Time-to-digital converter of very high pulse stretching ratio for digital storage oscilloscopes

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Abstract
A time-to-digital converter (TDC) for digital storage oscilloscopes has been developed. The TDC utilizes an analog interpolation scheme named a pulse stretcher of very high stretching ratio of about 5000 to achieve fine timing resolution. With the high stretching ratio, high single-shot accuracy can be achieved with a relatively low counting clock frequency. The start and stop inputs of the TDC are the trigger pulse and the 250 MHz sampling clock respectively, so they have at most a 4 ns time interval. The time interval between two inputs is stretched by a pulse stretcher with a high discharging–charging ratio, and the stretched interval is counted with a coarse counter. The resulting count is the output of the TDC which is proportional to the time interval between the two inputs. The least-significant-bit step and the root-mean-squared deviation of the TDC output were measured to be 7.80 and 14.1 ps. The TDC was used in a 250 MS/s data acquisition system for interpolating the 4 ns sampling clock period and its interpolation capability was verified.

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