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Thanks for improvements. Bottleneck is limited OTA performance, for larger frequency range I would consider precision analog multiplier. C3/R15 highpass doesn't affect circuit performance if I understand right, low pass path isn't required to realize a specific gain. High pass cancels frequency dependant OTA offset, increasing time constant also increases settling time.

I was thinking about a full featured SOGI (second order generalized integrator) FLL in the first place, it also uses a 90 degree detector for tracking control. We are using an all digital version of this filter for grid tied inverter reference signal generation. Then I found that simpler tuned all-pass already does the trick.

For reference, see SOGI-FLL diagram, copied from Theodoresco et al, Grid Converters for Photovoltaic and Wind Power Systems qv' is quadrature (90 degree) output

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