Accurate time interval measurement electronics for pulsed time of flight laser radar

A time-to-digital converter for pulsed time-offlight laser radar devices has been developed and six units have been constructed and tested thoroughly. The time interval between the start and stop pulses is digitized coarsely by counting the 100 MHz clock oscillator pulses. The +/-10 ns uncertainty is improved to +/-10 ps by means of an analogue interpolation method based on the discharge of a known capacitance by a constant current. The TDC is very stable due to the symmetrical construction of the interpolation unit, which cancels out the drifts of the two time-to-amplitude converters. The measured drift of all six TDCs is less than +/10 ps in a temperature range of –10°C +50°C, and some units have even zero drift. The single-shot precision of a TDC is between 6 and 12 ps, and can be improved by averaging successive single-shot measurement results, and about 70 fs precision can be obtained if about 65000 results are averaged. The linearity of the TDC is better than +/-20 ps in a measurement range of 0 – 1.3 μs.