Patent GB-1515850-A (GB)
Title: Methods and equipment for testing transmission lines
IPC: G01R31/11
Filed: 1974-10-09
Abstract: 1515850 Measuring coefficient of reflection LC HWA 30 Sept 1975 [9 Oct 1974 14 Nov 1974] 43699/74 and 49350/74 Heading G1U [Also in Division H4]  To locate a point of reflection in a transmission line, or alternatively to measure the coefficient of deflection of said point of reflection, a coded digital word is repeatedly transmitted down the line and reflections on the line repeatedly correlated with successively delayed replicas of the digital word. Each digital word, eg. 255 bits long, is successively transmitted 1024 times for correlation of its reflection with a corresponding train of 1024 digital words but delayed from the corresponding transmitted words by a constant delay. A correlation value is thus accumulated for said delay whereupon a second series of 1024 coded digital words are transmitted and the correlation repeated between the reflected pulses and the corresponding train but with a slightly different delay. This process is repeated until sufficient values of the delays have been used to cover the entire length of the transmission line whereby a correlation function Fig. 5 of a transmission line may be built up and displayed yielding the position of the fault by the peak of the correlation function and the coefficient of reflection by the shape of said correlation function. The output (i) Fig. 2 of a clockpulse generator A Fig. 1 and frequency divider B (ii) is fed as a driving train to an 8 stage ring counter C1 the output of the first stage thereof being fed as the driving signal for 8 stage shift register El in word generator E. Selected stages of the shift register are coupled back to the input of the register via exclusive OR gates connected in cascade E2 whereby the last stage of the register produces the desired 255 bit coded word. The start of each coded word is detected at F and produces a pulse (v) Fig. 3 which is fed to a 13 stage binary counter C3 the 10th stage thereof producing an output when 1024 pulses (v) and coded words have been transmitted. Each stage of the ring counter Cl produces a pulse train (ii), (iii) &c. Fig. 2 successively delayed by a bit duration. The output (ii) from the first stage is selected by the pulse from the 10th stage of binary counter C3 for the first group of 1024 transmitted words and the outputs of the later stages of counter C3 cause the other seven stages to be sequentially selected for the corresponding next 7 groups of 1024 transmitted words. The so selected ring counter stage outputs are fed (vii) Fig. 2 as time pulses to the reference word generator D, said generator being functionally similar to generator E. At the input to the transmission line there appears a mixture of the transmitted words and the distorted reflections thereof. To remove the transmitted words said words are inverted at K and combined with said mixture in differential amplifier L thereby leaving said echoes (ix) Fig. 4. 1024 successive echoes are then multiplied at Gl with 1024 successive reference words having a predetermined delay and the resultant integrated at G2. The next 1024 echoes are multiplied with the next group of differently delayed 1024 written words &c. A recorder or cathode ray tube display suitably synchronized with the ring counter Cl records or displays the output of the integrator G2. An exclusive OR circuit N and changeover switch G3 fed with the output of counter C3 enables out of phase interference to be removed.