Recently a fellow ham claimed that "envelope detection" doesn't really exist, and that the standard "rectification and filtering" explanation of how envelope detectors work (going back to Terman and beyond) is wrong. But here is a good demonstration of the envelope detection of an AM signal. It uses an infinite impedance detector built around an FET. As Baltic Lab notes in the video above, the action is essentially the same as what happens with a vacuum tube: The device is biased at cut-off. The negative portion of the ENVELOPE is discarded. The positive portion of the ENVELOPE is passed through the device and filtered. What remains is the audio -- the same audio frequency that modulated the carrier. ENVELOPE DETECTION.
I was thinking about how fortunate we were that this form of detection was possible. Fessenden used what were in effect diode detectors to envelope detect very early transmissions of radiotelephony. If simple envelope detection via rectification had not been possible, radiotelephony might not have been invented as early as it was.
Here is Baltic Lab's blog post on this: https://baltic-lab.com/2023/02/jfet-based-infinite-impedance-detector-for-am-demodulation/
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Nice article and video - thanks!
ReplyDeleteBy the way, the “Bezos Box” from your web page seems to have disappeared. I used it recently, but it has disappeared for me now.
73,
Mike K6STR
Thanks Mike. I don't know where the Bezos box went. It is a mystery. Thanks for trying. 73 Bill
DeleteDid you try it without the carrier? That's the envelope. Since the carrier is constant amplitude, it should merely provide a constant voltage offset at the detector's output.
ReplyDeleteBut you'll.never get proper demodulation without a carrier, from the transmitter or reinserted at the receiver. Thus the detector is mixing.
I've seen articles like this. But they are always from the old school where AM is distinct from SSB. Even though there's been theory about SSB since 1915. People write articles about the perfect AM detector, but they imagine recovering the envelope, rather than as a mixer.