Standard fan PWM frequency (no replies)

Hello everyone,

I have been looking on google for a while, what PWM frequency does the part fan run at? But I cant find answer. I am designing a closed loop speed control circuit, The circuit needs a steady input voltage, so the op-amp can do its thing. So I am adding a low pass filter before the circuit. It would be nice to know the PWM frequency so I dont have to hunt and peck to find the values for the filter.

Thanks