Schematic diagram of the time interleaved self-comparison measurement method. (a) Two independent digital feedback loop servos A and B alternatively probe the clock transition. The two correction frequencies f_A and f_B are fed back to the AOM12 alternately to keep in-loop operation. (b) The timing sequence of instability measurement. Each interleaved measurement cycle contains 4 clock interrogation cycles and the period of the single clock probe cycle is 1 s.