RFICDUDE
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The main reason for measuring IM products at drive levels significantly below the P1dB compression point is that the individual contributors to compression (IM3, IM5, IM7, etc.) are only clearly identifiable at the lower input amplitudes. That is to say, the classic 3:1 relationship in power for the third-order distortion components and subsequent 5:1 for 5th, 7:1 for 7th and so on are only directly measurable at lower drive levels. By the time compression sets in there are contributions from multiple orders that are significant making it difficult to separate the different order contributors at those power levels.
Also, the concept of intermodulation intercept point is explicitly defined in terms of individual order contributors. There are no composite intercept points which somehow account for all possible nonlinear contributors.
So, the idea is to measure the intermod products where their respective slopes match the order of the intermodulation distortion.
Hope this helps some.
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