GB629279A - Improvements in or relating to electrical measuring instruments - Google Patents

Abstract

629,279. Capacitance measurements. BEVERIDGE, H. N. Oct. 5, 1946, No. 29766. Convention date, Oct. 29, 1945. [Class 37] The capacitance between two terminals of a circuit element is measured by connecting them to the control-grid and anode of a valve arranged as a voltage amplifier, applying an alternating voltage to the control grid, and measuring the effective grid-cathode capacitance of the valve. A high-frequency voltage derived from a signal generator 1 is applied through a buffer amplifier 10 and a condenser 11 to the control grid of an amplifier valve 25. Between the control grid and the earthed cathode of the valve 25 are connected a resistance 13 in series with a switch 12, a calibrated variable condenser 16, a vacuum tube voltmeter 17, and an inductance coil 14 proportioned to resonate at the signal frequency for some intermediate setting of the condenser 16. The anode of the valve 25 is connected through a load resistor 24 to the positive side of a D.C. source 15. An inductance coil 27, variable condenser 29, short-circuiting switch 31, and valve voltmeter 32, are connected in parallel, one side of the circuit being earthed and the other side connected through a condenser 26 to the anode of the valve 25. The coil 27 is proportioned to resonate at the signal frequency for some median setting of the condenser 29. Small condensers 36, 38 are connected in series between the grid of the valve 25 and the condenser 29 and their junction is earthed through a low resistance 39. A radio receiver 40, tunable to the frequency of the signal generator 1, is connected across the resistor 39. The circuit element X, of which the capacitance is to be measured, is connected through terminals 33, 35 between the control grid of the valve 25 and the condenser 29. The switch 12 is closed so that the valve 10 becomes a low impedance voltage source supplying a small constant voltage to the grid of the valve 25. With the switch 31 open the condenser 29 is adjusted until a null or sharp minimum in signal strength is detected in the receiver 40, indicating that the anode circuit of the valve 25 is so adjusted that a 180 degrees phase relation exists between the plate and grid voltages. The switch 12 is then opened and the switch 31 closed, reducing the amplification of the valve 25 to zero. The condenser 16 is then tuned to resonance and the setting noted. The switch 31 is opened and the condenser 16 agains adjusted to resonance. The new setting is noted and the change of capacitance obtained by subtracting the second reading from the first: The readings of the voltmeters 32, 17 are noted and the amplification determined from their ratio. The value of the unknown capacitance is then obtained by dividing the change of capacitance by the amplification and subtracting the inherent grid-anode capacitance.