[TenTec] Re: [jupiter538] Re: IF Gain Adjustment - Reasons to be cautious

Tue Dec 30 15:01:44 EST 2003

Duane,

I did perform the mod to my Jupiter, while before it performed just below the ICOM 756ProII as far as sensitivity prior to the change, now it performs better than the ProII and has less strong signal overload than the Pro.   Also the AGC action is much improved than before the change.   Other than having to use the ATTN as I would a pre-amp, the radio is much improved..      

My observations only, your mileage may vary..

Thanks and 73's,
Dudley
WA5QPZ
jhurry at austin.rr.com

  ----- Original Message ----- 
  From: Duane Grotophorst 
  To: jupiter538 at yahoogroups.com ; rx350 at yahoogroups.com ; tentec at contesting.com 
  Sent: Friday, December 26, 2003 2:52 PM
  Subject: [jupiter538] Re: IF Gain Adjustment - Reasons to be cautious

  --- 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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