude with AM's modulating frequency and practical testing with instruments proves that, right down to the limit of the instruments. So, lowering the modulation frequency to very low, even sub-audio, doesn't change anything. The instruments run out of selectivity and start measuring the combination of all products at the same time. Instrumentation will observe time domain (the envelope) instead of frequency domain (individual sidebands). There's really nothing wrong with theory or the practicality of it all. The general equations for modulated RF use a single frequency for modulation in the textbooks because that is the easiest to show to a student. A few will show the equations with two, possibly three frequencies...but those quickly become VERY cumbersome to handle, are avoided when starting in on teaching of modulation theory. The simple examples are good enough to figure out necessary communications bandwidth...which is what counts in the practical situation of making hardware that works for AM or FM or PM. In the real world, everyone is really working in time domain. But, the frequency domain theory tells what the bandwidth has to be for all to get time domain information. In SSB with very attenuated carrier level, that single sideband is carrying ALL the information needed. We can't "hear" RF so the very amplitude stable receiver carrier frequency resupply allows recovery of the original audio. With very very stable propagation and a constant circuit strength, the original audio could go way down in frequency to DC. The SSB receiver could theoretically recover everything all the way down to DC...except the practicality of minimizing the total SSB bandwidth and suppressing the carrier puts the low frequency cutoff around 300 to 200 Hz. The carrier isn't transmitted, and it is substituted in the receiver at a stable amplitude in a SSB total circuit. Yet, theoretically it would be possible to get a very low modulation rate but nobody cares to do so. There ARE remote telemetering FM systems that DO go all the way down to DC...but most communications applications have a practical low-frequency cutoff. Theory allows it but practicality dictates other- wise. The same in instrumentation recording/observing what is happening...that also has practical limitations. If most folks stop at the "traditional" AM modulation envelope scope photos, fine. One can go fairly far just on those. To go farther, one has to delve into the theory just as deeply, perhaps moreso. Staying with the simplistic AM envelope-only view is what made a lot of hams angry in the 1950s when SSB was being adopted very quickly in amateur radio. They couldn't grasp phasing well; it didn't have any relation to the "traditional" AM modulation envelope concept. They couldn't grasp the frequency domain well, either, but that was a bit simpler than phasing vectors and caught on better than phasing explanations. :-) Basic theory is still good, still useable. Nothing has been violated for the three basic modulation types. Practical hardware by the ton has shown that theory is indeed correct in radio and on landline (the first "SSB" was in long-distance wired telephony). BLENDING two basic modulation types takes a LOT more skull sweat to grasp and nothing can be "proved" using simplistic statements or examples (like AM from just RF envelope scope shots) either for or against. I like to use the POTS modem example...getting (essentially equivalent) 56 K rate communications through a 3 KHz bandwidth circuit. That uses a combination of AM and PM. Blends two basic types of modulation, but in a certain way. Nearly all of us use one to communicate on the Internet and it works fine, is faster than some ISP computers, heh heh. So, the simplistic explanations of "one can't get that fast a communication rate through a narrow bandwidth!" falls flat on its 0 state when there are all these practical examples showing it does work. It isn't magic. It's just a clever way to blend two kinds of modulation for a specific purpose. It works. In the "single-sideband FM" examples, one cannot use the simplistic rules for FM in regards to bandwidth or rate. Those experiments were combining things in a non-traditional way. It isn't strictly single sideband, either, but many are off-put by the name given it. Len Anderson retired (from regular hours) electronic engineer person |
