What is PIM
PIM, which stands for Passive Intermodulation, is an undesired effect caused by a non-linear mixing of two or more frequencies at signal path junctions and in ferrous materials. PIM produces signals at unwanted frequencies that can interfere with RF signal reception and lead to dropped calls, reduction in cell capacity or limited coverage resulting in loss of mobile connection and consequently revenue.
What causes PIM
Production of PIM involves physical features and materials that cause passive components to become mixers, modulators and frequency multipliers. Examples of such features are loose metal to metal contacts, use of ferromagnetic materials, contamination, surface oxidation, insufficiently thick metal plating or contact between dissimilar metals. The frequencies of the signals produced by these elements can be calculated as 3rd, 5th, or higher order intermod products and if they fall into the receiver’s passband, can raise the noise floor of a system substantially.
Ensuring low PIM becomes especially relevant when the intermodulation causing products fall into the receiving band, thus changing the signal to interference ratio and generating problems such as reduction in cell/sector coverage and/or capacity. At times, one or more of the frequencies causing the intermod products may be coming from a nearby transmitter. Since PIM effects increase significantly as the average transmit power level increases, PIM interference becomes an increasingly disruptive problem as the base station becomes further loaded. Nevertheless, the coverage or reach of only lightly loaded base stations can still be affected by PIM.
Is there an acceptable level of PIM
Acceptable levels of PIM vary depending on the location of a given RF device. Located in transmit/receiving paths, the RF device’s PIM characteristics are particularly important because the bands are so closely spaced. Load levels of frequency bands and base stations are further determining the level of acceptable PIM. Common specifications generally allow levels of up to -110 dBm (with two +43 dBm per carrier tone). However, ultimately Wireless Service Companies specify values that they deem acceptable under all conditions considered. The premise will always be to avoid adding more equipment or installing new cell sites to cope with the reduction in cell capacity or coverage caused by the interfering signal.
How to avoid PIM
To adequately address the question of how to avoid or at a minimum reach an acceptable level of PIM, three aspects have to be considered.
Low PIM product design
Products designed specifically with a low or ultra-low PIM quality generally adhere to the following design principles:
- Avoidance of ferrous material
- Minimal number of contact junctions
- Any contact junctions are precise and maintain good contact under pressure
- Avoidance of dissimilar metals in direct contact and
- Surfaces are plated to avoid oxidation
Quality Manufacturing
A well designed product can still become a major PIM generator if the manufacturing process did not ensure highest quality standards. Poorly aligned parts, inadequately torqued screws and fasteners, bad solder joints, insufficiently cleaned parts prior to plating or poor plating adhesion can render the product ineffective for its PIM avoidance features.
Proper Installation and Maintenance
During the installation phase, the main source of problems arises at connection points where bent cables, over or under-torqued connectors and torsional loads on connector junctions may cause unwanted signal interference. Since some PIM producing elements are not easily visually detectable, strict adherence to manufacturer’s installation guidelines is important.
Having a well-designed product that is flawlessly built and properly installed may not, however; ensure low PIM performance over time. Natural elements, such as temperature variations, moisture, wind-induced vibrations and air-borne dirt continuously work to break down and erode exposed parts of the component, weakening joints, separating connections, oxidating materials and allowing moisture and dust to enter. Visually check your components regularly and consider various testing instruments to certify continuous low PIM performance.
Susann Ferrari
Protection Technology Group