A Varactor Null Detector for Aduio Frequency Capacitance Bridges

A two-varactor, double-sideband up-converter pumped at 30 MHz has been constructed. Operated at room temperature with a signal frequency of 104 rad/s, the device has an optimum source resistance of 105 ohms and a minimum noise figure of 0.01 dB. Immersed in liquid N2, minimum noise figures below 0.001 dB referred to a room temperature source have been measured. The device is particularly useful as a null detector for audio frequency capacitance bridges. At 104 rad/s, a signal current of 10-14 amperes through a capacitance of 1000 pF can be detected in less than one second with this instrument. Techniques for suppressing microphonics and other extraneous sources of noise are described.