a During the calibration process, both fceo and frep of the MLL are locked to the 10 MHz reference. The beat note between the MLL and the free-running TL is traced by an electronic spectrum analyzer (ESA) and measured by a frequency counter. A computer (PC) is used to tune the frequency f1 within the frep locking block and collect the real-time results from the ESA and the frequency counter. b Mode number root-mean-square (RMS) error with respect to averaging numbers. c Mode number calibration error after 1000 times of averaging (n = 936, M = 766,746).