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craigm · January 18th 07, 02:51 AM posted to rec.radio.shortwave

wrote:

Michael Black wrote:
"N9NEO" ) writes:
Greetings,

I have just got a sony ICF sw7600GR and it is a very nice radio. The
sync detector seems to take care of a lot of the distortion, but the
audio continues fading in and out and is quite annoying. Could the
fading be mitigated to any extent by using another stage of agc? I am
going to be doing some experiments with the 455kc if out on my Red Sun
RP2100 whenever it gets here. Detectors, filters, SSB, etc... I thought
that along with other experiments I might want to try some outboard
agc.

Synchronous detectors have never been about dealing with fading. They
are about ensuring there is enough "carrier" to beat the sidebands down
to audio.

Narrow band signal have less fading, thus sync demod will have less
fading. However, the result isn't all that significant since all you
have done is cut the bandwidth in half.

So there's fading on the incoming signal. That means the amplitude of
the sidebands is varying with that fading. A locally generated "carrier"
at the receiver ensures that there is something to beat those sidebands
down to audio, even if the transmitter's carrier has faded too much to
do the proper job. But a constant level "carrier" at the receiver beats
the sideband down to audio intact, ie an ideal mixer would not add
anything
to the signal. So if the sideband is fading, of course the audio output
of the receiver will vary with that fading.

With an envelope detector, the carrier isn't beating down the sideband.
If you just look at the math of AM modulation, you would see that the
carrier is just there for the ride.

If you look closely at the operation of an envelope detector, you find the
carrier is an important component.

What the sync detector brings you is the ability to decode that signal
even if the carrier goes missing, because of selective fading.

Dealing with the fading of the sidebands is in a different realm, and
obviously a miraculous receiver that eliminates fading has long been
sought after. Armstrong dealt with it in part, by moving to FM
and using limiters in the receiver, but that only works when the signal
is above a certain level. Below it, the signal levels are too low for
the limiters to kick in, and that fading is obvious.

Beyond a certain point, you get conflict. Have a scheme that does
a really good job of eliminating the fading, and likely that starts
affecting the "fidelity" of the signal, because how do you discriminate
between the voice at the transmitter end varying in amplitude, because
the speaker starts talking more quietly or even just because sounds are
made up of varying levels, and the signal fading as it travels to the
receiver? It's easy to counter some of the fading, but it gets harder
the more you try to conquer it.

Michael

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