If all you need to do is set a maximum frequency that is allowable to pass, you could design a low pass filter circuit to control the maximum frequecy that pass. This is just a combination of a capacitor and a resistor, but they must be the correct value to get the right cutoff frequency. Now after this cutoff frequency, I suppose you want the output to stay at the max value that is under the cutoff. I'm not sure how you'd design that, but it should not be too difficuilt with some research.

cutoff frequency: fc = 1/(2*pi*R*C) i think.

Jon