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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 |
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