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Capacitance Measuring Apparatus

IP.com Disclosure Number: IPCOM000079797D
Original Publication Date: 1973-Sep-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 2 page(s) / 55K

Publishing Venue

IBM

Related People

Radcliffe, JK: AUTHOR

Abstract

It is sometimes necessary to measure small changes in capacitance, as for example less than 0.1pf, at radio frequencies.(RF). Such measurement may be made by methods similar to the Hartshorn and Ward method, providing a suitable detector is available. Previously, detectors for frequencies in excess of 1MHz have not been available. Apparatus has been developed to extend the measurement range and has been successfully used to detect capacitance changes as small as 0.01pf, at frequencies to 500MHz.

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Capacitance Measuring Apparatus

It is sometimes necessary to measure small changes in capacitance, as for example less than 0.1pf, at radio frequencies.(RF). Such measurement may be made by methods similar to the Hartshorn and Ward method, providing a suitable detector is available. Previously, detectors for frequencies in excess of 1MHz have not been available. Apparatus has been developed to extend the measurement range and has been successfully used to detect capacitance changes as small as 0.01pf, at frequencies to 500MHz.

The operation may be understood with reference to the schematic of Fig. 1, wherein RF power is supplied to two legs by an amplitude modulated RF source 10, a power splitter 11 and attenuators 12 and 13 for isolation. The signals are then fed through two audio frequency (AF) detectors 14 and 15. The sample leg is fed through a length of coax 16 to two parallel coupled capacitors 17 and 18. Capacitor 17 is a precision calibrated variable capacitor, located very close to the unknown capacitor 18 to minimize lead inductance. The reference leg is composed of an open-circuit terminated variable-length airline 19. The presence of the capacitors 17 and 18 on the sample leg and the open circuit on the reference leg will produce standing waves on the airlines. This will affect the RF voltage seen at the detectors 14 and 15 and, therefore, the AF output of the detectors 14 and 15. The two AF outputs are fed to the AF null detector 20.

To perform a mea...