A Digital Interpolation Technique in Time Measurement

The problem of measuring a time interval using a crystal oscillator as a time base is discussed. A statistical analysis is made of the free-running oscillator and gated counter when the time interval to be measured is repetitive. Using the equivalent circuit of a quartz crystal, the difficulties involved in using a pulsed crystal oscillator as a time base are investigated. A method is described to measure, at a random instant, the instantaneous phase of a free-running oscillator and use this information as the last significant figure in a time interval or pulse delay method. The method consists of sensing the coincidence between oppositely-traveling pulses on a delay line. A digital-delay generator was constructed using this technique. The accuracy of the delay was measured as ±0.01 us ±crystal time-base accuracy. The construction and testing of this device is discussed.