[CQ-Contest] Re: 2-Radio Interference

Tom Rauch W8JI at contesting.com
Tue Dec 26 21:19:41 EST 2000

Hi Scott,

> The "rules" about the effect of attenuation on intermodulation products is
> just a mathematical consequence of nonlinearity.  You do a power series
> expansion of the nonlinear transfer function, apply some sine waves of
> various frequencies, apply some trig identities to the result, and you get
> the rule.  One way to look at it is that, for example, third order
> intermod results from three multiplications:  One signal squared times the
> second signal.  If you double the amplitude of the input, the third order
> product increase by a factor of 8.
> The change in signal level occurs AHEAD of the nonlinearity.  Saturation
> shouldn't be an issue unless some OTHER signal(s) are causing saturation,
> or saturation is occurring further down the signal path, like the last IF
> stage.

It seems to me those rules only apply when the stage or stages 
(and it's usually stages) generating IMD aren't in compression or 
limiting. I looked at that some time ago, and as I recall the stage 
couldn't be near saturation for either voltage or current. Most of the 
time where IMD is severe the stages are in compression, or very 
near it, in real world situations.

This is what prompted me to ask in a round-about way how often 
the "rule" really works in a practical case.

73, Tom W8JI
w8ji at contesting.com

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