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    Overcoming Passive Intermodulation

    Passive intermodulation (PIM) products are becoming a problem in wireless communications system design for the following reasons:

    • Regulatory bodies are maximizing spectrum capacity through reduced bandwidth.
    • Towers are being shared and the number of antennas used for transmit and receive functions reduced.
    • Equipment providers are increasing transmit power levels and reducing receiver sensitivity.
    • Equipment providers are providing trunked architectures, enabling multiple channel operation in VHF and UHF bands.

    With proper low passive intermodulation devices and installation procedures much can be done to reduce the effects of this problem.

    About Passive Intermodulation

    Passive intermodulation is the production of new, unwanted signal frequency components in passive, nonlinear devices commonly found in and around radio sites. These new signals can land on existing transmit or receive frequencies on the same or an adjacent site, causing interference.

    A partial list of devices that, when frequencies are mixed, could potentially generate passive intermodulation products, includes the following:

    • Cables
    • Connectors
    • Adapters
    • Tx line junctions (such as tee and stars)
    • Filters
    • Directional couplers
    • Antennas
    • Surge protectors

    Contributing Factors to Low-PIM Signal Generation

    While standards are still under development, an acceptable PIM level is  –150 dBc as the magnitude for third-order intermodulation products using two 20-watt transmitters. To achieve this low level of intermodulation, designers are focus

    Checklist of PIM-Reducing Factors

    • Avoid ferromagnetic materials.
    • Ensure sufficient plating thickness if ferromagnetic base material is used.
    • Minimize the number of joint or contact points.
    • Avoid using dissimilar metal contact.
    • Ensure sufficient separation between dissimilar metals.
    • Perform firm, complete soldering/welding or brazing of joints where possible.
    • Avoid improper solder joints as they can break and/or corrode.
    • Ensure that contact interfaces are free of debris.
    • Ensure high contact pressure.
    • Use silver-plated, soldered center pin, 7/16 DIN connectors whenever possible.
    • Specify torque values for butt joints.
    • Avoid crimp joints.
    • Avoid unsoldered braided coaxial cables and minimize flexing if used.
    • Ensure that plating solutions are not contaminated and do not use excessive “brighteners” that significantly decrease conductivity.
    • Make sure that surface finish is smoothed to 0.4 mm or better.
    • Select PC boards that use rolled, annealed copper foil rather than electro-deposit copper foil.

    Proper low passive intermodulation devices and installation procedures will help you reduce the effects of passive intermodulation in your systems.

     

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