What is Passive Intermodulation (PIM)?

Passive Intermodulation (PIM) is a type of Intermodulation Distortion (IMD) that occurs in passive, non-linear components in wireless systems. Passive components are those components which do not require electrical power to operate. Examples of RF passive components include power dividers/combiners, couplers, terminations, coaxial connectors, cables etc. Oxidation, internal resistance, or other effects in passive components can cause the generation of a non-linearity. Non-linearity of components also tends to increase with age. 

PIM is generated when multiple signals (two or more) are used in passive components with some non-linear elements. Those signals interfere with each other to create unwanted signals. The main sources of PIM are usually junctions between different types of passive components such as junction between connector and cable, two different types of cables made from different materials, connection between cable and antenna, etc.

The unwanted signals generated by intermodulation or interference between the signals can be the sum and difference of those individual signals. In the figure above, if f1 and f2 are the two signals present in the passive, non-linear component then they will create signals of different orders as shown. The amplitude of harmonics decreases with an increase in the order of harmonics, hence the higher order harmonics (4th order or higher) can be neglected.

PIM is an issue for almost every wireless system, but it is most noticeable in cellular applications (CDMA, HSPA, and LTE). This is because the frequency bands used by these technologies are very close to each other. For example, in LTE Frequency Band 2, the downlink frequency is from 1930 MHz to 1990 MHz, while the uplink frequency range is from 1850 MHz to 1910 MHz. If two transmitter carriers at 1940 MHz and 1980 MHz are transmitting from the base station system with PIM, their intermodulation will lead to a signal at 1900 MHz, which will fall into the receive band.

Passive intermodulation, falling back into the receiver band (Source: Analog Devices)

PIM lowers the reliability, capacity, and data rate of wireless systems by limiting the receive sensitivity. End results may include dropped calls, decreased system capacity, and decreased data rates.

The environment in which the system is operated also plays a role in the generation of PIM. Wide temperature variations, polluted air, excessive vibration, etc. can cause metal components to rust or lead to less than ideal connection at the junctions. These imperfections can lead to higher PIM generation.

PIM can be minimized by following taking certain precautions:

1. Remove bad RF connections
2. Connectors are securely tightened
3. Low PIM cables are chosen with fully soldered connectors
4. Ideal installation should be maintained without any loose connections
5. Proper cleaning and maintenance of components before connecting

PIM Standard and Testing

IEC 62037 standard is the international standard for PIM testing of RF equipment that describes the measurement setup & testing procedure. This standard specifies the use of two RF signals with a power level of 20 W (+43 dBm) on RF equipment such as cables, antennas, or other objects for PIM testing. It is done to check if any non-linearity is encountered in the propagation path (that leads to PIM signal generation). A spectrum analyzer with test instruments setup is used to display the signals.

Further Reading

Passive Intermodulation (PIM) Effects in Base Stations: Understanding the Challenges and Solutions.

Passive Inter-modulation (PIM) is the generation of interfering signals caused by non-linearities that occur in passive components such as antennas, cables, connectors, or duplexers with two or more high-power input signals. Two signals mix together (amplitude modulation) to produce sum and difference signals and products within the same band, causing distortion.