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July 7th 07, 05:26 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 frequencyonanastronomically-low carrier frequency
"isw" wrote in message ... In article , "Ron Baker, Pluralitas!" wrote: "John Fields" wrote in message ... On Thu, 5 Jul 2007 00:00:45 -0700, "Ron Baker, Pluralitas!" snip When AM is correctly accomplished (a single voiceband signal is modulated The questions I posed were not about AM. The subject could have been viewed as DSB but that wasn't the specific intent either. What was the subject of your question? Copying from my original post: Suppose you have a 1 MHz sine wave whose amplitude is multiplied by a 0.1 MHz sine wave. What would it look like on an oscilloscope? What would it look like on a spectrum analyzer? Then suppose you have a 1.1 MHz sine wave added to a 0.9 MHz sine wave. What would that look like on an oscilloscope? What would that look like on a spectrum analyzer? --- The first example is amplitude modulation precisely _because_ of the Is there multiplication in DSB? (double sideband) Yes, and in fact, that multiplication referred to above creates a DSB-suppressed-carrier signal. To get "real" AM, you need to add back the carrier *at the proper phase*. So does the multiplication in the first example really make it amplitude modulation? FWIW, if you do the multiplication and then add back a carrier which is in quadrature (90 degrees) to the one you started with, what you get is phase modulation, a "close relative" of FM, and indistinguishable from it for the most part. A true DSB-suppressed carrier signal is rather difficult to receive precisely because of the absolute phase requirement; tuning a receiver to the right frequency isn't sufficient -- the phase has to match, too, and that's really difficult without some sort of reference. A SSB-suppressed carrier signal is a lot simpler to detect because an error in the frequency of the regenerated carrier merely produces a similar error in the frequency of the detected audio (the well-known "Donald Duck" effect). Isaac |
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