The Hewlett-Packard Model 5370B Universal Time Interval (T.l.) Counter is capable of making single-shot T.l. measurements with ±20 ps resolution. It uses a phase-locked vernier interpolating technique in which the interpolating oscillators are locked to the time base thus retaining its basic accuracy at all times. The technique also allows positive, zero, and negative time interval measurements, and a resident microprocessor extends the usefulness of the instrument by offering statistical data such as mean, standard deviation, max, min, etc., for repetitive time intervals. Other features include push-button user-defined time interval reference for systematic error cancellation; "hysteresis" in arming circuitry eliminates possible random fluctuations between + and - measurements in repetitive time intervals. In addition to time interval, high resolution frequency and period measurements can be made with gates from one period to 1 second. Both time and event information are provided for interrogating complex waveforms. " The HP 5370B has a sensitive high-speed input amplifier with digital trigger level set and a precision quartz crystal oscillator for accurate long T.l. measurements.