Simple scheme: lock the input signal with cd4046, control the bandwidth of the phase-locked loop, and get a more stable square wave signal which is consistent with the average frequency of the input signal. Then, the square wave signal is subjected to frequency selective amplification, filtering and other operations to obtain a sine wave signal.

Complex scheme: digital scheme, in which the input signal is sampled and phase-locked by numerical method to generate a sine wave signal (digital signal) which is consistent with the average frequency of the input signal but more stable, and is converted into an analog signal by DAC devices.