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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