A frequency counter counts the number of cycles of a signal within a given time window. It calculates the frequency of the signal based on the number of counts and gating time.
Moku:Lab phasemeter implements a digital phase-locked loop (PLL) structure. It calculates the frequency based on the initial or previous frequency of a local oscillator (LO), and the real-time frequency difference between the input signal and the LO. It is able to track the frequency, phase, and amplitude of the signal with zero dead time. On the other hand, the PLL has a finite tracking bandwidth of up to 10 kHz. The PLL may fail if the input signal has significantly higher bandwidth.