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AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-low carrier frequency
"isw" wrote in message ... After you get done talking about modulation and sidebands, somebody might want to take a stab at explaining why, if you tune a receiver to the second harmonic (or any other harmonic) of a modulated carrier (AM or FM; makes no difference), the audio comes out sounding exactly as it does if you tune to the fundamental? That is, while the second harmonic of the carrier is twice the frequency of the fundamental, the sidebands of the second harmonic are *not* located at twice the frequencies of the sidebands of the fundamental, but rather precisely as far from the second harmonic of the carrier as they are from the fundamental. Isaac I can't speak to second harmonics of a received signal, though I can't think why they would be any different than an internal signal.. but: When you frequency multiply and FM signal in a transmitter (As used to be done on most FM transmitters in the days before PLL came along), you not only multiplied the extant frequency, but the modulation swing as well. i.e. if you start with a 1 MHz FM modualated crystal oscillator, and manage to get 500 Hz swing from the crystal (using this only as a simple example), then if you double that signal's carrier frequency, you also double the FM swing to 1 KHz. Tripling it from there would give you a 6 MHz carrier with a 3 KHz swing, and so on. |
#2
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AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-low carrier frequency
In message , Brenda Ann
writes "isw" wrote in message ... After you get done talking about modulation and sidebands, somebody might want to take a stab at explaining why, if you tune a receiver to the second harmonic (or any other harmonic) of a modulated carrier (AM or FM; makes no difference), the audio comes out sounding exactly as it does if you tune to the fundamental? That is, while the second harmonic of the carrier is twice the frequency of the fundamental, the sidebands of the second harmonic are *not* located at twice the frequencies of the sidebands of the fundamental, but rather precisely as far from the second harmonic of the carrier as they are from the fundamental. Isaac I can't speak to second harmonics of a received signal, though I can't think why they would be any different than an internal signal.. but: When you frequency multiply and FM signal in a transmitter (As used to be done on most FM transmitters in the days before PLL came along), you not only multiplied the extant frequency, but the modulation swing as well. i.e. if you start with a 1 MHz FM modualated crystal oscillator, and manage to get 500 Hz swing from the crystal (using this only as a simple example), then if you double that signal's carrier frequency, you also double the FM swing to 1 KHz. Tripling it from there would give you a 6 MHz carrier with a 3 KHz swing, and so on. For multiplying FM, yes, of course, this is exactly what happens. And as it happens for FM, it must also happen for AM. However, I feel that the subject of the effects of harmonics of an AM signal needs to be investigated. I think what you hear depends on how and where the harmonic is produced, and the characteristics of the receiver. In the good old days of AM, on those occasions when I listened to the 2nd harmonic of my transmissions, I got the impression that the quality of the audio was not very good, and that the mod depth was lower than on the fundamental. Assuming that the signal is coming from a 'normal' AM transmitter, you could have two scenarios: (a) In the first scenario, the signal is initially clean, but gets multiplied by two, along with the sidebands. [This may occur in the transmitter itself, or in the receiver, or in some external device.] In this case, the frequencies and bandwidth of the sidebands will be doubled (like FM multiplication). The signal should definitely be of poor quality (it should sound rather 'toppy'), but may still be fairly intelligible. If the bandwidth of the receiver is be insufficient to embrace the full (doubled) bandwidth of the signal, you will only hear the lower part of the audio spectrum. This will limit the toppiness, and the level will be rather low, but, in practice, the signal quality may be quite 'acceptable'. (b) In the second scenario, the 2nd harmonic is effectively present BEFORE modulation, so it gets modulated along with the fundamental. In this case, the lower frequencies of sidebands of the 2nd harmonic will be 'normal', and the signal will sound normal. In practice, both (a) and (b) probably occur together (certainly in the transmitter). Again, as the receiver will only select the lower part of the audio spectrum, what you hear might sound OK. I suspect that, if you 'off-tune' a bit, you will find a lot of sideband 'splash' either side of the signal. It should not be difficult to set up a simulation of the above, and do some quantitative tests. Any volunteers? Ian. -- |
#3
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AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-low carrier frequency
In article ,
Ian Jackson wrote: In message , Brenda Ann writes "isw" wrote in message ... After you get done talking about modulation and sidebands, somebody might want to take a stab at explaining why, if you tune a receiver to the second harmonic (or any other harmonic) of a modulated carrier (AM or FM; makes no difference), the audio comes out sounding exactly as it does if you tune to the fundamental? That is, while the second harmonic of the carrier is twice the frequency of the fundamental, the sidebands of the second harmonic are *not* located at twice the frequencies of the sidebands of the fundamental, but rather precisely as far from the second harmonic of the carrier as they are from the fundamental. Isaac I can't speak to second harmonics of a received signal, though I can't think why they would be any different than an internal signal.. but: When you frequency multiply and FM signal in a transmitter (As used to be done on most FM transmitters in the days before PLL came along), you not only multiplied the extant frequency, but the modulation swing as well. i.e. if you start with a 1 MHz FM modualated crystal oscillator, and manage to get 500 Hz swing from the crystal (using this only as a simple example), then if you double that signal's carrier frequency, you also double the FM swing to 1 KHz. Tripling it from there would give you a 6 MHz carrier with a 3 KHz swing, and so on. For multiplying FM, yes, of course, this is exactly what happens. And as it happens for FM, it must also happen for AM. If you start with, say, a 1 MHz carrier AM modulated at 1 KHz, tuning to the second harmonic gives you a 2 MHz carrier AM modulated at 1 KHz; not 2 KHz as your "must also happen for AM" would suggest. Isaac |
#4
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AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-low carrier frequency
In article ,
Ian Jackson wrote: (b) In the second scenario, the 2nd harmonic is effectively present BEFORE modulation, so it gets modulated along with the fundamental. In this case, the lower frequencies of sidebands of the 2nd harmonic will be 'normal', and the signal will sound normal. I believe that will be the likely scenario for any AM transmitter which uses plate modulation or a similar "high level modulation" system. If the RF finals are running in a single-ended configuration (rather than push-pull) even the unmodulated carrier is likely to have a significant amount of second-harmonic distortion in it... and I'd think that this would tend to grow worse as the audio peaks push the finals up towards their maximum output power. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
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