Patent GB-974060-A (GB)
Title: Improvements in or relating to testing apparatus for electric cables
IPC: G01R31/11
Filed: 1962-12-19
Abstract: 974,060. Radio echo pulse cable testing. SIEMENS & HALSKE A.G. Dec. 19, 1962 [Dec. 20, 1961], No. 48064/62. Heading H4D. An electric cable testing apparatus includes a pulse train generator for supplying test pulses to a cable and a receiver for the reflected pulses of the test pulse trains, the receiver including a variable gain device, wherein a sampling pulse train samples the reflected pulses at the output of the receiver at various positions in time between test pulses, and wherein the gain of the variable gain device is controlled in dependence upon the position in time of each sampling pulse between the test pulses. In the arrangement shown a pulse generator 1 synchronizes the transmission of pulses by a transmitter 2 to the cable under test X, via a balanced transmit/receiver bridge 3, and the echoes of these pulses are amplified in amplifier 5 and passed to an equalizer 6. The generator 1 also synchronizes the production of a train of sawtooth waves by a generator 12, the beginning of each sawtooth being delayed from the corresponding transmitted pulse by a variable delay circuit 11. A second pulse generator 15 produces pulses at a much slower rate than generator 11, which synchronize the production of long sawtooth waves in generator 14. The two trains of sawtooth waves are fed to a voltage comparator 13, which produces a pulse each time the signal from generator 12 becomes equal to that from generator 14. The resulting sampling pulses are increasingly out of phase with their respective transmission pulses, and sweep through the chosen part of the transmission period. The equalizer 6 may be in the form of a variable gain amplifier having an onlinear frequency response controlled by the sawtooth output from generator 14, on an adjustable four-terminal attenuation network, such that a received reflection pulse, caused by a fault in the cable at a distance corresponding to the position of the pulse from comparators 13, and deformed due to the frequency varying attenuation of the cable, is restored to its original shape at the output of the equalizer. The sampling pulses open a gating means 7 to let through any such equalized reflection pulses, to a band pass filter 8, where they are broadened out for display purposes. The output of the filter 8 is thus asimulation of the reflection signal received from the cable but at the much reduced repetition frequency of the generator 15, and with the distortion of the echo signals removed. The said output is fed via an amplifier 9 to the vertical deflector of a two co-ordinate recorder 10, whilst the horizontal deflection is controlled by the sawtooth wave from generator 14. The recorder thus shows the undistorted reflection signal from the cable. The delay introduced by delay circuit 11 determines how far down the cable the investigation will commence, whilst the duration of the sawtooth from generator 14 determines the length of cable investigated. The cable may be investigated in sections by varying the delay introduced by circuit 11, in steps, at the end of each sampling sweep, by pulses from generator 15. Generator 15 also feeds a generator 17, to produce a stepped voltage for application to the vertical deflector of the recorder to separate out each section into a separate line on the recorder and may also adjust the equalizer 6 in steps. The sections may overlap or have gaps inbetween them, depending on the duration of the sawtooth wave from generator 14. A marker pulse is produced by comparing a voltage produced by generator 19 with the sawtooth voltage from generator 12 in a comparator 20, and feeding the resultant pulse to the vertical deflector of the recorder. A long persistance C.R.T. 18 may be used in addition to the recorder.