20 ps resolution time-to-digital converter for digital storage oscilloscopes

A time-to-digital converter with 20 ps resolution has been developed to be used in digital storage oscilloscopes. It takes two independent inputs that have less than 4 ns time interval. The time interval is stretched by discharging-charging ratio with a pulse stretcher, and the stretched interval is counted with a coarse counter. The resulting count is proportional to the time interval of the two inputs. The LSB count of the TDC is 7.80 ps, and the RMS of standard deviation of the output is 14.1 ps. So the TDC can be used where less than 20 ps timing resolution is required. We have used it in a 250 MS/s data acquisition system for interpolating the 4 ns sampling clock period and verified its performance.