GB865947A - Improved apparatus for automatically measuring and recording electrical impedance values - Google Patents

Abstract

865,947. Electric selective signalling. BRITISH TELECOMMUNICATIONS RESEARCH Ltd. Feb. 2, 1960 [Feb. 3, 1959], No. 3860/59. Class 40 (1). [Also in Groups XXXV and XXXVI] In apparatus for automatically testing and recording the values of impedances and comprising an A.C. bridge provided with means, coupled to an analogue to digital converter, for continuously adjusting the variable impedance of the bridge between maximum and minimum values, a phase detector circuit in the bridge output initiates the selection of the required information from the analogue to digital converter as the bridge passes through balance, means being provided for recording the information obtained. Code disc DD, Fig. 3, coupled to a variable differential capacitor forming the variable impedance of the bridge, carries the instantaneous impedance in digital form on a pair of 5-bit binary code tracks (one bit being a parity bit for checking purposes) and phototransistors D2, D1, are energized through the code slots by a lamp LD, gates MG being opened to pass the code to stores SR in response to pulses generated in pulse generator PG when when the bridge passes through its point of balance. The generator PG receives a starting pulse from bi-stable circuit BS when the change of phase of the bridge output as it passes through balance is detected by phase detector PD. The code disc has an additional track producing clock pulses which through the interval signal circuit limits the generator pulse to an interval when the code can be read, and two sign tracks for use where the impedance changes smoothly from maximum-minimum-maximum and the bridge balances twice each revolution. The disc may be replaced by a linearly-moving coded plate and the code spacings may be non- linear to compensate for non-linearities in the capacitor. Five wire code outputs from the store are gated sequentially under the control of sequential toggles ST to a tape TP, or through a translator TR, which converts them to a potential on one of 16 wires, and gates GT to a typewriter TW. A five-digit code representing the position of a pre-set capacitor AC which constitutes the third digit in the capacitance value, and a five-digit tabulator signal are also applied to the sequence gates SG. The gates GT are opened for periods of about 30 millisecs. at 60 millisec. intervals by pulses from sequence strobe SS. A lamp LG is lit from SS on the completion of recording or in the case when balance is not achieved, a lamp in an indicator OI is lit in a manner indicating the direction of the error. A sign presence unit limits the recording to one of the positions of balance. The five wire codes from the store are converted to a ten-wire input for the translator by the use of a pair of inverter circuits. The additional capacity may be automatically adjusted by the " off scale " indicator circuit and reset of the various toggle circuits may be automatic or manual. The arrangement is said to be particularly useful for measuring the pair capacities of a quad cable for selective jointing and in such an arrangement the resetting pulse may be arranged to introduce a further impedance to be measured by stepping a selector. The oscillator input B is compared with the output A in a transistor phase-sensitive circuit, Fig. 1, pulses being applied to the bases of transistors L or M to produce a positive potential on leads U or V according to whether the output is out-of-phase or in phase with the input. A switching transistor H in the B input circuit permits only short duration comparison pulses.