The most critical technique of the phase-modulated continuous-wave (PhMCW) ranging method is to measure the pulse width of the intermediate frequency (IF) signal to obtain the optical time-of-flight. Time-to-digital converter (TDC) is used to measure the time interval (i.e., pulse width). The range and accuracy of the time interval measurement by a high-precision TDC module essentially determine the range and accuracy of the PhMCW lidar. The aim of this paper is to develop a large-range and high-precision TDC chip for time-of-flight measurement to support the development of high-performance PhMCW lidar for its application in the field of autonomous driving.

In this paper, we utilize Xilinx's Artix-7 series field programmable gate array (FPGA) chips to implement the TDC module design through the strict counting chain technique by utilizing the on-chip CARRY4 carry chain as the fundamental delay unit. This approach allows an expansion of the time measurement range by increasing only the number of bits in the first counter, achieving higher precision while utilizing fewer resources. The performance of the TDC module is tested by generating gate signals of varying lengths from the signal source, followed by experimental testing and data analysis. Finally, an actual lidar system is constructed for experimental demonstration.

Using the signal source to generate the measured signals with different pulse widths for practical testing, a time measurement range of 1.24 μs is achieved. The optimal value of measurement accuracy is 26.42 ps, corresponding to a ranging accuracy of 3.96 mm (Figs. 7 and 8), which is better than the existing commercial lidar metrics (50 mm). In order to further analyze the correlation factors of the measurement accuracy, we take the 200 ns pulse width measurement data as an example in the frequency domain for analysis, and find that the TDC test results are affected by the switching power supply noise (Figs. 9 and 10). A PhMCW lidar system is built for application verification, and the time-of-flight detection for the distance of 0.3?7 m is realized (Fig. 12).

In this paper, for the urgent need of high-precision TDC for PhMCW ranging, we adopt the strict counting chain method and realize the TDC module design based on FPGA development board. Using this TDC module, the time measurement range of 1.24 μs is realized, corresponding to a ranging range of 186 m, which can meet the demand of automatic driving for large-range detection. Using the signal source to generate the measured signals with different pulse widths for practical testing, a measurement accuracy of better than 133.62 ps is obtained, corresponding to a distance measurement accuracy of 20.04 mm, which meets the needs of automatic driving for high-precision detection. However, when the TDC module is demonstrated in a real lidar system, the analysis reveals that the commercial amplifier module currently used has a large impact on the test results. This problem will be solved by optimizing the design of the amplifier module, so as to obtain a high-precision and long-range PhMCW lidar system.