[TenTec] PSK-31 and ALC

[Dr. Gerald N. Johnson]

On Sat, 2006-12-30 at 09:20 -0500, wa3fiy at radioadv.com wrote:
> With all this interesting discussion re ALC vs PSK-31 vs distortion, I thought 
> I would re-post something I posted to the list five years ago.  Although my 
> test was using the Jupiter, I suspect that many Ten Tec rigs would yield 
> similar results.   Here is what I posted:
> 
> 
> >
> > I almost always read that one should not operate into the ALC 
> > range in PSK31 [and I suppose most other sound card modes].   
> > On the other hand, one of the outstanding characteristics of Ten 
> > Tec rigs for the last few generations, has been ALC stabilized 
> > output at any power level within the transmitter's power range.   
> > I have always operated PSK just on the edge of ALC control.   Is 
> > that good or bad?   Don't know...............so I decided to run a test to
> > see what effect ALC had on signal quality.
> > 
> >For the test, my main rig was Pegasus
> > connected to a dummy load with a very short piece of wire clipped to the
> > "hot" lead of the D/L and the power output set at 30 watts in the TUNE
> > mode.  My test receiver was an Argosy with about 4" of wire connected to
> > the antenna jack.  I have a 20db user selectable attenuator in the Argosy
> > so I had that in line.  With my Pegasus keyed and sending PSK, I got a S9
> > reading on the Argosy, so I clearly was not overloading it.   Both radio's
> > had computers attached running Digipan.  The interface between the Peg and
> > my computer is a home built isolator with a transformer, resistive
> > attenuator and trim pot level adjust.   I leave the mic setting on the Peg
> > just where it is  when I operate SSB.   I adjust the output level slider
> > on the sound card to 50% scale.   Then I adjust the trim pot on the
> > interface until I JUST come into the ALC range.  I then make any fine
> > adjustments in transmit level with the sound card level slider.  OK,
> > that's the setup.
> > 
> > 
> > The results of the test were quite interesting.  With the sound 
> > card adjusted so the ALC light never illuminated, I saw IMD 
> > readings on the receive channel of around -32 to -33db.   When I 
> > brought the sound card level up to the point where the ALC light 
> > just illuminated solidly, I saw -30 to -31db!   And when I opened 
> > the sound card level to full output, putting the Peg into deep ALC 
> > range, I still saw IMD levels in the -27 to -28db range!   And the 
> > waterfall confirmed the above readings by being almost text 
> > book perfect at all settings of the level control.
> > 
> > 
> > So the answer to the question is....not bad to run the 
> > Pegasus/Jupiter lightly into ALC range on PSK.   The transmitted 
> > signal will be one you can be very proud of and you will enjoy 
> > stable output power as the rig heats up during transmit cycles 
> > and the level adjustment will not be critical.> 
> > 
> 
> 73,
> 
> -Lee-
> WA3FIY
> 
While the intermod levels with lots of ALC aren't horrible by solid
state standards they aren't very good either. Remember the 32S-1 from
Collins did at least 10 dB better with tubes 50 years ago.

Your test do show an increase in intermod products with the increase
drive which is as I'd expect. Typically the ALC threshold is set at the
power that begins to show such increased intermod products.
-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer