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I am fixing up an old classic Yaesu FRG7 general coverage AM/SSB/CW HF receiver. The BFO section is a rudimentry 455 kHz IFT configured as a carrier reinsertion oscillator that that is made to run USB or LSB by switching a small trimmer cap either in or out of the circuit.The overall stability of SSB demodulation seems to be surprisingly good on 80m.The Wadley loop system used implies that the higher bands will also be good. Hence there is no real incentive to use two crystals for USB and LSB. We keep the BFO. AM detection is done with an envelope detector. I have not heard any AM on the amateur bands. I can get lots of SW broadcast stations coming in strong at night. There is a lot of fading. Would a synchronous detector help here? Would loosely coupling some 455 kHz IF to the BFO be a simple way of achieving this?

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  • $\begingroup$ You likely don't hear any AM on the amateur bands because there isn't any. Standard practice is single-sideband due to its higher efficiency. AM isn't forbidden and occasionally you will hear it, but it's scarce. There are scheduled events featuring AM. For example: amrally.com $\endgroup$ – Phil Frost - W8II Aug 11 '18 at 13:00
  • $\begingroup$ There IS some AM, almost every evening around 3880-3892 kHz (and/or in the 75m Extra portion) and 7285-7290. The ones on 75m are usually very loud in the eastern US. Other AM frequencies are 14.286, 21.420, and 29.0 thru 29.2. And there are other AM frequencies and other bands listed near the end of amrally.com. $\endgroup$ – Mike Waters Aug 11 '18 at 14:04
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AM synchronous detection will certainly reduce or eliminate selective fading. It generally isn't implemented simply due to the increased cost and complexity. But you will certainly be pleased with its performance if you enjoy listening to AM shortwave broadcasts.

I think you would find that the easiest design that you could retrofit to the receiver would be the addition of a high gain amplifier off of the IF. This is injected into a final mixer. Since the AM signal is hard limited by this amplifier stage, its output is the AM carrier without modulation. This makes it an ideal injection component for the final mixer to recover the baseband audio.

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