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#11
July 5th 07, 08:11 AM
posted to sci.electronics.basics,rec.radio.shortwave,rec.radio.amateur.antenna,alt.cellular.cingular,alt.internet.wireless
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AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-low carrier frequency
In article ,
"Ron Baker, Pluralitas!" wrote: "isw" wrote in message ... snip 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 Whoa. I thought you were smoking something but my curiosity is piqued. I tried shortwave stations and heard no harmonics. But that could be blamed on propagation. There is an AM station here at 1.21 MHz that is s9+20dB. Tuned to 2.42 MHz. Nothing. Generally the lowest harmonics should be strongest. Then I remembered that many types of non-linearity favor odd harmonics. Tuned to 3.63 MHz. Holy harmonics, batman. There it was and the modulation was not multiplied! Voices sounded normal pitch. When music was played the pitch was the same on the original and the harmonic. One clue is that the effect comes and goes rather abruptly. It seems to switch in and out rather than fade in an out. Maybe the coming and going is from switching the audio material source? This is strange. If a signal is multiplied then the sidebands should be multiplied too. Maybe the carrier generator is generating a harmonic and the harmonic is also being modulated with the normal audio in the modulator. But then that signal would have to make it through the power amp and the antenna. Possible, but why would it come and go? Strange. Hint: Modulation is a "rate effect". Isaac |
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