[TenTec] Re: IF Gain Adjustment - Reasons to be cautious

[Duane Grotophorst]

--- als_radios <aburzynski at swri.edu> wrote:
> Regarding the thread about increasing the IF gain, I
> decided to try 
> the same thing on my Ten-Tec RX-350 receiver. (The
> RX-350 receiver is 
> really the Jupiter with the transmitter section
> removed and a simpler 
> power supply installed.)
> 
> I connected the 12kHz IF output from the receiver to
> my pc and
> monitored the IF output using Spectran.

Using Spectran is part of the correct approach for
determining the effects of the infamous IF gain tweak.
However using the 12kHz IF signal as the test point
does not tell the whole story. The signal at that
point has not been through the A to D stage or have
had any DSP processing done to it. The real problem
with adjusting the IF gain to the max shows up as
greatly degraded IMD responses and images at several
+/- 12Khz intervals up and down the band from the
strong signal. These problems I beleive are all taking
place at the 12kHz IF.

To test this remove the antenna (better yet put a 50
ohm load on it if you can) then use the audio line out
of the radio to feed Spectran. This will assure that
you are testing all of the radio's RF/IF chain. With
RF gain set to max and the IF gain pot set at the
factory default setting you will likely see a steady
faint spur somewhere in the audio range that moves
around in relation to the VFO frequency. You can
easily get rid of that spur by increasing the IF gain.
BUT all that you've accomplished by doing so is to
make the DSP start applying AGC within the DSP code
itself because the AGC is now acting on the noise,
with the IF gain at the factory default setting it
does not. At first glance this may seem like a
universally good thing. Until that is you start
listening/looking at a strong signal, that's when the
major downside of this tweak will become evident (more
about that below).    

> I then listened to a signal
> on the receiver and adjusted the pot to full
> clockwise. I got about
> 10dB of gain in my IF signal but the noise floor
> also rose about
> 10dB. If I only listened to the change, it appeared
> that there was an
> increase in gain, but I belive there was no increase
> in the signal-to-
> noise ratio. The net result, according to my
> observations, was no
> real increase in signal strength, if you considered
> the noise.

Precisely, this tweak does nothing to improve
"sensitivity" when thinking about sensitivity in the
proper context of S/N ratio. The S/N ratio of all
these radios is established in earlier in the RF
chain. The IF gain pot only sets the level that the
following stages see, and most importantly the MAX
level that the A to D is allowed see. It is at the A
to D stage that you will see the big gotcha downside
to this tweak.

By design there is a somewhat precise balance between
the ~60 dB (if my memory serves correctly) of AGC
range contained within the DSP code itself, (the rest
of the overall AGC range is done by traditional analog
means). The IF gain tweak will make it possible to
subject the A to D stage to signal levels higher than
it is designed to handle. So with very strong signals
the A to D WILL be over driven, the net results is you
will hear clicks/pops +/- 10's of kHz from the actual
signal itself, especially so if the signal is CW. The
true nature of this problem will not be readily
apparent on a busy band with lots of other natural
noises present.

The only way to test this correctly and to see it for
yourself is on a quiet band with only one S9+20 or
stronger signal present. Once you have that test
configuration set up tune around a bit listen for the
presence of phantom signals and clicks with the IF pot
at the factory setting and then do it again with it
cranked up. Also look at Spectran window and notice
how much dirtier the audio gets after the IF gain is
tweak up, even with a clean strong CW signal as the
input. Try these tests with several different strength
signals being fed to the radio as well. The difference
is pretty dramatic even with a single strong signal.
With a multitude of strong signals present the problem
can only be worse.

To restore the factory IF gain setting use Spectran to
watch the audio output spectrum and back off the IF
gain pot until the weak spur just reappears. You will
also notice that the factory optimized "point" is real
close to a rather pronounced "knee" in the response
that the IF gain pot goes through as you adjust it. Be
sure to do this only without an antenna connected
(better yet with a 50 ohm dummy load).

Yes it is true that the RF gain control and attenuator
settings will help mitigate these problems but not
entirely so. The reason is that the original design
balance between the analog stages and the DSP has been
upset as soon s the IF gain is cranked up. This is
because the DSP IF stage will always be applying some
degree of AGC to all signals and/or background noise
no matter how quiet or noisy the band really is
otherwise.   

> On the Jupiter, was there a gain in the SNR of the
> IF output? or is this just an apparent gain.

Your observation is exactly correct; it is just an
increase of overall RX gain, NOT sensitivity. 

I'm pointing these things out because the last thing I
want to see is the RX350/Jupiter/Pegasus get a bum rap
for having really poor strong handling capability
because folks out there are indiscriminately cranking
things fully clockwise. By today's standards these
radios are already only mediocre performers for strong
signal handling and dynamic range.

Duane
N9DG


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