Jon says:
>THe FET's will be over-driven before the
>mixer is. That's why you can turn the preamp-off and get a
tremendous
>improvement in IP3 performance.
In any properly designed receiver, the IMD performance of the RF amp will be
a relatively small contribution. The mixer, and occasionally, high frequency
crystal filters are the major causes of IMD. Switching off the RF amp
reduces signal levels into the mixer, and thus mixer IMD. Getting the
performance in an RF amp is relatively easy, especially if you use noiseless
feedback and a good linear transistor running well in Class A with plenty of
current. (For the effects of current on IMD, see my paper in the WESCON 1981
session on High Performance Mixers). Crystal filters are a problem close in,
especially at higher frequencies. This is because the electric field stress
is higher for a given input power in a thinner crystal, and the stress
reaches the point where the crystal no longer follows Hooke's law. At one
well known company, filters were selected for IMD, and the worse ones went
into transmitters. Alsocrystal filters are not symmetrical in this respect,
and turning a filter round can improve matters.
>So while harmonic distortion doesn't affect your signal it might piss off
>someone trying to work a DX station on 10M that is on your harmonic!
I can hear some of my 'locals' second harmonics on 10 from 15 miles away, at
about S7. Trap beams don't help, of course, as they offer no antenna
selectivity.
73
Peter G3RZP
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