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March 13th 04, 04:19 AM

In article ,
(Michael Black) writes:

The Q-Multiplier will not do a thing for image rejection.

Not per se, but it will notch out a het from an image signal just as well as
from a legitimate passband QRM signal. Although image heterodynes do tend to
drift around a lot, making them harder to keep notched out.

FWIW, my HQ-160 has a Q-multiplier that works only to peak desired signals, not
notch out QRM. The passive slot filter does that (same as on the 170/180,
which do not have Q-mults). Maybe the 150 is different.

Anyway, a peaking Q-mult is perfect for CW and nice for SSB, but not much good
for AM.
--Mike K.

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

March 13th 04, 04:43 AM

Mike Knudsen ) writes:
In article ,
(Michael Black) writes:

The Q-Multiplier will not do a thing for image rejection.

Not per se, but it will notch out a het from an image signal just as well as
from a legitimate passband QRM signal. Although image heterodynes do tend to
drift around a lot, making them harder to keep notched out.

FWIW, my HQ-160 has a Q-multiplier that works only to peak desired signals, not
notch out QRM. The passive slot filter does that (same as on the 170/180,
which do not have Q-mults). Maybe the 150 is different.

Anyway, a peaking Q-mult is perfect for CW and nice for SSB, but not much good
for AM.

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

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. To knock out one
is to knock out the other. The situation is identical to that in
a direct conversion receiver, where you can get good selectivity with
an AF filter, but it won't do a thing for the audio image, since it's
right there in the passband. You can only get rid of that audio
image by front end selectivity, or phasing.

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.

Michael VE2BVW

March 14th 04, 03:12 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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