Modern wireless high-speed data networks use tightly grouped channels and complex modulation schemes to enable transmitting vast amounts of data. This in association with ultra-sensitive receivers may face unanticipated but serious capacity losses if the network is disturbed by Passive Intermodulation, or PIM for short. Generally, modulating RF signals is necessary to transport information, but arbitrary passive intermodulation is utterly unwanted. Unfortunately, it can happen whenever more than one signal is channeled through one RF path. As a result, we may see unwanted non-linear frequency responses of passive components including connectors and cable feeds. These components start acting like mixers, modulators and frequency multipliers creating unwanted spurious products.
PIM (Passive Intermodulation Distortion)
PIM may become a major problem when Tx and Rx signals share one RF path. Luckily, VSWR measurements are standard procedure after network installation. Such measurements determine how much RF energy the antenna emits, and how much unwanted energy is reflected back into the transmitter. VSWR meters are however not capable to detect non-linearity in system components. Validating PIM network quality requires special PIM test systems. The preferred scenario for best network quality is – preventing PIM in the first place. For achieving this, it is paramount to utilize only high quality, low PIM components, apply proper installation procedures and ensure excellent grounding of the RF system.
Why is it critical to eliminate PIM?
Intermodulation products may be generated whenever base stations transmit RF signals. The resulting intermodulation frequency products are often found within the receiving bands of a network. Since RX signals are by nature very low power, interference with regular voice and data traffic occurs. Unwanted PIM interference may desensitize one or more receiving channels to such degree that not only creates very high BER that reduce network bandwidth, but may even drop calls completely. It the worst case it can even lead to permanently unusable receiver channels. Loss of already sparse network capacity caused by PIM is in no way acceptable for high volume, high speed wireless data networks.
What causes Passive Intermodulation?
- Ferromagnetic metals, like iron, nickel and steel, show hysteresis effects with applied energy. The resulting signal levels are altered and the signal response is no longer linear.
- Dissimilar metal plating on connectors constitutes potential voltaic elements that act like a diode, causing unwanted random modulation effects.
- Corroded surfaces cause PIM. Corrosion may happen on unprotected component surfaces or by human influence (e.g. touching a connector pin with bare fingers).
- Irregular contact surfaces, even on a microscopic scale, can cause an inconsistent flow of charge carriers and generate inhomogeneous electromagnetic fields. Causes can be of mechanical or electrical nature: Low quality components, shearing by forced connections and disconnections of components, spark craters caused when “hot” connections are disconnected.
- Wind load and dissimilar expansion coefficients of tower and feed lines stress both, connectors and cables and will cause deteriorating connection quality.
Minimizing Passive Intermodulation (PIM) is critical for achieving maximal system capacity and efficiency of wireless high-speed networks. PIM awareness is paramount for PIM prevention. Installers need to be trained properly ensuring familiarity about PIM causes and expertise on how to prevent them. Manufacturers have to deliver products that not only come with low PIM characteristics but guarantee sustaining specifications over a long time under environmentally harsh conditions. System designers have to consider appropriate products, and they need to pay special attention to material and plating of mating component surfaces. Finally, wireless operators have to maintain their network ensuring that PIM behavior is not deteriorating during operation of the system.