I've read through some of the replies and didn't see what I thought was a good answer to "where can I find a good explanation". We've been doing a series of technical seminars at work, and one of the first ones covered AM and FM modulation. (FYI...we build equipment that is very good at analyzing spectral content of signals, so it's an area we care quite a bit about.) We used a vector diagram that I think is fairly easy to understand. Wish I could draw it here! I'll try to describe it verbally in a way you could draw it yourself, and think about it. For AM: Draw a vector starting at the origin and going one unit right. This is the carrier, at time=0. It rotates counterclockwise (by convention) at the carrier frequency. Now consider, say, 50% modulation with some sinewave, maybe 1/1000 the carrier freq. To represent this, draw two more vectors. The way we've done it is to start them both at the right end of the first (carrier) vector. Both are 1/4 unit long. To start, at time=0, draw them both further to the right from the carrier. Since they are both adding to the carrier, the net at that point in time is 1.5 units long. Now if the carrier didn't move (zero freq), one of the little vectors would rotate clockwise and one would rotate counterclockwise, at just the same rates. (Careful here! The one going clockwise represents your "negative freq" if you will, but there is NO MATH, just a picture, so don't let your mind lock up on this one!) They'd get to be both pointing to the left at just the same time, and at that time they'd subtract from the carrier and leave you with a vector 0.5 units long. But before you got to that point, you'd have one of them pointing straight up, and one pointing down, and they'd cancel out, leaving just the carrier. Now just imagine all that happening as the carrier rotates them around... it's all just the same but produces the carrier plus the two sidebands. One key thing to get from this picture is that the two modulation vectors always sum together to a vector which is parallel to the carrier vector (or else zero length). For FM: Draw the same picture, but now the modulation vectors both start pointing up, at 90 degrees to the carrier. As they rotate around, they always sum to something that is perpendicular to the carrier vector. Hmmmm...but notice that if they are very short, the net result is practically the same length as the carrier vector all the time, but if they are a bit longer, you'd have the carrier amplitude changing. Draw the picture to see that! Let's say that each of the two are 1/10 as long as the carrier, so that the result is a right triangle with the carrier 1 unit long and the modulation 1/5 unit long. So the net in that case would be sqrt(1^2 + 0.2^2) = 1.02. But this is FM, and the amplitude is not allowed to change. So we have to put in a correction. One way to do that is to add a couple more vectors which correct this initial error. If you think it through, you'll see they have to rotate twice as fast as the initial two modulation vectors. So the initial ones represent the first sidebands, and the next pair represent the second sidebands...and if you draw it out right, you'll be able to see how the whole set of sidebands comes about. So...why is it FM? Because the sidebands rotate the carrier phase. In fact, that's how you have to draw the set of modulation vectors: to sum up to a carrier whose phase is modulated (which is the same as FM, of course, for this single sine freq modulation). But notice that if the modulation is low enough, practically all the modulation energy is in those initial two sidebands, represented by the first two vectors. Now if you transmitted ONLY those two and removed the carrier, and someone on the other end inserted the carrier at t=0 pointing UP instead of to the right, why you'd have -- AM! Or at least something very, very close to AM. So, I think it should be clear from that, that single sideband FM (assuming very low modulation index) should be practically equivalent to single sideband AM. By the way, back several years ago there was a lot of interest in finding ways to make more efficient AM broadcast transmitters. If you use a class C power amplifier, you can get good RF-generator efficiency, but the modulator running class AB or B is inefficient. And if you do the modulation at a low level, you have to run the RF chain AB or B. So one of the ways invented to get AM was to generate two FM signals, which of course can be amplified by class C power amps, whose modulation was generated through a pretty special DSP algorithm, so that when you combined the RF outputs of the two FM transmitters you got, ta-da, AM! I always thought that was pretty cool, but I don't think it ever caught on in a big way, because folk have come up with other ways of efficiently generating AM. Cheers, Tom (Bruce Kizerian) wrote in message . com... Can anyone direct me to some good understandable references on single sideband frequency modulation? I have no real practical reason for wanting to know about this. It is interesting to me in a "mathetical" sort of way. Of course, that is dangerous for me because my brain gets very stubborn when I try to do math. Such ideas as "negative frequency" kind of send my mental faculties into total shutdown. But I read schematic very well. It is a visual language I can usually understand. Seems like years ago there was an article on SSB FM in Ham Radio. That would probably be a good start. If anyone can send me a copy of that article I would be much appreciative. Thanks in advance Bruce kk7zz www.elmerdude.com Cheers, Tom |
