In article . com,
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.


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.

What makes you interpret my remarks as the "atmosphere" treats the
carrier differently than the rest of the signal?

You mean ionosphere. Phase cancelation is a frequency dependent
phenomenon and the modulation scheme or part of it does not matter.

When phase cancelation is narrow and right at the carrier frequency then
the carrier level goes down where the rest of the side band signal is
still good.

You been talking to John Smith or something?

--
Telamon
Ventura, California

wrote:

Why do you insist that the atmosphere treats the carrier differently
from the rest of the signal?

Because it does. See below...

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.

Bzzzzt. Wrong!

Yes, the "atmosphere" [ionosphere] DOES "suck out a narrow band" or even
a single frequency. Ask any amateur radio operator that has used RTTY
(radio teletype).

The RTTY "modulation mode" used is FSK or frequency shift keying. At any
given instant, the transmitter is sending either a "mark" or "space",
essentially two carriers if you will, 170 Hertz apart that represent the
5-level Baudot code as used in ham RTTY.

As an aid to tuning, an oscilloscope is used as a tuning indicator; the
mark signal from your RTTY decoder is connected to the horizontal plates
of the 'scope, the space signal to the vertical plates. On the screen of
the CRT (due to the persistence of the CRT phosphors and your eyes),
this shows what appears to be a "+" sign, also known as the classic
"cross display". When you see the cross on your screen, you know you are
tuned in properly.

So, "What does this have to do with the discussion above?" you ask.

Remember, you are looking at essentially two "carriers", 170 Hz apart,
one on the horizontal axis and one displayed on the vertical axis.
During disturbed ionospheric conditions, many times you will see one
signal or the other disappear; i.e., the cross turns into a single line,
either a "-" or a "|", depending if the mark or space faded--and yes,
sometimes both fade, but it is more common to see one or the other
disappear.

This phenomenon is known to hams as "selective fading", is quite common
and is interesting to observe.

So, yes, the ionosphere CAN suck out one signal separated from another
by as little as 170 Hz.

Carter
K8VT

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


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.

When the signal exists over more than one path there is a difference
in phase between the two signals that algebraically cancels some parts
of the signal and strengthens other parts.

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.

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

N9NEO wrote:
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.

regards,
NEO

The sync' is doing what it's designed for by reducing the distortion
caused by selective fading, but you need a longer time constant (release
time) for the AGC, to help smooth out the fading. You could lengthen the
time constant of the AGC circuit in the 7600GR but then it wouldn't work
well for other conditions where a shorter time constant is needed. This
is why a good table-top receiver has more than one AGC rate, which can
be selected by the user. The AGC in the Drake-R8 series uses a decay
rate of 300-ms for the fast setting and a much longer rate of about
2-seconds in the slow mode. The latter really aides in reducing the
effects of rapid fading.

Ok, I got it. I kinda figured the band was very narrow where you would
lose carrier. The fade is probably moving around the transmitted
spectrum like a game of musical chairs. I figured I was going to have
to play with AGC some so I'll probably throw an agc amp on the board as
well.

73
NEO


HFguy wrote:
N9NEO wrote:
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.

regards,
NEO

The sync' is doing what it's designed for by reducing the distortion
caused by selective fading, but you need a longer time constant (release
time) for the AGC, to help smooth out the fading. You could lengthen the
time constant of the AGC circuit in the 7600GR but then it wouldn't work
well for other conditions where a shorter time constant is needed. This
is why a good table-top receiver has more than one AGC rate, which can
be selected by the user. The AGC in the Drake-R8 series uses a decay
rate of 300-ms for the fast setting and a much longer rate of about
2-seconds in the slow mode. The latter really aides in reducing the
effects of rapid fading.