November 7, 2016
The advent of 4G brought greater awareness and concern for Passive Intermodulation (PIM). While PIM existed in earlier wireless networks, it has much greater negative effects on LTE networks, creating headaches for operators, engineers, and field technicians alike. As networks continue to become more complex and bridge technologies to 5G are deployed, PIM will only become a potentially larger problem for wireless professionals.
PIM is a concern because it raises the noise level at the site, which increase the Signal-to-Noise ratio. The result is slower data throughput, especially for high-bandwidth services. Mobile users also experience shorter battery life on their devices because the UE must increase its signal level so it is “heard” above the increased noise caused by PIM. For these reasons, mobile operators have become more stringent in establishing PIM guidelines that must be maintained.
For engineers and field technicians, selecting the proper test solution is imperative to locating PIM sources efficiently so networks can perform according to specification. Even with the best solution on the market, users must still prepare the instrument properly for the best results. Here is a checklist to set up your PIM analyzer to take the most accurate measurements.
Calibrations – Let the analyzer warm up for 10 minutes before calibrating it to extend the useful temperature range, which may eliminate the need to re-calibrate later in the day. The Anritsu PIM Master™ battery-operated portable PIM analyzer (figure 1) has a 12-hour calibration limit, as long as the instrument temperature doesn’t change by more than 20° C. The half-day limitation is intentional to assure measurement accuracy. If you are measuring very low level PIM signals (below -120 dBm) calibration may be required more frequently to achieve very low residual PIM.
Keep it Clean – RF connectors on the analyzer, low PIM terminations, adapters, and test cables should be cleaned by pushing a lint-free wipe moistened with alcohol through the connectors. Use a non-metallic push stick and press hard into the corners to remove metal flakes. Wipe the mating surfaces to remove any oils or oxidation, as well.
Inspection – While cleaning the connectors check for signs of damaged spring fingers, mating threads, and mating surfaces. Replace if necessary.
Validate Test Parameters – Before taking a single measurement make sure the test parameters, including test power, test frequencies, IM order, test band, and pass/fail limit, are correct. To simplify this step, create a SET-UP file with all the necessary settings, and RECALL it prior to use.
Confirm Residual PIM – Once the calibration is complete, attach a low PIM termination, call up the PIM vs. TIME measurement mode, and perform a PIM test. Lightly tap on the low PIM termination during the test to measure the residual PIM of the instrument (figure 2). Per IEC 62037, the residual PIM of the instrument must be 10 dB lower than the Pass/Fail limit you plan to certify. If it fails, check to see if there are metal flakes inside the RF connectors, a worn connector saver or low PIM termination, or loose screws where the RF connector is attached to the analyzer.
Verify PIM Standard – Perform a 2x20W PIM test and measure the IM3 response of the PIM standard installed on the analyzer. The measured value should be within 3 dB of the expected value. When the value is greater than the standard allows, check to see if the power was set at 20W, if the IM order was IM3, if metal flakes are inside the RF connector, there is oxidation on the PIM standard mating surface or if the PIM standard was improperly torqued.
Test Lead Verification – The last step is to make sure that the test lead and any attached RF adapters are low PIM. Connect the test lead to the instrument test port and install the low PIM termination to the end of the test lead. Perform a PIM vs. TIME test while gently flexing the test lead at both ends. Per IEC 62037, the residual PIM of the analyzer must be 10 dB lower than the Pass/Fail you intend to certify. Common causes of an unacceptable PIM spike are metal flakes in the RF connectors, and worn RF adapters or test cable.
Once you have completed these steps you are ready to conduct accurate PIM measurements at your site. Anritsu offers certified PIM training courses that provide more details on these steps, as well as best methods for conducting measurements. You can also learn more by visiting our PIM page.