What is PIM

PIM, also known as Passive Intermodulation, is a type of signal distortion. Since LTE networks are extremely sensitive to PIM, how to detect and reduce PIM has received more and more attention.

PIM is generated by nonlinear mixing between two or more carrier frequencies, and the resulting signal contains additional undesired frequencies or intermodulation products. As the word “passive” in the name “passive intermodulation” means the same, the above-mentioned nonlinear mixing that causes PIM does not involve active devices, but is usually made of metal materials and interconnected devices. Process, or other passive components in the system. The causes of nonlinear mixing can include the following:

• Defects in electrical connections: Since there is no flawless smooth surface in the world, there may be areas with higher current densities in the contact areas between different surfaces. These parts generate heat due to the limited conductive path, resulting in a change in resistance. For this reason, the connector should always be accurately tightened to the target torque.

• At least one thin oxide layer exists on most metal surfaces, which can cause tunneling effects or, in a nutshell, lead to a reduction in conductive area. Some people think that this phenomenon can produce the Schottky effect. This is why rusted bolts or rusted metal roofs near the cellular tower can cause strong PIM distortion signals.

• Ferromagnetic materials: Materials such as iron can produce large PIM distortion, so such materials should not be used in cellular systems.

Wireless networks have become more complex as multiple systems and different generations of systems have begun to be used within the same site. When various signals are combined, PIM, which causes interference to the LTE signal, is generated. Antennas, duplexers, cables, dirty or loose connectors, and damaged RF equipment and metal objects located near or within a cellular base station may be sources of PIM.

Since PIM interference can have a significant impact on LTE network performance, wireless operators and contractors attach great importance to PIM measurement, source location and suppression. Acceptable PIM levels vary from system to system. For example, Anritsu’s test results show that when the PIM level increases from -125dBm to -105dBm, the download speed drops by 18%, while the former and the latter Both values ​​are considered to be acceptable PIM levels.

Which parts need to be tested for PIM?

In general, each component undergoes a PIM test during design and production to ensure that it does not become a significant source of PIM after installation. In addition, since the correctness of the connection is critical to PIM control, the installation process is also an important part of PIM control. In a distributed antenna system, it is sometimes necessary to perform PIM testing on the entire system as well as PIM testing on each component. Today, people are increasingly adopting PIM-certified devices. For example, antennas below -150dBc can be considered PIM compliance, and such specifications are becoming increasingly stringent.

In addition to this, the site selection process of the cellular site, especially before the cellular site and antenna set up, and the subsequent installation phase, also involves PIM evaluation.

Kingtone offers low PIM cable assemblies, connectors, adapters, multi-frequency combiners, co-frequency combiners, duplexers, splitters, couplers and antennas to meet a variety of PIM-related requirements.

Post time: Feb-02-2021