Passive intermodulation


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Passive intermodulation - 1

White paper Excellence in Connectivity Solutions

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3 Passive intermodulation (PIM) 4 What is passive intermodulation (PIM)? 5 Where and why is PIM a problem? 5 What causes PIM? 7 What do we do against PIM? 8 What must be considered when using low PIM products? Passive intermodulation (PIM) Intermodulation is a system characteristic, where new frequencies in the output are created. A common technical realisation for intermodulation is to use non-linear components, such as diodes. While intermodulation is important for certain applications, it is not in every case. One important example is a communication system, where transmission and...

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What is passive intermodulation (PIM)? Passive Intermodulation is generated in a system, if it is non-linear. This causes the shape of the output signal to be changed. Figure 1 shows that positive components of the amplitude are much more strongly amplified than negative ones. This signal also differs in its frequency spectrum. h Uout t Output signal Uin Figure 1: t Non-linear system characteristic Input signal Mathematically, we can explain this as follows: let h be a non-linear system, then the output is given as uout = h (uin) (1) To understand why additional frequencies are generated,...

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Passive intermodulation Where and why is PIM a problem? Whenever a high power and a low power signal is transmitted at the same time but with slightly different frequencies, PIM may become a problem. It is a problem, for example, in a full duplex radio base station but not in a radar application. (because transmission and received signals are not simultaneous) In order to reduce the number of components, it is desirable to use the antenna and cabling for both transmission and received signals. It is the case that the amplitude of the transmitted and received signals differ in order of many...

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What causes PIM? In the previous section, we mentioned non-linearity as a root cause of PIM. As a manufacturer of low PIM components, it is important to understand all possible sources of nonlinearity. Material and plating Both dielectric and metallic materials could cause PIM, if they show any non-linear behaviour. In practice, dielectrics are not a big problem as typical plastics behave very well. Only exotic materials could cause problems, e.g. semi-conducting materials. In special and/or very high power applications, corona discharge and multipaction can be a problem. Metallic...

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Passive intermodulation Galvanic contacts The contact parts of a connector are particularly sensitive to PIM, because of the high current densities in this region. Spring contacts typically found on centre conductors have only one mechanical contact point (green in figure 4) per contact finger. The electrical conducting area (red in figure 4) is a fraction of the contact area, which itself is a fraction of the contact finger area. Figure 4: A-spot Close to the conducting area, electron tunnelling is possible in the thin dielectric coating, which is highly non-linear. The a-spot area has a...

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What is done against PIM? Knowing the potential sources of PIM is not enough – we also need to know how to design a component to prevent PIM. Material and plating On the material side, we use high quality and very pure base material. If problems occur, we have the option to easily measure contamination and magnetism, in order to locate the source of problem. HUBER+SUHNER SUCOPRO plating uses a non-magnetic diffusion barrier. Silver plating is technically the best solution and can be used if cosmetic aspects are not critical. SUCOPLATE is a good and economical alternative. Galvanic contacts...

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Passive intermodulation What must be considered for low PIM products? In the previous chapter, we discussed techniques we apply for low PIM designs. As for those high performance products, correct handling by the user is essential to achieve the performance. Here are some recommendations on how you can optimise the performance of the product. Cleanness Any dirt within the signal path is a performance killer. Non-conducting substances on contact spots decrease the number and size of a-spots, which results in poor PIM performance. Dirt can enter the interface when it is not mated. Therefore,...

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Conclusion Some applications are very sensitive to intermodulation products. Although passive components are considered to be linear in theory, such components exhibit some non-linearity in practice. We have discussed several physical phenomena, which are root causes. The most important aspects are non-linear magnetisation of ferromagnetic materials and a-spots in galvanic contacts. A careful design is a prerequisite, but correct connector care – such as a clean environment and correct torque – are also important. Author Adjan Kretz received a MSc degree in electrical engineering from the...

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WAIVER It is exclusively in written agreements that we provide our customers with warrants and representations as to the technical specifications and/or the fitness for any particular purpose. The facts and figures contained herein are carefully compiled to the best of our knowledge, but they are intended for general informational purposes only. HUBER+SUHNER AG Radio Frequency Division Degersheimerstrasse 14 9100 Herisau Switzerland Tel. +41 71 353 4111 Fax +41 71 353 4444 4069/RF/02.2012 HUBER+SUHNER is certified according to ISO 9001, ISO 14001, ISO/TS 16949 and IRIS....

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