[Collins] new subscriber with a 75A4

[Dr. Gerald N. Johnson, electrical engineer]

In a post a couple hours ago, AF2HD said his experience with the A4 was
as I alleged. I didn't allege specifically for the A4 because it
accompanied Collins first amateur SSB transmitter and I'd have hoped it
copied SSB with good AGC. I know older receivers did not work well on
SSB with full RF gain simply because of lack of a fast attack, slow
decay SSB time constant that came in with the S-line as far as I know.

As there were several versions of the A4 (and with varying values at
dealers according to serial number so that Art decreed the S-line would
have randomly ordered serial numbers) some of the changes could well
have involved the later dual time constant AGC of the S-line that works
so well.

The AGC problem is true of ALL receivers using BFO injection to the
diode detector (which provides AGC as well as audio) on SSB. There has
to be a trade off of the BFO injection level. Low distortion requires
large injection which swamps the AGC, low injection to not swamp the AGC
then means signal that reaches the diode can't be as strong. Another
problem that often accompanies strong BFO injection is that strong
injection comes from a relatively large coupling capacitor that also
couples IF signal energy to the BFO tending to pull it by injection
locking that adds frequency distortion. Making the diode detector
receive SSB well is full of compromise and large IF signal never makes
it work well. But it can take a large IF signal to activate the AGC, and
if the time constants are based on AM and carrier they won't be fast
enough for SSB allowing each syllable to overdrive the last IF and the
detector. Doesn't matter if its an amateur receiver from Heathkit,
Hammarlund, Hallicrafters, Collins or a 51J4 or R390(a). Diode detectors
and SSB work better when the RF gain is backed off to limit the signal
at the detector.

73, Jerry, K0CQ

-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.