I've read through some of the replies and didn't see what I thought was a good answer to "where can I find a good explanation". We've been doing a series of technical seminars at work, and one of the first ones covered AM and FM modulation. (FYI...we build equipment that is very good at analyzing spectral content of signals, so it's an area we care quite a bit about.) We used a vector diagram that I think is fairly easy to understand. Wish I could draw it here! I'll try to describe it verbally in a way you could draw it yourself, and think about it. For AM: Draw a vector starting at the origin and going one unit right. This is the carrier, at time=0. It rotates counterclockwise (by convention) at the carrier frequency. Now consider, say, 50% modulation with some sinewave, maybe 1/1000 the carrier freq. To represent this, draw two more vectors. The way we've done it is to start them both at the right end of the first (carrier) vector. Both are 1/4 unit long. To start, at time=0, draw them both further to the right from the carrier. Since they are both adding to the carrier, the net at that point in time is 1.5 units long. Now if the carrier didn't move (zero freq), one of the little vectors would rotate clockwise and one would rotate counterclockwise, at just the same rates. (Careful here! The one going clockwise represents your "negative freq" if you will, but there is NO MATH, just a picture, so don't let your mind lock up on this one!) They'd get to be both pointing to the left at just the same time, and at that time they'd subtract from the carrier and leave you with a vector 0.5 units long. But before you got to that point, you'd have one of them pointing straight up, and one pointing down, and they'd cancel out, leaving just the carrier. Now just imagine all that happening as the carrier rotates them around... it's all just the same but produces the carrier plus the two sidebands. One key thing to get from this picture is that the two modulation vectors always sum together to a vector which is parallel to the carrier vector (or else zero length). For FM: Draw the same picture, but now the modulation vectors both start pointing up, at 90 degrees to the carrier. As they rotate around, they always sum to something that is perpendicular to the carrier vector. Hmmmm...but notice that if they are very short, the net result is practically the same length as the carrier vector all the time, but if they are a bit longer, you'd have the carrier amplitude changing. Draw the picture to see that! Let's say that each of the two are 1/10 as long as the carrier, so that the result is a right triangle with the carrier 1 unit long and the modulation 1/5 unit long. So the net in that case would be sqrt(1^2 + 0.2^2) = 1.02. But this is FM, and the amplitude is not allowed to change. So we have to put in a correction. One way to do that is to add a couple more vectors which correct this initial error. If you think it through, you'll see they have to rotate twice as fast as the initial two modulation vectors. So the initial ones represent the first sidebands, and the next pair represent the second sidebands...and if you draw it out right, you'll be able to see how the whole set of sidebands comes about. So...why is it FM? Because the sidebands rotate the carrier phase. In fact, that's how you have to draw the set of modulation vectors: to sum up to a carrier whose phase is modulated (which is the same as FM, of course, for this single sine freq modulation). But notice that if the modulation is low enough, practically all the modulation energy is in those initial two sidebands, represented by the first two vectors. Now if you transmitted ONLY those two and removed the carrier, and someone on the other end inserted the carrier at t=0 pointing UP instead of to the right, why you'd have -- AM! Or at least something very, very close to AM. So, I think it should be clear from that, that single sideband FM (assuming very low modulation index) should be practically equivalent to single sideband AM. By the way, back several years ago there was a lot of interest in finding ways to make more efficient AM broadcast transmitters. If you use a class C power amplifier, you can get good RF-generator efficiency, but the modulator running class AB or B is inefficient. And if you do the modulation at a low level, you have to run the RF chain AB or B. So one of the ways invented to get AM was to generate two FM signals, which of course can be amplified by class C power amps, whose modulation was generated through a pretty special DSP algorithm, so that when you combined the RF outputs of the two FM transmitters you got, ta-da, AM! I always thought that was pretty cool, but I don't think it ever caught on in a big