> From: Peter Chadwick <Peter.Chadwick@gpsemi.com>
> To: amps <amps@contesting.com>
To: <amps@contesting.com>
> Date: Tue, 17 Jun 97 08:32:00 +0000
> I'd say 'it all depends'. If you want to evaluate your entire SYSTEM
> under two tone test conditions (and I'd say that's a useful, but not an
> infallible guide), then that's a good start. However, it's worth looking
> at the method of generating two AF tones without IMD on them, and that's
> not always too easy - lots of attenuation is useful before the combiner,
> or make a 2 to 4 wire hybrid in the same way as the telephone people do,
> using a transformer and passive components.
That's true, evaluating the exciter you do have to use two audio
tones, but mix them cleanly before the rig dirties them up.
> Tom says:
>
> >I don't think much of that method, because the exciter always has
> >IMD distortion and less than perfect sideband and carrier
> >suppression. Most tube PA's will go far beyond what a modern solid
> >state rig will do in IMD performance.
>
> I'm not sure I agree totally, but Tom has a very valid point, especially
> if the exciter is running hard.
Most of our solid state amateur rigs I've tested are not all that
"clean". Not only that, they don't necessarily get "cleaner" as
you turn the power down. (That's why I believe your results with the
AB2 PA, I've seen it happen often during IMD tests.)
My IC-751A gave best IMD performance at 50-60 watts PEP output in a
two tone test. IMD performance was MUCH worse at 10 watts and only a
little poorer at 100 watts. From the factory it was in the -25 dB
range below one tone. My 747 and 757 Yeasus behaved similar, as
does my IC-706 mobile rig, producing best IMD neither at full or
minimum power.
Solid state rigs are so bad that the ARRL and others now measure them
below PEP instead of the commercial "below one tone of a two equal
tone" test!!! If you want to compare your modern solid state rig to
an old Collins 32S3, be sure to add 6 dB of extra distortion to the
-35 db specs, for a "real commercial standard" IMD of -29 dB.
All tubes in Eimac spec sheets are measured dB below one tone, so a
tube like the 3CX1200A can actually be in the area of -40dB in the
"modern" rating system. The 8877 (with all its NASTY grid current)
can be in the area of -50dB third order (using the new below PEP
amateur rating method) if a cathode feedback resistor is added.
I cringe when I see articles recommending a PA design that runs real
low drive powers. They often use the excuse low drive "cleans up"
the exciter. In most cases it would greatly increase the level of
exciter IMD.
The optimum power setting is one that keeps the exciter out of the
worse IMD net power generation range for the longest time window of
modulation, and that usually winds up(from the dozen or so
solid state exciters I've measured) being near but slightly below the
full PEP rating in normal speech, and somewhere around half power
with processed speech.
73, Tom W8JI
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|