A fairly common frequency to voltage converter, often called a "pulse
counting FM demodulator" consists of a monostable multivibrator followed
by a low pass filter. Since the DC component of the output of the LPF is
proportional to the duty cycle of its input, and the duty cycle of a fixed
pulse width stream is proportional to its frequency, you get frequency to
voltage conversion. This was frequently used for precision FM demodulation
in broadcast instruments (before it all went DSP). For a sample circuit
(largely using discrete transistors), see the FMM-1 at
http://louise.hallikainen.org/BroadcastHistory/index.php/Belar%20Electronics%20Laboratory
. I recently spoke with the designer of the FMM-1. He told me their later
monitor used a 74121 monostable instead of the discrete monostable and got
better performance with the 74121.

So, doing this with op amps, I'd make a monostable with one op amp, then
an LPF with another.

Harold



-- 
FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
opportunities available!
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist