A phase measurement system for measuring phase between an input signal having a frequency and a reference signal having a substantially different frequency includes: a phase comparator having an output representing a time between a crossing of a first threshold by a clock derived from the reference signal and a predetermined time along an interval from a first sample to a second sample, where the input signal crosses a second threshold on the interval; an interpolator having an output indicative of an interpolated time of the second-threshold-crossing on the interval in the input signal; and a phase calculator which computes the phase difference by combining the phase comparator output and the interpolator output.