Voltage-controlled phase shifter for measuring transfer function in the presence of noise

A linear voltage-controlled phase shifter has been constructed for use in a feedback loop to measure the amplitude and phase of a signal buried in noise. The feedback circuit has a phase marker output with an accuracy of 0.0025 rad while automatically following an arbitrarily large phase shift. In addition it has one output voltage proportional to the amplitude of the signal and another proportional to its phase with automatic translation in increments of π. A voltage-controlled oscillator is also described whose frequency and amplitude are proportional to their respective control voltages. The frequency control voltage for this oscillator is also used by the phase shifter circuit to make the proportionality constants for the outputs of the feedback circuit independent of frequency. Thus the circuits described make possible the highly accurate measurement of the transfer function of a two-port system that outputs more noise than signal.