Impedance Bridge Balancing Using Perturbation Theory

This paper describes a technique for separating the two signal voltages necessary for balancing an ac impedance bridge from its detector output voltage. The technique is unique in that the necessary information is derived from modulations on the generator voltage. These modulations are formed by perturbing the variable arms at a fixed frequency but phased 90° apart. The two signals are then separated by phase detectors referenced to the modulation frequency and the proper phase. The perturbation increases and decreases the arm by a small amount, comparable to the action of a human operator increasing and decreasing the variable arm slightly to see which direction appears to lead to a null. Although no attempt was made to automate a bridge, this paper discusses the theoretical considerations for deriving the proper control signals for automating a bridge as well as experimental results obtained from a working model of such a system.