(a) Principle of digital calibration. The dotted line explains that Δphase2 contains the information about the phase fluctuations of Δ f ceo but is not used for extraction. (b) Process of digital calibration. LPF: lowpass filter. N₁: the coefficient of Δphase in the signal used for mixing aim to avoid frequency aliasing. N₂: the coefficient of Δphase in the signal used for resampling.