Michael Black wrote:
) 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.
DSB never has a carrier. There is no such thing as DSB and DSB without
a carrier, just DSB.
Think of a mixer. If the signal are zero mean, you get DSB. If there is
a DC offset on the modulating signal, you get AM.
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.
No, AM and SSB are different.
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.
And when did I comment about SSB?
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
Christ all mighty, what is with the verbal diarrhea? You are trying to
analyze modulation by looking at demodulation. This is wrong thinking.
You analyze modulation by looking at modulators.
AM:
One mixer. Feed it a carrier and signal. The carrier is zero mean. The
signal has a DC bias sufficient that the signal always remains
positive. Congrats, you gave birth to AM.
DSB:
One mixer: Feed it zero mean carrier and signal. Out pops DSB.
SSB:
Slightly more complicated since it involves Hilbert transformers and
quadrature mixers. I can do an explanation, but it might take a
paragraph.
My original comment still stands. The author of the paper confused AM
and DSB.