PIM comes from two or more strong RF signals mixing in a non linear device. These non linear devices, or junctions, occur in improperly tightened, damaged, or corroded connectors or in damaged antennas. Rusty components, such as mounts and bolts, are also suspect when hunting for sources of PIM.
Many common frequency combinations can produce PIM in a cell’s receive band. Signals in the cell’s receive band will raise the receive noise floor, increase the bit error rate, and shrink the reception area for cellular communications. Avoiding PIM starts with frequency assignments that put potential PIM products outside of receive bands. However, increasing capacity, new services, and aging infrastructure are all working against this strategy, making PIM testing more important every day. It is apparent that most onsite PIM issues that affect service are derived from the sidebands of internally generated interference, not the calculated frequencies themselves.
Proper care and maintenance of connectors is essential to keeping PIM low. Inspection and cleaning is a central part of good performance. Proper torque is also important, as this keeps the center connector from damage.
PIM testing is becoming more critical as cellular systems age and the carrier count is increased. A test that was not as important when cellular systems were lightly loaded is becoming a critical part of modern cellular maintenance.
Ubi-Tech are the sole training partner for Anritsu for the UK and Europe and are able to deliver certified training on how to test for PIM interference as well as Certified Line Sweep SiteMaster.
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