Abstract: Apparatus is described for measuring the time interval between the occurrences of an electrical signal at two different points in a circuit or electrical system. Each signal drives a receiver containing a high-Q coaxial resonator. The resonators begin to oscillate in timed relation with the reception of the associated signals. Each receiver signal is mixed with the output signal of a common local oscillator and the respective mixed signals are amplified by IF amplifiers. The phase difference between the output signals of the IF amplifiers is representative of the difference in time of reception of the signals, but on an expanded time scale. The phase difference is detected and used to gate the output signal of an oscillator into a counter circuit. Thus, the accumulated contents of the counter is representative of the time difference of reception. Circuitry is also provided for indicating the polarity of the phase shift.