Passive intermodulation (PIM) is evidence of nonlinear behavior in a communications system and some of its components. PIM is caused by the mixing of multiple tones, which results in unwanted signals. If the levels of these signals are high enough, the operation of a cellular base transceiver station (BTS) will be impacted. The result will be dropped calls for cellular customers and lost capacity for cellular network operators. In a five-page white paper, Fred Hull, Director of Engineering at San-tron, explains how various mechanical structures can contribute to high PIM levels. Current flow through certain materials also can produce PIM signal energy.
The paper, titled "Minimizing PIM Generation from RF Cables and Connectors," begins with an explanation of PIM. When two signals (f1 and f2) mix, they can produce third-order, fifth-order, seventh-order, and higher-order harmonic-signal products. The third-order products, which are at the highest power levels, pose the greatest threat in terms of potential interference. These third-order PIM products can be produced by a combination of 2f2 f1 or 2f1 f2.
The nonlinear characteristics that cause PIM stem from the effects of corona generation, the use of paramagnetic materials, and the effects of current saturation. To avoid corona generation, one should develop RF geometries that support an application's expected power levels. Simply put, paramagnetic materials should not be used in any components that are intended for low-PIM communications applications. To deal with current saturation, additional mating force can be used at junctions between conductors. Micro-mountain tops can then be flattened to increase the contact surface area and reduce current saturation within the junction or connection.
The bulk of the paper is devoted to a discussion of coaxial cable assemblies with braids, as they can present challenges when trying to achieve low PIM performance in a communications system. When the cables undergo flexure, for example, the braids rub over each other and are constantly repositioning themselves. Ground currents are continuously re-routed through the fine braids, which invites current saturation and non-transverse-electromagnetic (non-TEM) conditions or eddy currents within the cable braid structure. The paper emphasizes that braid structure is critical to the development of low-PIM system integration. Armed with an understanding of the mechanical tolerances, coaxial design details, and connector materials used in cable assemblies, designers can produce communications equipment with the lowest possible PIM levels.
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