Pete Beals ) writes:
I like to try the quasi sync circuit with a limiter and product detector on
AM band. Will this work with a TRF receiver?
This probably won't work with weak signals but has anyone tried this?

An obvious point about using it as a TRF is how much selectivity will
you be putting at the front end?

Any time you are using a mixer to convert down to audio (as with SSB
or synchronous detection of some kind for AM), you can get selectivity
at audio, since it's a frequency translation.

But, if you've just got a limiter open to the world, at the very least
it's going to limit on the strongest local signal. If you don't have
enough front end selectivity, I would think you would get relatively
little tuning ability. The strongest signal will "capture" the limiter,
and that will be the "carrier" that beats against the incoming signal
in the mixer.

When people made synchronous receivers out of the old Signetics analog
PLL ICs, at least the PLL had a lock range that provided "selectivity".
so they could run with little or no front end selectivity. Except for
mixer overload, what you got out of the mixer was limited by the audio
circuitry and your hearing, since only one frequency (plus the image)
could convert down to say 0 to 3KHz from the oscilaltor frequency. The same
applies to direct conversion CW/SSB receivers.

A quasi-synchronous receiver will work the same way, in that the output
of the mixer will be within the audio range from the "carrier" out of
the limiter. But if you have no selectivity on the input to the mixer,
enough to separate out signals, then the strongest signal into the
limiter will prevail.

Much of what I've seen about "quasi-synchronous" detectors have placed
them in the IF of a superheterodyne receiver, where the IF bandwidth
ensures that it only reacts to the wanted signal. The only TRF that
I can think of seeing was in an old Technical Topics book from the RSGB,
where there was an MC1330 (which is a quasi-synchronous detector, for
TV use, though I believe the datasheet simply calls it synchronous)
as the detector, but there was an stage of amplification (I think
an MC1350P) ahead of it, with a couple of stages of tuned circuits.
With a quasi, you do indeed need some TRF before the detector.

Michael VE2BVW