I imagine Jerry is right.
But my comment is that high power at ALC limit levels is not needed or
desirable in PSK.
As most here already know, it is regular practice to operate with only a
very modest power output.
It is common for people to see 5 or 6 discrete signals in the passband, and
click on each one in turn and work each one, without re-tuning.
Someone who comes on at high power, just blots out a wider swath of spectrum
and forces people to move apart and waste the space.
Especially during a contest, this is most undesirable.
Its true that one can crank in all kinds of filters. But the running of
high power is still poor practice.
Besides we all remember that we are to run the lowest power needed at all
times, right ? :)
73 de Gary, AA2IZ
----- Original Message -----
From: "Dr. Gerald N. Johnson" <geraldj@storm.weather.net>
To: <tentec@contesting.com>
Sent: Thursday, January 31, 2008 10:55 PM
Subject: Re: [TenTec] Jupiter w A/T ettings for Digital(PSK31)
Communications
> On Wed, 2008-01-30 at 23:45 -0600, Stuart Rohre wrote:
> > Henry,
> > I would expect if you are into the ALC active region of your rig, you
are
> > possibly distorting the signal, and that is bad for digital encoding.
Very
> > little power is needed for perfect copy PSK 31, and less is more, (more
> > successful).
> >
> > 73,
> > Stuart
> > K5KVH
>
> The theory is (and its proven regularly by waterfall spectrum displays)
> that driving up to ALC leads to third order intermod being only 25 or 30
> dB down and with most transmitters running lower power than being
> limited by the ALC results in lower intermod products. That's presuming
> that the transmitter is not setting reduced power by the ALC. If a
> transmitter is setting reduced power by the ALC circuitry, the intermod
> improvement should be the same because the amplitude of digital signals
> like PSK-31 is constant and so the detected ALC reduces the gain of some
> driver or exciter stage.
>
> There can be defects in that scheme if the ALC threshold is practically
> at the clipping point of the PA (usually the least linear stage) or the
> gain reducing stage looses linearity as the gain is reduced. In those
> cases reducing the audio at the mic gain or the sound card output
> interface or in software is most effective.
>
> But I've seen data for some solid state amplifiers that did not improve
> 3rd and 5th order intermod performance by reducing the level, sometimes
> those intermod products increased at lower output power levels. Some of
> the RF power modules often used at VHF and UHF come to mind having that
> undesired characteristic. Yet similar modules used for cable TV
> distribution amplifiers have superb 3rd order intermod specs but only
> while delivering an output power 20 dB down from P1 dB (where the gain
> has dropped one dB from limiting or compression).
>
> 73, Jerry, K0CQ
>
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>
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