Patent GB-1271359-A (GB)
Title: Apparatus for locating faults on wires and cables
Inventor: SHALYT GERMAN MIKHAILOVICH
IPC: G01R31/11
Filed: 1970-05-21
Abstract: 1,271,359. Locating faults in cables; transistor pulse and sampling circuits. VSESOJUZNY NAUCHNO - ISSLEDOVATELSKY INSTITUT ELEKTROENERGETIKI. 21 May, 1970 [21 May, 1969], No. 24684/70. Headings H3T and H4D. Apparatus for locating faults on a cable Fig. 1, comprises a clock pulse generator triggering a control pulse unit 2 which produces a train of bipolar pulses at a first output b and unipolar pulses at a second output c; said first output being connected to the control inputs of a selector 4 and two switches 10, 12; said second output c being connected to a voltage generator 3 which produces a pulse on a first output d which on passing through said selector 4 triggers a transmission pulse and said generator 3 also supplies from a second output f a linearly rising voltage to a comparator 6 connected to the input of a strobe pulse generator 7 coupled to a first input of a modulator 8 which input is biased by the output of the second switch 12, whose inputs are connected to the terminals of two capacitors in an integrator 11, to the outputs of the first switch 10 and to a D.C. meter 13; said modulator 8 also having a second input connected to the cable and an output j connected through a pulse stretcher 9 to the inputs of the first switch 10. Power supply unit 1 converts the D.C. supply to a rectangular bipolar waveform which is transformed and rectified to produce D.C. voltage supplies for the apparatus (Fig. 2, not shown). The rectangular waveform is also supplied as the clock waveform to control unit 2 which generates bipolar pulses b (see also Fig. 5 (not shown), at half the clock frequency and unipolar pulses c at the clock frequency which are supplied to voltage generator 3. This produces a pulse d every clock period but selector 4 is only open every other period, thus generator 5 transmits a pulse during every odd period. Generator 3 also produces a sawtooth voltage which triggers a pulse from comparator 6 when it reaches the reference voltage set by potentiometer 23. This pulse generates a strobe pulse from generator 7 the timing of which relative to the transmission is dependent on the D.C. bias applied by potentiometer 23 which is calibrated in distance. The strobe pulse is multiplied by the cable 22 voltage in modulator 8 whose output is expanded and applied through switch 10 to one of the two capacitors 11 the outputs in successive clock periods being applied to alternate capacitors. These are connected to opposite terminals of meter 13, thus any noise received is cancelled. However if a strobe pulse coincides with a reflection from a cable fault, the voltage is stored only on one of the capacitors 11 (since there are no transmissions during even clock periods) and the meter deflects. The D.C. bias applied by potentiometer 23 is thus indicative of the distance of the fault along the line. The integrated voltages are fed back through switch 12 to provide negative feedback which adjusts the bias of the modulator 8 input to improve stability. Sawtooth and trigger pulse generator, Fig. 3.- The input from unit 2 opens a switch shorting capacitor C 1  which charges up through R 1  and R 2 . When the base voltage is high enough the transistor 19 turns on sending a pulse to the selector 4. Capacitor C 1  continues to charge up through R 1  and diode 18 becomes reverse biased thus isolating the transistor 19 from the capacitor and avoiding distortion of the linearly rising voltage.