Evaluating Effects Of Passive Intermodulation

Intermodulation distortion can stem from both active and passive components, although spurious signals from passive sources are often more difficult to track and control in a communications system.

Passive Intermodulation (PIM) is a nonlinear effect in passive components, including cables and connectors, which can degrade communications system performance. The intermodulation occurs between two or more signal tones at sufficiently high power levels, and results in the generation of additional tones which can appear as signals or interference within a communications system.

Although the levels of PIM products are usually low compared to intermodulation (IM) products generated by a communications system's active components, such as amplifiers, the IM products from active sources can usually be removed or attenuated by filtering, whereas PIM products may occur at a point in the system's transmit section that is difficult to filter.

In communications systems, PIM is often referred to as "the rusty bolt effect" because of its association with less-than-ideal metal interfaces. It can be caused by the interface of two dissimilar metals in the signal path of a communications system, or even by a poorly fitting and improperly torqued connector interface. For example, a study performed by Justin Henrie and associates from Purdue University (available online) noted that because of the large difference in power between transmit and receive signals in modern communications systems, Pim levels as low as -150 dBc can work as interference for a system, especially where the transmit and receive frequency bands are closely spaced.

The Purdue study involved development of a model that could be used to predict the total PIM of a system comprised of many different PIM sources. This model helped the researchers to understand the effects of different metal-metal connector interfaces in the system, and how the use of some metals could impact the levels of PIM in the system. By creating connector interfaces with a hard-soft metal interface, a wiping action is created at the connector interface that serves to minimize PIM.

Several years ago, researchers at North Carolina State University analyzed the link between electro-thermal effects in high-power passive components, such as attenuators, and the generation of PIM.1 They found that thermally induced PIM was related to the thermoresistance effect in certain materialsessentially the self-heating of resistive materials such as those used in attenuators. Modeling the effect is difficult because the transfer of heat through the device doesn't take place instantaneously, but is a function of time.

The researchers used a parameter called thermal capacity to describe the capability of a material to store heat as a rise in temperature and to conduct heat to the surrounding environment at a given rate. They were able to create a compact electrical model of the self-heating effect in a resistive element, comparing it to a resistive-capacitive (RC) filter with dissipated power providing a current into the thermal resistance and thermal capacity of the device under analysis.

Research performed several years ago at the Queen's University of Belfast (UK) on various laminate materials has shown that the metal-dielectric interface in planar printedcircuit boards (PCBs) can also give rise to PIM at certain frequencies and power levels.2 Various dielectric laminate materials in combination with transmission lines, such as microstrip, can have a third-order nonlinearity behavior in such circuits as antennas and filters. As a result, PCB laminate suppliers now offer low-PIM materials based on the use of specially treated metal foils for the conductive traces.

For improved PIM performance, Taconic developed their RF-301 copper-clad PTFE-based material with low loss and dielectric constant of 2.97 at 1.9 GHz. Rogers Corp. created their RO4000 LoPro family of thermoset hydrocarbon resin laminates with reverse-treated conductive foil. Ideal for antennas and amplifiers, the specially treated conductive layers help minimize the generation of PIM at higher power levels.

Of course, understanding PIM means being able to measure it, and several companies have developed effective test solutions for analyzing PIM, like Summitek Instruments, which recently joined its sister businesses Triasx, Allrizon Communications, and the telecommunications division of TRAK Microwave under the KAELUS brand name.

The firm offers testers targeting many of the wireless cellular communications bands, including AMPS, GSM, and PCS. Their PIM analyzers are designed to generate two 25-W test carriers and measure PIM products with -140 dBm receiver sensitivity. The company also offers a free white paper on PIM testing, available.

Boonton, part of the Wireless Telecom Group of companies, offers a group of instruments under the PIM 31 Passive Intermodulation Analyzer, with frequency coverage similar to those of the Summitek Instruments testers. Rosenberger also supplies a PIM analyzer, its portable Passive Intermodulation Analyzer (PIA), which covers 800 to 2200 MHz with typical receive sensitivity of -168 dBm.

Earlier this year, Anritsu Company announced their MW8219A PIM Master (see figure), a portable tester for locating the source of PIM in the field. As with the other PIM testers, it is available in versions for various cellular frequencies, and is designed to work with the company's BTS Master, Site Master, Spectrum Master, and Cell Master handheld signal analyzers.

1. Jonathan R. Wilkerson, Kevin G. Gard, and Michael B. Steer, "Electro-Thermal Passive Intermodulation Distortion in Microwave Attenuators," Proceedings of the 36th Annual European Microwave Conference, Manchester, United Kingdom, 2006, pp. 157-160.
2. Aleksey P. Shitvov, Dmitry E. Zelenchuk, Alexander G. Schuchinsky, and Vincent F. Fusco, "Passive Intermodulation Generation on Printed Lines: Near- Field Probing and Observations," IEEE Transactions on Microwave Theory & Techniques, Vol. 56, No. 12, December 2008, pp. 3121-3128.

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