Patent GB-692894-A (GB)
Title: Improvements in electric measuring apparatus of the bridge type working with alternating currents
Inventor: CLARKE WALTER WILSON HUGH
IPC: G01R27/32
Filed: 1951-03-09
Abstract: 692,894. Bridge circuits. BRITISH INSULATED CALLENDER'S CABLES, Ltd. March 7, 1952 [March 9, 1951], No. 5772/51. Class 37. In a bridge for comparing the response of a circuit element with that of a standard to different intensities of an A.C. influence, the element and standard are submitted in the bridge to cyclically varying intensity of such influence, the bridge output is fed to one deflecting system of a cathode-ray tube and a source of variable voltage is connected to the other deflecting system, the voltage variation being controllable simultaneously with and in definite relation to the variation of the A.C. influence so that the bridge output appears on the screen as a panoramic display along a base-line. The bridge shown is for testing a coaxial cable pair with A.C. of constant voltage at a continuously varying frequency and consists of two equal resistance arms R, an arm containing a model or simulator S with which the cable is intended to conform at all points of the test range, and an arm containing the coaxial pair cable C under test terminated by a simulator S1. The two simulators are identical and each comprises a network designed to represent over the required range of frequency an infinite length of cable of the kind being tested. The junction 1 of the arms R is connected through a cathode follower CF1 and an amplifier A1 to the upper plate 2 of the vertical pair of deflecting plates of a cathode-ray tube CRT. The junction 3, between the simulator S and the cable C, and the other deflecting plate 4 of the vertical pair are connected to earth. The A.C. supply is applied to the other diagonal points 5, 6 of the bridge through a transformer, the primary winding 8 of which is fed from the final cathode follower CF2 in a mixing and amplifying chain which also includes a mixer M1, filter F1, and amplifier A2. This chain combines the output from a variable frequency oscillator O and a fixed frequency oscillator O1. The variable condenser of the oscillator O is driven from a motor 9 through a shaft 10 and continuously varies the frequency between a minimum and a maximum value. The horizontal line base is produced by a time base valve generator T which produces a saw-tooth voltage wave across the plates 11, 12. This wave is synchronized with the frequency variation in the oscillator O by means of a cam 13 on the driving shaft 10 operating micro-switches. The variable frequency oscillator O is connected to a mixer M2 through a filter F2. Between the mixer M2 and the cathode follower CF1 is a filter F3. The output of the mixer M2 is at a fixed frequency, which is reduced before feeding to the amplifier A1 by an amplifier A3 and a final mixer M3 and a second fixed frequency local oscillator 02. The variable and fixed oscillators O, O1 generate frequencies which, when combined in the mixer M1, produce the required range over which the cable C is to be tested. Shields 15, 16 are connected together and to the output terminal 1 of the bridge. A second screen 17 is connected to the terminal 1 through a cathode follower CF3. A third screen 18, which is earthed, surrounds the whole and has a branch 19 passing between the transformer windings 7, 8. The linearlyvarying potential for obtaining the horizontal line base may alternatively be provided by means of a potentiometer across a battery.