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?
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.