When it comes to PIM testing, the necessary steps to identify passive intermodulation (PIM) and locate its source are similar to interference hunting. In the end, PIM is an internal source of interference and can have a significant impact on network performance.
Before starting with PIM measurements, however, it is recommended to test return loss. The PIM analyzer transmits high power; therefore, before PIM testing, any high-power reflection, for example, generated by a pinched cable, must be located and repaired. Return loss can be easily measured over a wide frequency range with a cable and antenna analyzer such as the R&S®Cable Rider ZPH or the R&S®ZVH.
1. Test equipment verification
Why take the time to verify your equipment? The simple answer: the test equipment and related accessories needed for testing are also subject to passive intermodulation. We want to measure PIM on the BTS, not PIM generated in our test setup.
RF connectors have a specific number of mating cycles, which refers to the number of times a physical connector can connect (mate) to its counterpart. After reaching that maximum number, they may become a PIM source. Dirty or damaged connectors also generate PIM, so make sure that all connectors – on the instrument, cables, and adapters – are clean and intact.
It is also important to verify the test equipment itself. The receiver should not be influenced by test transmitted signals (residual PIM), and its sensitivity should be at least 10 dB lower than the defined PIM limits for the BTS components. The transmitter should generate the desired output power.
How to check the instrument? We can use a PIM standard together with the PIM load. Basically, the PIM standard generates a known PIM level at a specific IM3 frequency and test power. This is an easy verification that should be done before starting a working day in the field.
2. Static PIM tests
Now, it is time to connect the PIM analyzer to the system and verify PIM on the site. An antenna or a PIM load (when the components are tested separately) must always terminate the system under test. Existing on-air signals increase the noise level on the receiver, making it more challenging to identify PIM. By configuring the IM3 to fall into a guard band frequency, the noise level on the receiver will significantly decrease.
In general, –97 dBm (– 140 dBc) with 2 x 43 dBm (20 W) test signals are estimated as the pass/fail limit for a system. For individual components and feed systems terminated into low PIM terminations, a –107 dBm (–150 dBc) level is expected.
In case of small cells, if we follow the 1:3 dB rate (raising the TX power by 1 dB to increase the PIM energy in the UL by 3 dB) as explained in an earlier post, the result is as follows:
- Macrocell criteria remain: –97 dBm at 43 dBm test power
- Small cell test power: 37 dBm (5 W)
- Delta test power: 43 dBm minus 37 dBm = –6 dB
- Specification reduction due to change in power = 3 dB/dB times –6 dB = –18 dB
- New pass/fail level at 37 dBm (5 W) test power: –97 dBm plus –18 dB = –115 dBm
Finding a faulty component in a complex system can be time-consuming. An easier way to accomplish this is by using the PiMPoint tool or distance-to-PIM measurement available on the PiMPro Tower. A piece of steel wool in front of the antenna can be used as a marker to determine if the PIM occurs before or after the antenna.
When testing an antenna, place it on a non-metallic pedestal (or other support) so that it is at least 30 cm off the ground – the higher, the better. A clear sky and an unobstructed view are required to avoid the excitation of secondary PIM sources, which could cause false PIM failures. Therefore: no fences, no metal objects, no equipment and storage sheds, etc. Isolate the feeder system using PIM load, and repair PIM points by process of elimination before moving on to dynamic tests.
3. Dynamic tests
During dynamic tests, lightly tap all connections on the main feeder line (tap without excessive force to not damage connectors; but tap with sufficient force to excite potential PIM problems). If PIM products exist or increase to a failed state, rework the connections.
Finally, repeat the static and dynamic tests, and do not forget to record your results.
PIM testing – best practices in a nutshell:
- Clean, clean, clean connectors …Use isopropyl alcohol
- Torque all connections
- When mating connectors, quality workmanship counts: it significantly reduces PIM!
- Verify component PIM specification
- Always pay attention to high power RF radiation from the analyzer and antennas
- Use PIM standard and load to validate system performance
- Measure twice