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Subject: [AMPS] IMD
From: (Peter Chadwick)
Date: Tue, 18 Sep 2001 13:28:44 +0100
Jon says:

> That sort of sweet spot for IMD is typical, so dropping power may cause IMD to
>worse for a bit but then it should eventually get better as stages are taken
out of 

If you take a 3/2 power curve for your 'perfect' triode, and do the analysis,
you see the 
3rd order products dipping and then getting worse as power increases. Eimac did
this in 'care and feeding', and they show the various order products going
through minima at differing points. However, even at the worst points, the
products should be low enough to meet a reasonable spec - I take Tom's point
about the TS2000 being particularly bad, but the quotation 'forty million
reasons but not one excuse' comes to mind.

Steve commented about the 'banana curve', which could give the effect of IMD not
improving at lower power,  but it doesn't alter the fact that it should get
better! Gain compression shouldn't come into it, as the stages should be
operating well below gain compression.

I suspect that gain and gain control distribution in transmitters hasn't
received the attention that it has had in receivers - although there seem to be
very few receivers where gain control distribution has been done properly. From
a 20dB S+N/N, an increase in signal of 20dB should result in at least a 35dB
S+N/N, and the ultimate SNR should be 45dB or better.


Peter G3RZP

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