The Video Return-Loss Bridge

Abstract

The accurate measurement of impedance errors which cause television signal degradation is important in distribution systems of broadcast studios and network facilities. Methods of measurement which use impedance bridges or other devices are generally time consuming and no one device will yield complete data over the whole video-frequency spectrum. An alternative technique which has become popular in other countries is to measure impedance error in terms of return loss. With respect to transmission lines, return-loss is defined as the ratio of the incident signal to the reflected signal expressed in decibels. The return-loss concept may also be extended to measure impedance errors of two terminal devices. A new version of a return-loss bridge uses a high-gain, wide-band oscilloscope differential preamplifier as the error detector. The characteristics of this bridge are: (a) passive components, (b) extremely simple construction and (c) great sensitivity and bandwidth. With this bridge a single measurement can cover the complete video spectrum. Return-loss errors of 60 dB can be measured over the bandwidth of 0 to 8 Mhz.