Minimizing PIM Generation From RF Cables and Connectors
The published specifications for leakage current in RF power devices are often a source of concern and confusion for engineers and technicians. This paper examines the real meaning behind the leakage current specifications and offers guidance on properly testing a device for leakage current.
Communications system interference can come in many forms, but one of the most difficult to defeat is passive intermodulation (PIM) distortion. Unlike noise, which tends to cover broad bandwidths and is easily filtered, PIM results in discrete signals that fall within the operating bandwidth of a wireless communications receiver, capable of blocking desired signals. At sufficiently high magnitudes, PIM distortion can desensitize a communications receiver, resulting in poor quality of calls in a cellular system or even dropped calls. Reliably good communications system performance depends on minimizing PIM. And choosing the right cable assemblies can go a long way towards helping to shave PIM distortion.
What is PIM? It is the result of the nonlinear characteristics of the components and materials that channel communications signals. It is caused when multiple tones mix to generate unwanted harmonic signals. Sometimes referred to as “ghost” signals, these harmonic signals can interfere with the proper operation of a communications system when they occur at high enough levels. For example, for a wireless cell tower and its base transceiver station (BTS) attempting to process the very low-level signals from handheld portable cell phones, unwanted PIM harmonic signals can block the desired cell-phone signals from ever reaching the cell-tower receiver.