Measuring The Passive Intermodulation Performance Of RF Cable Assemblies

By Brad Deats, Director of Engineering, and
Rick Hartman, Director of Sales & Marketing,
Summitek Instruments, Inc.

As an increasing volume of voice and data information must pass through a fixed bandwidth in wireless communication systems, passive intermodulation distortion has become one factor which limits system capacity. Just as in active devices, passive intermodulation (IM) occurs when signals at two or more frequencies mix together in a non-linear fashion to produce spurious signals. When these spurious IM signals fall within the receive (uplink) band of a base station, receiver desensitization can occur. This can degrades call quality, or degrade the system C/I thus reducing the capacity of the communications system.

Passive IM is caused by a number of factors. A few of these include poor mechanical contact, ferrous content of conductors in the RF path, and contamination of the RF conducting surfaces. As it is difficult to predict the exact level of passive IM in a device, measured data is commonly used to characterize devices. Because IM performance can vary significantly with only minor changes in construction technique, some manufacturers are utilizing 100% production inspection of RF devices used in base station applications to ensure the passive IM levels are within specification.

Every component and subsystem located in the high power transmit path of a base station generates IM distortion when two or more frequencies are present. This article focuses on just one such component: cable assemblies. Understanding that IM distortion generated within cable assemblies is both directional and frequency dependent is an important factor in the specification and use of cable assemblies for communication base stations. Understanding that IM distortion generated within cable assemblies is both directional and frequency dependent is an important factor in the specification and use of cable assemblies for communication base stations.

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