I just re-read this important thing:
Quote:
Charles,
Thanks for an excellent circuits archive. I'm writing because I may be able to throw some light on the reason why Paul Beaumont's all band receiver can demodulate FM, despite having an AM-only detector. I believe it is due to multipath reception. Consider this:
Your antenna receives an FM signal via the direct path, and also via an indirect path (perhaps a reflection off a tall building). The extra distance for the indirect path might be 300 meters. This corresponds to a delay of 1us for the indirect path. If the FM signal is at 100MHz, then the 1 us delay is exactly 100 cycles of the carrier. The two signals arrive in-phase and the signals produce a strong signal in the antenna. If, however, the FM carrier moves to 100.5 MHz, then the 1 us delay corresponds to 100.5 cycles of the carrier. The signals arrive anti-phase and they cancel (to some extent at least). Now this is not so extreme as it seems. FM signals vary in carrier frequency by 0.15 MHz (+/-75 kHz). So if *any* multipath occurs, there is likely to be some AM resulting. Under the ideal conditions described, there would be considerable AM (perhaps 60% modulation depth). Hope this provides a plausible explanation!
Regards,
Karen
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Yay, finally an explanation as to why I can hear all these voices on the FM band and not on the AM band and how they are demodulating the FM signals at a rate of 200 megacycles per second in a backward sweep.
So again, I return to my 'comfort zone' within the confines of the FM radio band...I feel so at home here...lol