Design and experimental verification of automatic relocking technology for phasemeter in space laser interferometry
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摘要:
面向空间激光干涉的相位计，当锁相环路发生失锁时，现阶段普遍采用FFT（Fast Fourier Transform，快速傅里叶变换）测频法重新完成信号捕获，该方法存在测频精度偏低（100 Hz量级）、重锁耗时较长（约7 ms）等技术问题。本文提出一种与FFT协同部署的自动重锁技术，该技术采用瞬时频率值与频率变化率相结合的失锁检测策略，同时选取环路滤波器原始数据与CIC（Cascaded Integrator-Comb，级联积分器梳状滤波器）降采样数据两类数据源完成失锁判断，失锁发生后通过复位操作清除积分误差，并接收频率预测算法输出的预测值。该频率预测算法针对周期信号采用波形生成算法，针对非周期信号采用二阶多项式预测算法，同时结合插值技术生成对应的频率预测值。该自动重锁技术与FFT采用并行部署的方式且形成明确的功能分工，其中该技术依托信号自身的规律性开展频率预测，负责处理所有规律信号的失锁场景（无论失锁时长）以及短时（<1 s）非规律信号的快速重锁，FFT则负责处理非规律信号以及长时复杂失锁场景下的信号重新捕获，二者形成优势互补的工作模式。实验验证结果表明，在规律信号失锁的场景下，本研究提出的算法平均重锁时间为32 μs，最大重锁时间为60 μs，相较于FFT方法提升两个数量级，且重锁速度与失锁时长无关联，即在失锁时长达到10 s时仍能保持数十微秒量级的重锁速度，同时在−10~10 dB的信噪比范围内，频率估计误差稳定在10 Hz以下，即使信噪比低至−10 dB时仍可实现稳定锁定。这种与FFT协同部署的架构在保留FFT宽频捕获能力的基础上，显著提升了规律信号场景下的快速重锁能力，为空间引力波探测任务提供了高精度、快响应、强稳定性的相位测量技术支撑。
Abstract: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.
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图 1 DPLL相位计原理框图
Figure 1. Schematic diagram of a DPLL phase meter (PD: Phase Detector, NCO: Numerically Controlled Oscillator, PI: Proportional-Integral Controller, ADC: Analog-to-Digital Converter, PIR: Phase Increment Register, PA: Phase Accumulator, LUT: Look-Up Table, Q: Quadrature signal, sin: Sine wave)
图 5 频率估计误差随信噪比变化散点图
Figure 5. Scatter plot of frequency estimation error varying with signal-to-noise ratio
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