Temperature Stability of a Wave Union Time-to-Digital Converter Core Implemented in a Commercial FPGA and Its Space-Grade Equivalent for Time Tagging Applications

This paper presents a comparative evaluation of a wave union (WU) time-to-digital converter (TDC) implemented on two Microchip flash-based field-programmable gate arrays (FPGAs): the radiation-tolerant RTG4 (RT4G150-1CG) and the low-power SmartFusion2 (M2S150TS-1FCG1152). Both implementations use an identical VHDL architecture consisting of parallel tapped delay lines (TDLs) each with a WU pattern generator, edge-coded logic encoding, and real-time statistical bin width calibration. Single-shot precision (SSP), defined as the standard deviation of consecutive period measurements derived from calibrated timestamps, is evaluated across four independent input channels. Measurements are performed at five input frequencies (1, 2, 10, 20, and 40 MHz) and six ambient temperatures ranging from 20 °C to 60 °C. At a low input frequency, the RTG4 implementation achieves a mean SSP of 6.97 ps, while IGLOO2 yields 10.12 ps under identical conditions. As the input frequency increases, the SSP of both platforms decreases and converges to approximately 4.5 ps. However, at elevated temperatures, both devices experience observable degradation in SSP. To quantify thermal robustness, a thermal sensitivity coefficient (TSC) is introduced, defined as the rate of SSP variation with temperature. The results show that the same WU TDC core implemented on a space-graded FPGA exhibits improved thermal stability and reduced channel-to-channel variance compared to its equivalent on a commercial platform.