A high-performance digital phase comparator

A digital phase comparator has been developed that is especially well suited to making precision phase measurements at high frequency. The circuit consists of two standard phase comparators that are interconnected so that their outputs are averaged to give the desired phase measurement. A clipping circuit consisting of back-to-back Schottky diodes is used to square up the output pulses, which improves the linearity. A variable-modulus digital prescaler is used to change the range from ±1 fringe to ±32 fringes. Instruments using this circuitry are being used to measure transient phase signals from a microwave interferometer that measures the plasma line density on the magnetic-fusion tandem mirror experiment. The resolution is less than 0.1 degree at a carrier frequency of 40 MHz with a maximum nonlinearity of ~0.5%.