way, because folk have come up with other ways of efficiently generating AM. Cheers, Tom (Bruce Kizerian) wrote in message . com... Can anyone direct me to some good understandable references on single sideband frequency modulation? I have no real practical reason for wanting to know about this. It is interesting to me in a "mathetical" sort of way. Of course, that is dangerous for me because my brain gets very stubborn when I try to do math. Such ideas as "negative frequency" kind of send my mental faculties into total shutdown. But I read schematic very well. It is a visual language I can usually understand. Seems like years ago there was an article on SSB FM in Ham Radio. That would probably be a good start. If anyone can send me a copy of that article I would be much appreciative. Thanks in advance Bruce kk7zz www.elmerdude.com Cheers, Tom |
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In article , Gary Schafer
writes: Let's start at the other end and see what happens; If we have a final amp with 1000 dc volts on the plate and we want to plate modulate it to 100% or very near so, we need 1000 volts peak to peak audio to do it. On positive audio peaks the dc plate voltage and the positive peak audio voltage will add together to provide 2000 volts plate voltage. On negative audio peaks the negative audio voltage will subtract from the dc plate voltage with a net of zero volts left on the plate at that time. (or very nearly zero volts if we do not quite hit 100%) How does the tube put out any power (carrier) at the time there is near zero plate voltage on it? The negative audio cycle portion is going to be much longer than many rf cycles so the tank circuit is not going to maintain it on its own. Why does the carrier stay full? Gary, you are trying to mix the frequency domain and time domain information...and then confusing steady-state conditions in the time domain with repetitive conditions. The "carrier amplitude is constant" holds true over repetitive audio modulation. In conventional AM, with repetitive modulation from a pure tone, there are three RF spectral products. If you deliberately notch out the carrier component in a receiver, and then reinsert a steady-state, synchronized carrier frequency component in its place, you will recover the original modulation audio. The receiver demodulator sees only a steady, constant-amplitude carrier frequency component. There is absolutely no carrier amplitude variation then. But the original modulation audio is demodulated exactly as if it were the done with the original transmitted carrier. SSB reception is done all the time that way (except the carrier amplitude is so low it might as well be zero). That's a practical test proving only that the carrier amplitude does not have any change insofar as demodulation is concerned. As a practical test of just the transmitter, let's consider your basic old-style AM description...Class-C RF PA with linear plate volts v. power output characteristic, modulation by the plate voltage. That plate voltage is 1 KV steady-state. In steady-state, RF output has a single RF component, the carrier frequency. One. RF spectral component will follow the general time-domain RF equations with no modulation. [easy math there] Apply modulation to the plate voltage with a pure tone. Plate voltage swings UP as well as DOWN equally. [theoretical perfect linear situation] Same rate of UP and DOWN. [start thinking dv/dt] Look at the spectral components with this pure tone modulation. Now we have THREE, not just one. Any high resolution spectrum analyzer sampling the RF output will provide practical proof of that. So, if you want to examine the total RF in a time-domain situation, you MUST examine it as amplitude versus an infinitely-thin slice of TIME. You cannot take a finite time chunk out of the RF envelope and "prove" anything...anymore than you can justify the existance of three RF components, not just TWO. [if this were the real classroom, you would have to prove that on the whiteboard and justify it in full public view...and maybe have to show the class the spectrum analyzer output]. Remember that the modulation signal also exists in a time domain and is constantly changing. If the "carrier sinewave goes to zero and thus power output is zero," how do you justify that, a half repetition time of the modulation signal later, "carrier sinewave goes to twice amplitude and power output is double"? You are trying an analogy that has a special condition, by neglecting the RATE of the modulation. It is always changing just as the carrier frequency sinewave is changing. You want to stop time for the modulation to show repetitive RF carrier sinusoids and that is NOT modulation. It is just adjustment of the RF output via plate voltage. No modulation at all. The basic equation of an AM RF amplitude holds for those infinitely- small slices of TIME. The series expansion of