John? G8FPN
In Hans` QCX receive section the quadrature detector is so close to the antenna that it
seems you would not have adequate amplification to drive an AM detector at that point.
Also, there is really no tuning for receive at that detection point because the receive
side RF input is broadband with only a Low-Pass filter between there an the antenna.
Anything to the right of the Quadrature Sampling Detector is already audio derived
from mixing incoming RF with the Si5351A synthesizer output.? This would make
any recovered AM audio contain an undesirable beat note.?
It might be possible to do something really fancy with the micro-processor control of
the synthesizer and arrive at some sort of synchronous detection, but that would be
complex, and probably beyond the capability of the small ATMega328 processor.
But, having said that, it might be that with the GPS frequency stabilization you could
tune close enough to the desired signal that the beat note would be below normal
audible range and thus not noticeable.? This though causes another problem in that
there would be a slow QSB (fading) of the signal due to the local oscillator being
close but not exactly on the AM carrier frequency.? QSB rate would be the difference
between LO and AM carrier frequency.
Other transceiver or receiver designs which follow closer to conventional super-
heterodyne configurations might be a better choice if you want to receive AM signals.?
I don't want to discourage your interest, but a direct-conversion receiver seems
like a difficult direction if the intent is for AM capability.?
It should be possible to design and build a true multi-mode receiver using a
super-heterodyne configuration, but the detectors would be after the IF stage(s).
You could still use quadrature detection for SSB and CW, along with turning the
BFO off and using a diode detector for AM.? FM might also be possible with a
typical FM limiter and FM detector stage at the same point.?
Arv? K7HKL
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