Frequency-Voltage conversion circuit
I see that D4 is shorted by having both pins to ground.If D1-D4 form a limiter/squarer, the connection is wrong. Take a look at this:
http://electronicdesign.com/digital-ics/whats-all-frequency-voltage-converter-stuff-anyhow
Plus, I don't agree with U3 directly on C2. I suggest to put a resistor, so it becomes a simple integrator.