that basic equation will show the spectral components that exist in the frequency domain. Nothing has been violated in the math and practical measurements will prove the existance and nature of the spectral components. For those that like the vector presentation of things, trying to look at a longer-than-infinitely-small slice of time or just the negative or positive modulation swings is the SAME as removal of the modulation signal vector. Such wouldn't exist in that hypothetical situation. It would be only the RF carrier vector rotating all by itself. In basic FM or PM, there's NO change in RF envelope amplitude with a perfect source of FM or PM. "The carrier swings from side to side with modulation," right? Okay, then how come for why does the carrier spectral frequency component go to ZERO with a certain modulation/deviation level and STAY there as long as the modulation is held at that level? RF envelope amplitude will remain constant. Good old spectrum analyzer has practical proof of that. [common way of precise calibration of modulation index with FM] The FM is "just swinging frequency up and down" is much too simple an explanation, excellent for quick-training technicians who have to keep ready- built stuff running, not very good for those who have to use true basics for design, very bad for those involved with unusual combinations of modulation. If you go back to your original situation and have this theoretical power meter working with conventional AM, prove there are ANY sidebands generated from the modulation of plant voltage...or one or two or more. :-) Going to be a difficult task doing that, yet there obviously ARE sidebands generated with conventional AM and each set has the same information. Lose one and modulation continues. Prove it solely from the time-domain modulation envelope. Prove the carrier component amplitude varies or remains constant. Hint: You will wind up doing as another Johnny Carson did way back in 1922 (or thereabouts) when the basic modulation equations were presented on paper. [John R. Carson, I'm not going to argue the year, that's in good textbooks for the persnickety] With conventional AM the CARRIER FREQUENCY COMPONENT amplitude remains the same for any modulation percentage less than 100. Period. I not gonna argue this anymore. :-) Len Anderson retired (from regular hours) electornic engineer person |
In article , Gary Schafer
writes: Let's start at the other end and see what happens; If we have a final amp with 1000 dc volts on the plate and we want to plate modulate it to 100% or very near so, we need 1000 volts peak to peak audio to do it. On positive audio peaks the dc plate voltage and the positive peak audio voltage will add together to provide 2000 volts plate voltage. On negative audio peaks the negative audio voltage will subtract from the dc plate voltage with a net of zero volts left on the plate at that time. (or very nearly zero volts if we do not quite hit 100%) How does the tube put out any power (carrier) at the time there is near zero plate voltage on it? The negative audio cycle portion is going to be much longer than many rf cycles so the tank circuit is not going to maintain it on its own. Why does the carrier stay full? Gary, you are trying to mix the frequency domain and time domain information...and then confusing steady-state conditions in the time domain with repetitive conditions. The "carrier amplitude is constant" holds true over repetitive audio modulation. In conventional AM, with repetitive modulation from a pure tone, there are three RF spectral products. If you deliberately notch out the carrier component in a receiver, and then reinsert a steady-state, synchronized carrier frequency component in its place, you will recover the original modulation audio. The receiver demodulator sees only a steady, constant-amplitude carrier frequency component. There is absolutely no carrier amplitude variation then. But the original modulation audio is demodulated exactly as if it were the done with the original transmitted carrier. SSB reception is done all the time that way (except the carrier amplitude is so low it might as well be zero). That's a practical test proving only that the carrier amplitude does not have any change insofar as demodulation is concerned. As a practical test of just the transmitter, let's consider your basic old-style AM description...Class-C RF PA with linear plate volts v. power output characteristic, modulation by the plate voltage. That plate voltage is 1 KV steady-state. In steady-state, RF output has a single RF component, the carrier frequency. One. RF spectral component will follow the general time-domain RF equations with no modulation. [easy math there] Apply modulation to the plate voltage with a pure tone. Plate voltage swings UP as well as DOWN equally. [theoretical perfect linear situation] Same rate of UP and DOWN. [start thinking