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matt weber · January 19th 07, 06:28 AM posted to rec.radio.shortwave

On 17 Jan 2007 20:54:27 -0800, wrote:

Telamon wrote:
In article . com,
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.

Narrow band signals do not have less fading.

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.

Selective fading occurs when conditions cause a very narrow band of
frequencies to be received at very low amplitudes where most of the
side band information is present at levels that your receiver can
ordinarily demodulate properly.

When part of the side band is being notched out it does not sound all
that bad but when the carrier gets weakened then the AM demodulator
can't process the side band information properly and there is
horrendous distortion. The carrier which is at the right frequency and
phase relative to the side band information keeps the detector in the
linear region so distortion is minimized.

A sync detector uses a local oscillator in a similar to the way SSB is
detected with the difference that it is phase locked to the signal
carrier and mixed with it so when the carrier fades out this near
perfect copy of the carrier allows the demodulator to continue to
detect the side band or bands without distortion during a carrier
fading condition. Here this necessary frequency and phase information
carried by the "carrier" is retained by the sync circuitry.

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

Snip

Michael has it right.

--
Telamon
Ventura, California

Why do you insist that the atmosphere treats the carrier differently
from the rest of the signal? Geez. You have a spectrum produced by
modulation. If the modulation is AM, then a carrier is present. Now you
are saying the atmosphere is sucking out the narrow band carrier and
leaving the wideband spectrum untouched. Fiction at best.
No, it is called selective fading, and it is a real phenomenon.
You can think of the two sidebands for AM as creating constructive and
destructive interference with the carrier. Any time the sideband
energy exceeds the carrier energy you get the equivalent of over
modulation. DSB and an envelope detector isn't a good combination.

As others have pointed out, what Synch detection does is to insure
that the sideband energy can never exceed the carrier energy. While it
is preferable to have it in phase, which a synch detector does, in
human speech, phase carries no information. As a result SSB can be
used to communicate. A product detector (which is used for synch
detection) simply produces phase distortion if the local carrier isn't
in phase with the original carrier. The phase distortion is a fact of
life in SSB-SC communication. If you Independent side band, and
generate the two independent side bands at quardrature, it is possible
to determine, and lock the local carrier in phase with the original
carrier.