GB720151A - Improvements relating to impedance measuring devices - Google Patents

Abstract

720,151. Capacitance measurement; detecting changes in the proximity or nature of surrounding objects. FIELDEN (ELECTRONICS), Ltd. Sept. 17,1953 [June 18, 1952], No. 15320/52. Class 37. The impedance to be measured causes a difference voltage to appear across two rectifier circuits whereupon a motor responsively to this voltage drives a variable impedance in one of the rectifier circuits until the difference voltage is zero. The extent to which the variable impedance is adjusted is a measure of the unknown impedance. The rectifier circuits are fed with low-frequency (e.g. mains) modulated oscillations. The invention is described with reference to capacitance measurement. In the embodiment shown pick-up coils L2, L3 receive from a Hartley oscillator A radiofrequency oscillations modulated at mains or supply frequency and L2 is connected to a capacity electrode by a screened lead B. The coils L2, L3 respectively feed series-connected junction-earthed load resistors R1, R1A through rectifiers X1-X3 and X4-X6 preceded by phase-adjusting combinations L4, C5 and L5, C6 and variable capacitors CV1 and CV2, CV3. The R.F. carrier is filtered out by C1, C2 and normally equal and opposite voltages demodulated at supply frequency appear across R1, R1A. The capacity of the electrode varies with changes in the proximity or the nature of the surrounding objects and CV1 is scaled in appropriate units. To condition the apparatus for use, CV1 is set to maximum capacity with the test electrode at minimum capacity and CV2, CV3 giving coarse and fine control respectively are adjusted until zero voltage exists across the outers of R1, R1A. If the electrode capacity increases the R1 voltage exceeds that of R1A and the difference via transformer/amplifier/transformer T1/D/T2 operates reversible motor M and the latter adjusts CV1 to zeroize the difference voltage. The motor M is damped by D.C. drawn from rectifier E. The apparatus may be used for measuring the capacity of a test capacitor connected across CV1.