dv/dt] Look at the spectral components with this pure tone modulation. Now we have THREE, not just one. Any high resolution spectrum analyzer sampling the RF output will provide practical proof of that. So, if you want to examine the total RF in a time-domain situation, you MUST examine it as amplitude versus an infinitely-thin slice of TIME. You cannot take a finite time chunk out of the RF envelope and "prove" anything...anymore than you can justify the existance of three RF components, not just TWO. [if this were the real classroom, you would have to prove that on the whiteboard and justify it in full public view...and maybe have to show the class the spectrum analyzer output]. Remember that the modulation signal also exists in a time domain and is constantly changing. If the "carrier sinewave goes to zero and thus power output is zero," how do you justify that, a half repetition time of the modulation signal later, "carrier sinewave goes to twice amplitude and power output is double"? You are trying an analogy that has a special condition, by neglecting the RATE of the modulation. It is always changing just as the carrier frequency sinewave is changing. You want to stop time for the modulation to show repetitive RF carrier sinusoids and that is NOT modulation. It is just adjustment of the RF output via plate voltage. No modulation at all. The basic equation of an AM RF amplitude holds for those infinitely- small slices of TIME. The series expansion of that basic equation will show the spectral components that exist in the frequency domain. Nothing has been violated in the math and practical measurements will prove the existance and nature of the spectral components. For those that like the vector presentation of things, trying to look at a longer-than-infinitely-small slice of time or just the negative or positive modulation swings is the SAME as removal of the modulation signal vector. Such wouldn't exist in that hypothetical situation. It would be only the RF carrier vector rotating all by itself. In basic FM or PM, there's NO change in RF envelope amplitude with a perfect source of FM or PM. "The carrier swings from side to side with modulation," right? Okay, then how come for why does the carrier spectral frequency component go to ZERO with a certain modulation/deviation level and STAY there as long as the modulation is held at that level? RF envelope amplitude will remain constant. Good old spectrum analyzer has practical proof of that. [common way of precise calibration of modulation index with FM] The FM is "just swinging frequency up and down" is much too simple an explanation, excellent for quick-training technicians who have to keep ready- built stuff running, not very good for those who have to use true basics for design, very bad for those involved with unusual combinations of modulation. If you go back to your original situation and have this theoretical power meter working with conventional AM, prove there are ANY sidebands generated from the modulation of plant voltage...or one or two or more. :-) Going to be a difficult task doing that, yet there obviously ARE sidebands generated with conventional AM and each set has the same information. Lose one and modulation continues. Prove it solely from the time-domain modulation envelope. Prove the carrier component amplitude varies or remains constant. Hint: You will wind up doing as another Johnny Carson did way back in 1922 (or thereabouts) when the basic modulation equations were presented on paper. [John R. Carson, I'm not going to argue the year, that's in good textbooks for the persnickety] With conventional AM the CARRIER FREQUENCY COMPONENT amplitude remains the same for any modulation percentage less than 100. Period. I not gonna argue this anymore. :-) Len Anderson retired (from regular hours) electornic engineer person |
Avery Fineman wrote:
So, if you want to examine the total RF in a time-domain situation, you MUST examine it as amplitude versus an infinitely-thin slice of TIME. You might want to remind everyone that the mathematical Fourier transform of a signal is an integral that extends from time=minus infinity to plus infinity. Since Real Spectrum Analyzers (or network analyzer) need to produce results in something, oh, less than infinite time (probably less than the time between now and the next donut break), they're necessarily limited in the low frequency detail they can provide. True, if Gary's transmitter is transmitting a zero at the moment he connects a spectrum analyzer, he won't see anything at all on the display, but as you point out -- this is an equipment problem, not a mathematical one. I'm still a believer in SSB-FM, BTW. :-) But I have enough respect for you that I won't attempt to argue it further without first finding the time to prepare a few drawings to demonsrate why! ---Joel Kolstad |
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