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Of course the circuit need some improuvement.
I have used the following schematic ( used on the Marconi 2305 modulation analyzer ) that converts voltage to frequency.
You need in front of it to apply a current to voltage converter but i don't think this is difficult.
Otherwise use the following link to National applications notes:
**broken link removed**
Hope to be helpful.
Mandi
The basic idea of such a converter is to charge a capacitor with a constant current [Q=C*U and i=C*(du/dt) --> dU/dt=i/C] and probing the voltage across the cap with a schmitt trigger.
Thats it. The more current the faster the limit of the schmitt trigger is reached. So all you need is a current source and a cap.
A voltage controlled current source is made for example using an opamp with an output transistor (e.g MOSFET) and an low ohmic shunt resistor. The feedback of the opamp is directly connected to this resistor.
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