Terminator (electrical)

In electronics, electrical termination is the practice of ending a transmission line with a device that matches the characteristic impedance of the line. This is intended to prevent signals from reflecting off the end of the transmission line. Reflections at the ends of unterminated transmission lines cause distortion which can produce ambiguous digital signal levels and mis-operation of digital systems. Reflections in analog signal systems cause such effects as video ghosting, or power loss in radio transmitter transmission lines.

Signal termination often requires the installation of a terminator at the beginning and end of a wire or cable to prevent an RF signal from being reflected back from each end, causing interference, or power loss. The terminator is usually placed at the end of a transmission line or daisy chain bus (such as in SCSI), and is designed to match the AC impedance of the cable and hence minimize signal reflections, and power losses. Less commonly, a terminator is also placed at the driving end of the wire or cable, if not already part of the signal-generating equipment.

Radio frequency currents tend to reflect from discontinuities in the cable such as connectors and joints, and travel back down the cable toward the source causing interference as primary reflections. Secondary reflections can also occur at the cable start, allowing interference to persist as repeated echoes of old data. These reflections also act as bottlenecks, preventing the signal power from reaching the destination.

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