we have been set a question at college to design a frequency to voltage converter. we have been trying to do this for over a month now but have been unsuccessful. any help would be appreciated

input = 20hz - 20khz
output = .25v/khz

please please please help.

edit: we also cant use dedicated IC's to do this, just basic op-amps, monostables etc.

 

How about this rather simplistic approach ...

Edge trigger a 25uS, 10volt monostable circuit with the 20-20kHz signal.

Use a LPF to extract the mean of the monostable output.

At 20Hz mean value = 10 x 25e-6 x 20 = 5mV

At 20kHz mean value = 10 x 25e-6 x 20e3 = 5V

Each output complies with the 0.25V per Khz spec.

Designing the filter is probably the more challenging part.

 

How about this rather simplistic approach ...

Edge trigger a 25uS, 10volt monostable circuit with the 20-20kHz signal.

Use a LPF to extract the mean of the monostable output.

At 20Hz mean value = 10 x 25e-6 x 20 = 5mV

At 20kHz mean value = 10 x 25e-6 x 20e3 = 5V

Each output complies with the 0.25V per Khz spec.

Designing the filter is probably the more challenging part.

I didn't do math for this circuit but this is the right one as the circuit doesn't need any input but only small trigger and the output will be fixed till you send another trigger

 

A phase locked loop will perform this function.

 

Would one HPF and one LPF in parallel do the job? You put at the end of each filter a FET and give the right voltage to the output?