Phasemeter based on second harmonic signal filter for space-based gravitational wave detection

The space gravitational wave detection aims to detect gravitational waves in the mHz band in order to study supermassive black hole mergers, galaxy evolution and the structure of the early universe. One of its core payloads is a transponder-type interstellar laser interferometer, designed to measure relative displacement changes at the pico-meter level. Among its components, phasemeter is tasked with extracting the phase and frequency of the interference signal. Currently, phase-locked loop (PLL) phasemeters are commonly employed. However, the second harmonic signal generated by the mixer can restrict both the dynamic range and phase measurement accuracy of the phasemeter. This paper analyzes the interstellar laser interferometer and the impact of the second harmonic signal on the phasemeter’s performance. To address these challenges, a phasemeter incorporating a second harmonic signal filter is proposed. This new design mitigates second harmonic disturbances within the phasemeter’s bandwidth by dynamically adjusting the filter’s cutoff frequency to track the input signal frequency, thereby suppressing the second harmonic signal in real time. Theoretical and simulation analyses demonstrate that the proposed phasemeter with a second harmonic filter significantly enhances the dynamic range. Finally, experimental results verify that the phasemeter can achieve the tracking of sudden frequency changes up to 4.8 MHz.