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Finding The Importance Of Pim

What is PIM?

Passive Intermodulation (PIM) is the unwanted signal or signals generated by the non- linear mixing of two or more frequencies in a passive (or linear) device such as a connector or cable. These nonlinearities can occur due to poor-quality material, ferromagnetic materials, corrosion, or improper connections. In integrated base stations, significant levels of PIM can be generated within any of the passive components between the high-power amplifiers and the receiver filter, as well as the antenna.

PIM Causes Degraded System Performance

PIM can impact system performance when unwanted intermodulations generated by multiple signals in the downlink appear in the uplink, reducing overall quality of service. Additionally, PIM can cause interference in nearby systems, including Public Safety solutions, reducing their overall coverage or signal quality.

Low PIM is Important for:

  • Improved system performance. Components with PIM levels below the noise floor allow for maximum system performance.
  • Near-term system quality. High-quality, compatible materials guaranteeing system performance when the system is initially powered up.
  • Long-term reliability. Low risk of loose connections or corrosion guaranteeing system reliability for the long haul.

Testing PIM Compliance

Recently, the engineers at Cellular Specialties, Inc. were at a jobsite troubleshooting a DAS (distributed antenna system) with performance issues. A suspect directional coupler was tested and the PIM measurement levels did not meet the standard carrier PIM measurement of -153.

The non-PIM-compliant components were removed and replaced with PIM-compliant products that met or exceeded the standard carrier performance levels. Once the system was turned back on, nearby interference decreased, and caller quality immediately improved.

Improving PIM in Your DAS

Consider the following when designing your next DAS:

  • Low-PIM components at the BTS interface will eliminate self-interference due to the BTS downlink generating intermodulations that fall into the BTS uplink.
  • Low-PIM antennas will eliminate self-interference, as well as interference with nearby systems.
  • The use of high-quality connectors (DIN or N type) are necessary to achieving low PIM.
  • High-quality assembly techniques including cable prep tools and proper torque wrenches are a must.

www.tessco.com/go/csi to learn more