Swept PIM measurements are made by holding one test tone fixed and sweeping the other test tone across a range of transmit frequencies causing the IM frequency to also change. When a system has one dominant PIM source, the magnitude of the PIM tends to be relatively flat versus frequency. When more than one PIM source is present in a system, you may see that the PIM signals add or subtract based on the phase difference between the two signals. This is what is shown in the sample measurement that is shown in Figure: Swept PIM IM3 Measurement.

In this measurement, two PIM sources are spaced 6 meters apart. Each source is a loose RF connection, and they create PIM sources of approximately equal magnitude. The limit line is set at –97 dBm (–140 dBc), which is typical for site testing. You can see that at some frequencies, the PIM signals subtract (resulting in a Pass measurement), and at other frequencies, they add together (resulting in a Fail measurement). If this test had used fixed, two‑tone testing and happened to select two transmit tones that put the IM signal into one of the nulls, then the trace would have indicated that this site was very good, when it is actually bad. Using Swept PIM is a method to verify this condition.