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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 |
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