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March 14th 04, 04:12 AM

In article ,
(Michael Black) writes:

But I was making the point that the Q-multiplier has nothing to do
with image rejection.

Agreed, it does nothing to keep the image signal from entering the IF. But it
can keep it out of the detector.

Your scenario is about treating the image like any unwanted signal, and
notching it out or peaking elsewhere in the receiver's passband. But
surely that only works if you're lucky. An image by definition is
a frequency that translates to the same IF frequency, and there's no
way you can knock out a signal on the image frequency if it ends up
right where your wanted signal is in the IF passband.

If the image is CW, narrow-shift RTTY, or AM that isn't too heavily modulated,
you have a steady heterodyne signal that can be notched out the same as a
"legit" signal. If it's full of sidebands or laid over a critical part of the
desired signal, then I agree, you wish you had image rejection in your front
end.

Of course a lot of "legit" QRM is for the same reasons impossible to notch out.

On a different note, one use for a Q-Multiplier on AM is for
improved exalted carrier reception. The narrow peak works to
boost the carrier in reference to the sidebands, but the skirts
of the Q-Multiplier are relatively weak so it doesn't affect
the sidebands too much. You are in effect regenerating the
carrier, so the distortion seen with fading is reduced. A variant
on "synchronous detectors" but something in place long before such
detectors were practical.

Yes -- I once had a CV-157 which had a very narrow (100 Hz) filter right down
the middle, for that purpose (among other modes, that being the simplest).
The problem with doing this with most Q-mults is the tremendous boost to the
bass frequencies, producing muffled, booming audio.

If you had notch filters on either side of the carrier, to delete audio freqs
below 300 Hz, this would work. In fact, the CV-57 ran the sidebands thru the
ISB upper and lower filters to do just that. You could also have a receiver
audio section that attenuated bass freqs, though very few comm rx have a switch
or knob for this (Drake R8 is a welcome change).
73, Mike K.

Oscar loves trash, but hates Spam! Delete him to reply to me.

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