A high-precision time-to-digital converter for pulsed time-of-flight laser radar applications

A time-to-digital converter (TDC) has been designed, and six units have been constructed and tested. It consists of integrated digital time-interval measurement electronics with a +/-10-ns resolution that can be improved to +/-10 ps by an analog interpolation method. Identical construction of the interpolation electronics leads to stable performance of the TDC, The drifts of all six TDC units remain within +/-10 ps over a temperature range from -10 degrees C to +50 degrees C. The stability can be improved further by a real-time calibration procedure developed here. The single-shot precision of the TDC is better than 15 ps (standard deviation), but precision can be improved to below 0.1 ps by averaging about 10 000 measurements at the maximum measurement speed of 100 kHz, The time range of the TDC is currently 2.55 mu s, but this can be increased by adding an external digital counter. The TDC suffers from a periodic nonlinearity over the measured time range from 0 to 1.3 mu s, the amplitude and period of the nonlinearity being +/-20 ps and 40 ns, respectively. The reason lies in the integrated digital part of the electronics. The linearity of interpolation in the 10-ns time range improves, however, as more results are averaged.