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Terminatingresistorsareresistorsat the ends of thecablewhich correspond to theimpedanceof thecableand terminate it with the correct impedance. Due to the impedance-correct termination, the waves transmitted on thetransmission mediumrun out to infinity and cannot be reflected. Not even partially.

To avoid reflectionsat the ends of transmission media, injunction boxesor in activequadripoles, these must be terminated with the intrinsic impedance of the transmission medium. For this purpose, there are terminating resistors, terminating impedances orbus terminations. Technically, a termination is an input port which is determined by thescattering parameterS11=0 by definition. It represents the input reflection factor with matched output and should terminate the waves on the cable without reflections.

F terminating resistor with 75 Ohm, photo:

F terminating resistor with 75 Ohm, photo:

A terminating resistor has the impedance of the transmission medium and ensures that no reflections occur at the end of the cable. For example, if anantenna cablehas an impedance of75 ohms, then the terminating resistor must also have 75 ohms. If the impedance of the termination differs from the cable impedance, then parts of the transmitted oscillation are reflected and run in the opposite direction in the transmission medium back to the feed point.Standing wavesare generated which impair the transmitted signal. The reflection behaviour is expressed in thereturn loss. A return loss of 20dBcorresponds to areflection factorof 0.1 of the signal amplitude.

With digital signals, no visible reflections occur if the terminating resistor is incorrect or faulty, but the possible cable length is reduced.

Inmeasurement technology, problems can occur with low-impedance input impedances ofthemeasuring instruments. However, since most multimeters,oscilloscopesand voltmeters have ahighimpedance, this also compensates for the impedance of the measuring cable.

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Englisch: terminator
Updated at: 08.07.2019
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