An FPGA-Integrated Time-to-Digital Converter Based on Two-Stage Pulse Shrinking

We present the design and test results of a new time-to-digital converter based on the cyclic pulse shrinking method and implemented in a field-programmable gate array (FPGA) device. The pulse shrinking is realized in a loop containing two complementary delay lines. The first line shrinks, and the second line stretches the duration time of a circulating pulse; hence, the length ratio of the lines determines the pulse-shrinking capability of the converter. This resolution control mechanism is different from the bias adjustment typically used in complementary metal-oxide-semiconductor (CMOS) application-specific integrated circuit (ASIC) pulse-shrinking elements. Two forms of representation of a measured time interval (the pulse width and the time interval between two pulses) are utilized in the described converter to improve the efficiency of resolution control. To diminish the jitter of the edges of a circulating pulse and, consequently, to increase the precision of the converter, a two-stage conversion is introduced. The first stage, having a low resolution, rapidly shortens the measured pulse, thus limiting the number of cycles, whereas the second stage provides a final high resolution within a narrow time interval range. The optimal resolution of the first conversion stage is theoretically derived. Mostly, the same logical resources of the FPGA device are utilized in both conversion stages; thus, the overall area overhead of the converter is reduced. The converter has a resolution of 42 ps and a measurement uncertainty of below 56 ps within the measurement range of 11.5 ns. The converter has been implemented in a general-purpose reprogrammable device Spartan-3 (Xilinx).