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Tracking all-pass filter can basically achieve what you want

[ATTACH=full]198604[/ATTACH]

Brief desciption

- U1 forms programmable low-pass (gm-C filter)

- C3/R15 cancels DC offset

- U2 combines low-pass output with input signal forming an all-pass

- U3, U4 + XOR A1 measures phase shift, XOR output has 50 % duty cycle at 90 degree

- U5 integrates error signal

- R16/R4 reduces 2*f0 ripple that causes signal distortion at low signal frequencies

- control signal tunes low pass

Example waveform, settling for 10 kHz input:

[ATTACH=full]198606[/ATTACH]