IEC 62037-5-2021 Passive RF and microwave devices, intermodulation level measurement – Part 5: Measurement of passive intermodulation in filters.
4 General comments on PIM testing of filter assemblies
4.1 Sources of error: back-to-back filters Testing filter assemblies for PIM can be error prone if certain precautionary guidelines are not followed. Since PIM can be a frequency-dependent phenomenon, mathematically related to the harmonics of the input signals and combinations thereof, consideration should be given not only to the behaviour of the test set-up under fundamental stimulation, but also its harmonic performance. In particular, consider a receive-band PIM test set-up as shown in Figure 1 . As shown, this set-up could be used to measure the PIM in a two-port device under test (DUT); however, the accuracy of the measurement could be in question due to the back-to-back filters (diplexers) used.
While the diplexers certainly appear as a matched load around the fundamental frequencies and receive-band IM products, they can be very poorly matched at the harmonics of the fundamentals. A poor match will set up a standing wave at the harmonic frequencies which can re-illuminate any PIM sources within the DUT with higher-than-typical current densities. Furthermore, the measured IM response will become highly dependent upon the electrical length of the DUT because the locations of the peaks and valleys of any standing waves will move with respect to the PIM sources as the electrical length of the DUT changes.
4.2 Environmental and dynamic PIM testing Environmental and dynamic PIM testing, which can include placing vibrational or thermal stresses upon filter assemblies while concurrently measuring the PIM produced, may not give accurate or repeatable results. There are several significant factors affecting the results of these types of PIM tests.
a) DUT/test system isolation
– it is highly desirable that any environmental and dynamic stresses placed upon a DUT be isolated from the test system such that there are no measurable residual effects. This not only addresses the practical issues of test system reliability and maintenance, but it directly affects the issue of measurement repeatability. That is, should a particular piece of the test system require replacement after a set number of trials, then the results of subsequent measurements may be skewed by the performance of the replaced part.
b) Measurement repeatability
– it should be possible to repeat the results obtained from a particular measurement within a specific precision. However, the inherent sensitivity of the PIM response can prevent a desired precision from being achieved.IEC 62037-5 pdf download.