Design and experimental verification of automatic relocking technology for phasemeter in space laser interferometry

This paper studies the phase meter applied to space laser interferometry. The phase-locked loop will suffer from lock loss in actual operation. Researchers commonly adopt the FFT frequency measurement method to re-acquire the signal at the present stage. This method has obvious technical defects. Its frequency measurement accuracy is low at the order of 100 Hz, and the relocking time is long about 7 ms. This paper proposes an automatic relocking technique deployed in collaboration with FFT. This technique adopts a lock-loss detection strategy that combines instantaneous frequency values and frequency change rates. It selects two data sources to judge lock loss, including the original data of the loop filter and the down-sampled data of CIC. It clears the integration error through the reset operation after lock loss occurs, and it receives the predicted value output by the frequency prediction algorithm. The frequency prediction algorithm uses the waveform generation algorithm for periodic signals. It uses the second-order polynomial prediction algorithm for aperiodic signals. It also combines interpolation technology to generate the corresponding frequency predicted value. The automatic relocking technique and FFT are deployed in parallel, and they form a clear functional division. This technique performs frequency prediction based on the inherent regularity of the signal. It deals with lock-loss scenarios of all regular signals regardless of the lock-loss duration. It also realizes fast relocking of short-time irregular signals within 1 s. FFT is responsible for signal re-acquisition in irregular signal scenarios and long-time complex lock-loss scenarios. The two methods form a working mode with complementary advantages. Experimental verification results show that the algorithm proposed in this study has an average relocking time of 32 μs and a maximum relocking time of 60 μs in the scenario of regular signal lock loss. The performance is improved by two orders of magnitude compared with the FFT method. The relocking speed has no correlation with the lock-loss duration. It can still maintain the relocking speed at the order of tens of microseconds when the lock-loss duration reaches 10 s. The frequency estimation error is stably controlled below 10 Hz in the signal-to-noise ratio range from −10 dB to 10 dB. The system can still achieve stable locking even when the signal-to-noise ratio is as low as −10 dB. This architecture deployed in collaboration with FFT retains the wide-band acquisition capability of FFT. It significantly improves the fast relocking capability in regular signal scenarios. It provides high-precision, fast-response and high-stability phase measurement technical support for space gravitational wave detection missions.