GB566022A - Improvements in or relating to apparatus for testing cables - Google Patents

Abstract

566,022. Thermionic circuits. STANDARD TELEPHONES & CABLES, Ltd., and WEAVER, L. E. June 4, 1943, No. 8995. [Class 40 (v)] [Also in Group XXXVI] Relates to apparatus for locating and detecting impedance irregularities in transmission lines by sending pulses into the line and indicating the time-lag of the reflected pulses as described in Specification 551,818, [Group XXXVI) ; and comprises improvements designed to square up the shape of the reflected pulse ; to give equality of height on the pulse indicator no matter at what distance the impedance irregularity may be situated ; and to mark an accurate time scale on the indicating oscillograph. A pulse generator PG1, Fig. 1, controlled by an oscillation generator CO sends a train of short square pulses into the line L which is bridged by an impedance-matching network CN. The transmitted pulses, together with any reflected pulses that may arise, pass through an attenuator VN and amplifier EA to one pair of plates VP of an oscillograph CRT. The other pair of plates HP operate to provide a time base, being fed from the oscillator CO through a phase-shifter PS1. The gain of the amplifier EA continually increases between the sending of each pulse and the receipt of a reflected pulse from the remotest point on the line, this gain variation being effected by an element CVG giving an approximately sawtooth output voltage synchronized through a phase-shifter PS2 from the oscillator CO. In this way, reflected pulses as seen on the screen of CRT are all of substantially the same height. The element CVG may comprise a condenserrelaxation oscillator, Fig. 6 (not shown), arranged to give an expotential output voltage, Fig. 5; or the curve of Fig. 5 may be obtained by superposing on a linear saw-tooth voltage a voltage derived by full-wave rectification of a sine-wave of half the pulse frequency, Figs. 8 and 9 (not shown). In another arrangement, the element CVG is omitted, and the amplifier EA includes the valve circuit shown in Fig. 12, in which the pulses are applied to the grid through a time-constant network C1, R1 of.the same order as the pulse period. The initial pulse on the grid causes grid current, giving a negative charge to the grid which leaks away during the inter-pulse period and causes the gain of the valve to rise. The amplifier EA may comprise a forward amplifier V5, V6 provided with a negative feed-back path V7, V8, Fig. 10, the control voltage for regulating the gain being applied at F to suppressor grids in the feed-back valves V7, V8. The networks N1, N2 in the anode circuits of the forward amplifiers may be simple resistances if it is desired to regulate only the amplitude of the reflected pulses. If it is desired to square up the pulses by correcting for loss of the higher frequency components, the networks N1, N2 are designed to give this effect. Alternatively, the correcting networks may be in the anode circuits of the valves V7, V8. The time-base of the oscillograph may be calibrated by superposing a high harmonic of the pulse frequency on the vertical plates VP, Fig. 1, so as to produce a trace as shown in Fig. 4, enabling graduations in microseconds to be marked on the screen. This harmonic may be selected by a selective circuit TFS from the pulse sequence generated by PG1 ; or the timing wave may be generated by applying the pulses to a tuned circuit or a crystal, Fig. 3 (not shown). If it is desired to examine one particular reflected pulse, instead of the whole series which are visible when there are numerous impedance irregularities in the line, the beam of the cathode-ray tube may be cut off, except at the desired interval, by squaretopped pulses derived through a phase-shifter from the oscillator CO, Fig. 13 (not shown). The pulse amplifier EA may then have its gain adjusted by hand instead of automatically, a series of networks being provided and selected by switching according to the distance along the line of the impedance irregularity under investigation.