GB671133A - Improvements in and relating to capacity-meters for measurement of low electric capacitance values - Google Patents

Abstract

671,133. Capacitance measurements. COMPAGNIE . DES FREINS & SIGNAUX WESTINGHOUSE. Jan. 30, 1950 [Jan. 29, 1949], No. 2370/50. Class 37. An instrument for measuring low values of capacitance consists of a high frequency input circuit, a measuring circuit to which the high frequency input is applied and including the capacitance to be measured and a calibrated variable measuring element, a local oscillator providing a low frequency output, a mixer fed with the high frequency output from the measuring circuit and with the low frequency output of the oscillator, and means for indicating the resulting medium frequency output from the mixer as an indication of the value of the capacitance to be measured. The capacitance to be measured is that of a semiconductive rectifier 1, which is connected in parallel with a measuring circuit including an inductor 2 and a calibrated variable capacitor 3. The measuring circuit is coupled with an input circuit 4 having a high frequency of the order of 60 m.c., and through an inductor 5 and, variable capacitor 6 with the control grid 7 of a mixer pentode 8 in which the portion beyond the grid 7 operates as a local oscillator. The local oscillator circuit 9 is connected with the plate 10 and grids 11, 12 and in series with an inductor 13, resistance 14, and capacitors 15 to earth. The cathode 16 is connected to a medium frequency circuit 17 including an adjustable capacitor 18, the output being applied to an amplifier 20 and a diode 21, the rectified output of which is measured by a microammeter 22. The rectifier 1 is connected to earth through a capacitor 23 so as to permit the application of a bias voltage through a potentiometer 24, a milliammeter 26 and filter 27 being connected as shown. A voltmeter 28 is connected across one section of the potentiometer 24. A buffer circuit 29 is coupled with the circuit of the control grid 7 to prevent any charge from the local oscillator from being applied to the rectifier 1. With the rectifier 1 in position the pentode 8 and amplifier 20 are connected and the buffer circuit 29 is tuned to the local oscillator frequency in order to bring the rectified current to a minimum, as indicated by the milliammeter 26. The high frequency supply is then connected through the input circuit 4 and is so tuned by the capacitor 6 that on mixing with the local oscillator frequency a medium frequency output is produced such that the microammeter 22 gives maximum deflection. The rectifier 1 is then removed and the measuring circuit 2, 3 is tuned to the same resonance value as the mixer circuit in order to obtain maximum deflection of the microammeter 22, i.e. to obtain a reference point for the capacitance to be measured. The rectifier 1 is then replaced and the potentiometer 24 is adjusted to obtain any desired point of the rectifier characteristic. The capacitor 3 is then adjusted to give maximum deflection of the microammeter 22 and the capacitance is read on the dial of the variable capacitor 3.