On-site PIM testing using a CCI PIMPro analyser
Passive intermodulation (PIM) effects are caused by non-linearities in passive products such as connectors. When two (or more) signals are present the PIM products from a non-linearity are just like those from a mixer i.e. the sum and difference products of the two frequencies. This unwanted frequency generation can be tolerated in some systems as the PIM signal levels are low, often 100dB or so down on the main signal levels.
In systems where transmitter and receiver share the same antenna PIM can be particularly problematical; as the noise floor rises the sensitivity of the receiver is degraded. This is why wireless base station system components often have PIM specified at -160dBc (3rd order with two 20W signals).
This means that on-site testing is required to check PIM performance, either at the installation stage or later fault finding activity. It’s worth noting here that there are various possible non-linearities to consider; dirty connectors, oxidisation of connectors, poor quality feeder, loose connections or connectors with imperfect contact, moisture ingress etc.
In a recent example a network operator noted noise on an uplink channel at a particular site. Initial investigation gave the cause as faulty high power loads but using a CCI PIMPro it was quickly identified that the loads were meeting the -160dBc specification. Subsequent investigation revealed that the connectors mating with the loads had been degraded and were the cause of the problem.
Unlike other PIM testers the CCI PIMPro tower series are truly portable, supplied in a rugged back pack and are ideal for use at conventional base station or in-building Distributed Antenna (DAS) sites.