January 30, 2017
Passive Intermodulation (PIM) has been a well-known problem in cellular systems for more than 20 years. As wireless networks advanced to LTE and move towards 5G, however, the importance of PIM testing only grows. For this reason, it is essential that mobile operators minimize the amount of PIM in their networks to meet KPIs and lower customer churn.
Even if an antenna system has low internal PIM levels, external sources of PIM can wreak havoc on network performance. Loose metal-to-metal contact is the typical cause of external PIM. Some easily identifiable sources include air handling equipment on the rooftop near the system, overlapping layers of metal flashing, sheet metal cable trays, sheet metal roof vents, and rusty metal objects. Some sources are not as obvious. These include loose metal-to-metal connections hidden by roofing materials or loose cable hangers behind the antenna.
Though third order intermodulation product (IM3) levels below -100 dBm (-143 dBm) are specified, external PIM sources can generate IM3 as high as -50 dBm (-93 dBc). Testing for external PIM typically involves employing a solution that combines a PIM analyzer, such as the PIM Master™ MW82119B, and a PIM Hunter™ probe (figure 1).
Test equipment such as the examples mentioned above work in tandem to inject two high-power CW test signals that travel through the antenna feed system and radiate any PIM sources in the RF path. PIM sources beyond the antenna send intermodulation product signals in all directions that travel back to the antenna feed system and into the PIM analyzer receiver. A PIM probe connected to a spectrum analyzer can help locate the exact location of a PIM source.
When conducting PIM tests on wireless networks, there are several factors to take into consideration. Let’s discuss some that may be overlooked but certainly shouldn’t.
Technicians performing PIM tests must avoid excess exposure to RF fields in accordance to the local jurisdiction’s maximum allowable occupational exposure limit. In the U.S., FCC occupational / controlled exposure limits are shown in table 1.
Field personnel should know local exposure requirements and maintain safe working distances. That’s why a PIM probe should be about one meter long, so it is possible to test locations near antennas while remaining in compliance with occupational exposure limits. Table 2 demonstrates the recommended safe distances depending on antenna type and frequency of the signal being tested.
Technicians should keep their bodies either behind or to the side of the antenna if areas close to the front of the antenna need to be probed. Along with safety considerations, standing between the antenna and PIM source can interfere with the PIM test, so it is important to avoid obstructing the probe.
The receiver on the spectrum analyzer can be one of the worst PIM sources at the cell site. High levels of PIM can occur within the spectrum analyzer, if the high-power test tones used to excite external PIM sources reach the receiver front end. To prevent this, install a high-quality bandpass filter between the PIM Hunter probe and the input port of the spectrum analyzer. The filter used must allow the IM3 signals to pass while, at the same time, attenuating the two high-power test signals generated by the PIM analyzer.
Spectrum Analyzer Settings
To provide a good starting point for an external PIM hunt (and improve the chances of success), the spectrum analyzer must be set to the proper parameters. A spectrum analyzer user guide will detail instructions on how to set the instrument to the following settings:
- Center frequency = IM3 frequency from PIM analyzer
- Span = 100 kHz
- Amplitude reference level = 20 dB higher than peak IM3 measured on PIM Master
- Scale = 10 dB/div
- Auto Atten = ON
- Auto RBW = ON
- Auto VBW = ON
- Pre-amp = ON
- Trace A Operations = Average –>A
- # of averages = 2
- Trace B Operations = Max hold–>B
- Sweep Mode = Burst Detect
- Upper Limit = ON
- Limit level = 10 dB lower than peak IM3 measured on PIM Master
- Limit Alarm = ON
- Volume = set to maximum (found in the system menu)
If the Anritsu PIM Master is used, it is important to note that the analyzer pulses the CW PIM test signals on and off continuously during the test to reduce battery consumption. The result is that the IM3 signal being hunted will appear as a pulsed signal. Because of this, it is recommended to use a spectrum analyzer equipped with burst detect to capture the pulsing signal. Averaging two or three sweeps will smooth the resulting magnitude while maintaining quick response to changes in PIM level.
In most cases, the IM3 signal being hunted will be relatively high (-50 dBm to -80 dBm). Using the above settings, the spectrum analyzer’s noise floor should be well below -100 dBm, so the IM3 signal should appear on the spectrum analyzer display as soon as the PIM Analyzer is powered up. When the probe tip is close to the PIM source, the level will rise rapidly. A spectrum analyzer with a limit alarm provides an audible notification when this occurs, so technicians can listen for PIM instead of constantly checking the spectrum analyzer screen.
Max hold on trace B can be used to compare maximum IM3 level detected to the current IM3 level. If an audible alarm sounds while scanning across an area, slow down and scan more deliberately to find where the maximum level occurred. Once the PIM source location is marked, a Reset Trace function shows the maximum trace to be reset before looking for more PIM sources.
Following these important guidelines can help ensure safe and successful exterior PIM tests, which will improve the overall performance of the antenna systems. For more information and important considerations for conducting PIM tests, you can download our white paper entitled Identifying Sources of External PIM. You can also learn more about the topic on our PIM page.