Is There a Difference in Forward and Reverse (Reflected) IM Levels?
Because the intermodulation signals created at various PIM sources within an assembly are vectorial in nature, their relative phase relationships will determine the overall magnitude of (scalar) PIM measured at a particular location in a Device Under Test. Using the model developed in the application note Measuring the Passive Intermodulation Performance of RF Cable Assemblies, we find that all the IM responses arrive in-phase at Port 2 of a through IM measurement, independent of the IM frequency, while the reflected IM response present at Port 1 is a combination of the Port 1 response plus a phase-shifted response from the IM sources at Port 2. Because there is a vector combination of IM sources with differing phases, it is expected that the reflected IM response is a function of both frequency and the electrical length of the assembly. In the real world, however, the forward IM response may not measure as being frequency independent and may not be the worst-case IM response. This is
