[R-390] R-390A IF gain setting and S/N measurement

[Craig]

Charles,

Great reading! This is stuff worth printing and keeping inside the three
ring binder with the rest of the Y2K. As David Wise commented, QRM/Part 15
RFI makes some of this discussion interesting. At this QTH the plasma TV's
rule!

Many thanks on the theory of how it all works.

Craig,



-----Original Message-----
From: R-390 [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Charles
Steinmetz
Sent: Wednesday, June 22, 2016 1:48 PM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] R-390A IF gain setting and S/N measurement

David wrote:

> It's been stated many times by Roger Ruszkowski - who ought to know - that
the 150uV/7V IF gain spec in the manual is far above the gain that yields
best S/N.  (My radio agrees.)  Were those over-hot IF's simply following the
manual, or were they cranked even higher?

I concur with Roger that 150uV/7v is too hot.  However, you can't back the
IF gain all the way down to the point that maximizes S/N without also
compromising the overload performance of the receiver.  This is because the
stage that overloads first is upstream of the IF.  If you set the IF gain
lower, the AGC just cranks up the overall gain to compensate.  This means
that the RF gain runs higher and, therefore, that the mixer overloads on
weaker signals than it did before the IF gain was lowered.  Since the
390/390A front end is prone to overload to begin with (despite what sellers
say in their ebay listings), you don't want to compromise its overload
performance any more than absolutely necessary.

This is the art of balancing a radio's DR -- to get it sensitive enough that
the practical limit is always atmospheric noise coming in on the antenna,
never the front end's self-noise, while keeping the overload point as high
as possible.  This is the fallacy of pursuing maximum S/N and sensitivity --
you throw away overload performance that you desperately need, to gain
sensitivity that is useless in practice because the atmospheric noise was
already 10-30dB louder than the radio's own input noise.

Once some basic design choices are made -- in particular, the number and
general type of active devices and the impedance levels of the plate loads
and tuned circuits -- the best possible DR has been preordained. 
All you can do beyond that is (i) make sure you actually get the best
possible DR out of the parts (i.e., make no design blunders), and (ii) slide
the DR up and down the input signal range to optimize the tradeoff between
weak-signal and strong-signal performance.

In the real world of antennas, QRN, and QRM, there really isn't much room
for debate about what constitutes the best compromise for any particular
value of DR the radio has.  And to change DR, you need to change some really
fundamental things about the topology you are using, such as the impedance
levels and standing current at each stage.  As Larry is finding, simply
changing the stage gains by using different tubes doesn't buy you anything
if the radio was well designed in the first place.  After you adjust
everything so it works again, you're right back where you started -- at the
limits of the fundamental circuit elements and stage impedances the original
designers chose.

I once spent a few days with several 390s and 390As, a suite of signal
generators and combiners, a spectrum analyzer, and a distortion analyzer to
determine the optimum IF gain.  I can't find my notes right now, but I do
recall it was less than the Collins spec but not by all that much.

As to the IF gain settings I've observed in radios "as found," it appears
that once someone gets it into their head that more IF gain is a Good Idea,
they are not bound by any sense of moderation.  The ones I've found with
too-high IF gain are generally 6 to 20dB higher than 150uV/7v.  I'd be
interested to know how a radio set up according to Roger's "alternate
procedure" compares to the "by the book" 150uV/7v with respect to input
level and AGC voltage -- just how much lower is the IF gain?  Has anyone
checked this after performing the alternate procedure?

Best regards,

Charles


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