## Intermodulation distortion of a two-tone input to a nonlinear amplifier

Started by 6 years ago4 replieslatest reply 6 years ago209 views

I consider a two-tone input to a nonlinear amplifier.

In a paper I have read (unfortunately I cannot upload the paper), there is a formula mentioned for the calculation of the third order intermodulation distortion of a two-tone input:

c_1 is the so called amplitude compression and k_p1 is the so called AM-PM conversion factor of the modulation of only one carrier. Here's my question: What does "modulation of only one carrier" mean ? Does it mean, I am changing the frequency of only one carrier and the frequency of the second carrier stays the same? Or does it mean the second carrier is not influenced by any nonlinear distortion?

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One would have to read the paper to know what the authors had in mind.  I expect, though, that they meant that they're AM modulating one carrier and seeing how much that AM modulation turns into PM modulation (thus capturing the fact that some nonlinearities affect how quickly an amplifier reacts more than how much it reacts).

Note that the above equation is NOT universal.  It's going to work for amplifiers that tend to distort in ways that the author's amplifiers do.  It may well be good for most class $$\mathrm{AB}_{\mathrm{1}}$$ amps, for instance, but not necessarily anything else.  Or it may apply only to bipolar transistors, or only to MOSFETS, or only to good old fashioned toobs.  I'm not enough of an RF guy to know; maybe someone will weigh in.

I do know that I once asked myself "what is the 3rd-order IM behavior of an amplifier that does perfect clipping of the waveform?" -- the answer was "3rd-order IM distortion is a meaningless concept to apply to such amplifiers".

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The paper is about TWTAs. And the formula is stated to be valid for a two-tone input.

Let me see, if I understand your reply correctly: I am given the two-tone input. I am then changing the amplitude of one of the sine-waves, while the amplitude of the other sine-wave stays the same. Now you say, the AM modulation turns into a PM modulation. I don't understand how that is possible, can you explain it? - Maybe for further information: The D3 is the intermodulation distortion, i.e. the power ratio of the signal at the fundamental frequency and the 3rd order intermodulation product.

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