GB1382087A - Method and apparatus for testing transmission lines and channels - Google Patents

Abstract

1382087 Measuring variation of attenuation with frequency POST OFFICE 29 March 1973 [30 March 1972] 15176/72 Heading G1U The attenuation caused in a transmission line by multiple even order reflections - which arise from local irregularities in the line - is determined by applying a burst of a carrier frequency to the line, thereby causing at the output end of the line a resulting signal which is notionally divided into two portions, namely a primary signal being that part of the resulting signal that coincides in time with the applied carrier burst, and a secondary signal, being that part of the resulting signal that starts at the end of the carrier burst, and by measuring the ratio of the amplitudes of the secondary and primary signals. As described bursts of a swept frequency source 4, Fig.2, are gated 5 to a line 1 under test, the bursts being sufficiently short in relation to the sweep frequency that each can be regarded as being of a constant frequency. The output from the line is passed via an equalizer 7, a calibrating attenuator 8, an amplifier 9, a sampling circuit 10 controlled to sample either the primary output signal or the secondary output signal, a detector and an x-y plotter which also receives a control signal synchronous with the sweep control. In use first a variable delay 11 is set so that the primary output signal is sampled and a complete sweep is made. Then the delay is set so that the secondary output signal is sampled and a second complete sweep is made. Since the secondary output signal is considerably smaller than the primary, the calibrating attenuator 8 is also adjusted for this sweep to bring the secondary signal into a more convenient range. Finally, the two output plots are utilized as the basis of a series of calculations, from which the variation of attenuation with frequency is determined.