We usually measure just amplitude intermodulation products, are phase intermodulation products present and are they a problem?
In many amplifiers phase intermodulation exists because the semiconductor junction capacity is a function of the applied RF voltage. The resultant products are as real as the amplitude intermodulation products and at exactly the same frequencies. The PM and AM intermodulation products are usually not in phase but do contribute to the overall distortion. The analog signals are almost immune to closely spaced pairs of phase intermodulation products (see MTN-111) but QAM signals can be affected. Widely spaced PM products behave as AM products and because of phase offsets may add or subtract amplitude.
