A double phase-sensitive detector for bridge balancing

The general considerations involved in the design of low-frequency phase-sensitive detectors are reviewed. These include the equivalence of multiplier- and gating-type detectors, of pre- and postdetection filtering with regard to both signal and noise, and of half- and full-wave detection. The various configurations of full-wave chopper circuits using similar or complementary bipolar transistors are compared from the point of view of offsets. A question that has not been previously discussed-the degree of prefiltering needed to avoid noise saturation-is analyzed. These principles are applied to the design of a fixed-frequency detector that is especially adapted to bridge-balancing applications.