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Telamon · December 1st 06, 11:50 AM posted to rec.radio.shortwave

In article ,
"Brenda Ann" wrote:

"Michael Black" wrote in message
...
) writes:
wrote:
I found this web page while looking for a nifty audio filter I
found last year. At the very least it gives food for thought.

http://www.radiointel.com/phil/phils_radio_tuning_tricks.pdf

Terry

Eh, the author confuses DSB and AM. I wouldn't put much faith in
his/her analysis.

Huh? IN what way?

I glanced at it and maybe missed something, but DSB is AM. And he
certainly says it at the outset, and when he's talking about the
components he's talking about 2 sidebands and a carrier.

Now, "DSB" often has fallen into the meaning of "DSB with no
carrier", but technically one should specifically define that there
is no carrier.

Some of the problem with AM reception discussion is that it was
defined in a certain set of terms, for decades and then even for
beginners up till recent times (and maybe even today). So they'd
define AM as a signal that is amplitude modulated, and that sets
things up for the vision that the carrier amplitude goes up and
down. Then when SSB became commonplace, instead of going back from
the beginning and redefining it all, a separate set of definitions
gets tacked on.

This leaves people thinking they AM and SSB are two different
things, when they are basically the same.

Then when discussion of "low distortion AM detectors" comes along,
it isn't even clear what people are talking about. Because one is
not using a certain type of detector for AM (ie 2 sidebands with a
carrier), and a different type for SSB. The talk of "Amplitude
Modulation" invokes a vision of a detector that is following the
voltage variations of the signal. But that's not the case at all.

The carrier mixes with the sideband in the "envelope detector" and
that beating is what brings the modulation back down to "baseband".
It's just not a good mixer.

Listen to an SSB signal without a carrier or BFO. That's the sound
of the envelope varying according to the modulating signal, and
there's no way to make sense of it without a carrier. No differing
loads on the dioded detector, no precision half wave detector (with
the diode in a feedback loop), no forward biasing of the diode,
can ever make up for the lack of carrier.

The carrier of an AM signal is needed to beat with the sidebands
and get it back to audio. If the carrier fades in comparison with
the sidebands, you start hearing things like that SSB with an
"envelope detector", because the carrier is no longer strong enough
to mix the sidebands down to audio, and the "envelope detector" is
actually following the envelope of the signal.

The basic concept of demodulation is no different whether the
signal is AM (with carrier), DSB (with no carrier) or SSB (with no
carrier). They all need the carrier, or a locally synthesized
equivalent, to beat the sideband(s) down to audio.

If things were spoken of that way from the beginning, then there'd
be less of a leap to the "synchronous detector". No only would a
universal set of concepts be applied to all modes, but the point of
a synchronous modulator would become clear.

A single diode is a lousy mixer. On the other hand, since the
carrier of an AM signal comes in with the sidebands, there's no
reason for having a second and isolated input for that carrier.

But, long ago, people would mess with "exalted carrier reception",
which would be the first step up from those "envelope detectors".
They'd turn on the Q-multiplier, which had a narrow peak but a wide
skirt, and that would boost the incoming carrier in reference to
the sidebands, so there was a stronger carrier feeding into the
"mixer".

It seems that only when SSB came along, and there were design
reasons to go to better mixers for the demodulation, that two input
mixers started being used, commonly called "product detectors".
There were design reasons for going to those, but the basic concept
of a locally generated carrier did not require anything more than
the single diode "envelope detector". Indeed, the concept had been
there back in the days of regen receivers, and every superhet that
could be used for CW had a BFO that would feed into the "envelope
detector", to give to provide a beat with the incoming signal. I
should point out that when the synchronous detector was described
in CQ magazine in the late fifties, the actual mixers were single
diodes.

But once you had product detectors, that opened things up. The
notion of boosting the incoming carrier for better mixing action
became more clear. I've said before, there was an article in QST
about an advanced receiver in the fifties, and it had two parallel
IF chains. One wide for voice, the other narrow for CW. But, it
also allowed the output of the narrow chain to feed the product
detector, and there was the "quasi-synchronous detector" before
anyone came up with the name.

For AM (with carrier), you had two choices. You could strip off
the extra sideband and carrier, then the incoming signal was the
same as an SSB signal, and then demodulate it as an SSB signal.
This saw a lot of useage in the sixties, when SSB only ham rigs hit
the market, and yet AM was still common. People needed a means of
demodulating the AM signals, and that worked. While I think it got
discussed in the fifties as a better means of AM demodulation,
nobody in the sixties was talking that way. It was just a means of
demodulating AM signals when there was no means of doing so. (Not
only did the SSB-only receivers have narrow IF filters, but often
there was no way of turning off the BFO, and the product detectors
were a type of mixer that required having that second signal at the
second input; without it, you'd get little or no output even with
an AM signal that brought it's own carrier.)

But if you didn't want to do that, you had to deal with getting the
"locally generated carrier" in the right place. Not just so it
wouldn't beat against the incoming carrier (when it was strong
enough) but if it wasn't placed in the right place, the sidebands
would not be translated back to audio in the same places. (So if
you sent a 1KHz tone, and the "locally generated carrier" was not
right in the middle between those sidebands, one sideband would
translate down so that 1KHz tone was 1010Hz while the other would
translated down to 990Hz, which would obviously clash with each
other.)

That's what the "synchronous" bit is about. It's about putting the
same sort of BFO that you'd use with SSB (which would feed the same
sort of product detector used for SSB), with the addition of
circuitry to synchronize the BFO with the carrier of the incoming
AM Signal.

If the discussion had started with the AM detector as a mixer, then
there'd be little magic about "synchronous detectors". The whole
process is simply about getting the "carrier" strong in reference
to the sidebands, so good mixing happens in the detector. The
"synchronous" bit is only a secondary thing, a need because you
want the locally generated carrier in the right place.

There have always been various means of getting better mixing
action at the demodulator. But the important thing has always been
about doing that.

Michael

Damn good explanation, Michael. And 100% accurate.

He always writes good posts.

--
Telamon
Ventura, California