Residual Swept Frequency Error Power Spectral Density Method for Evaluating the Performance of FMCW Signal
Qiupin Wang, Pu Ou, Yingke Xie, Chaotao He, Maorong Zhao, Ziyi Kang, Fengling Zhang, Guang-Qiong Xia, Zheng-Mao Wu · Journal of Lightwave Technology · 2025
In this work, we propose a residual swept frequency error power spectral density (RSFE-PSD) method to evaluate the performance of the frequency-modulated continuous-wave (FMCW) signal. For such a method, the RSFE-PSD is extracted from a beat signal of the same FMCW after passing through two different delayed lengths. The results demonstrate that, through combining RSFE-PSDs and its related analysis, the nonlinearity and phase noise of the FMCW signal can be simultaneously characterized in both pre-distortion and optical phase locked loop (OPLL) systems. For the pre-distortion system, it primarily improves low-frequency noise, and the low-frequency noise decreases with the increase of the order of pre-distortion iterations. For the OPLL system, the low-frequency noise within the loop bandwidth can be further suppressed. However, an inappropriate loop gain exacerbates the frequency noise, especially outside the loop bandwidth. Moreover, after calculating fast Fourier transform spectra of the beat signal (FFTS-BS), the correlation between the RSFE-PSD and FFTS-BS is revealed. The low-frequency noise in RSFE-PSD correlates with the shape of the peak in FFTS-BS, and a weak low-frequency noise in RSFE-PSD is accompanied by a sharp peak emerging in the FFTS-BS. In contrast, the high-frequency noise affects the noise floor level in FFTS-BS, and a weak high-frequency noise in RSFE-PSD corresponds to a high signal-to-noise ratio in FFTS-BS.