# How to calculate a frequency - VTF Conversion

Discussion in 'Homework Help' started by rafaello, May 12, 2013.

1. ### rafaello Thread Starter New Member

Mar 10, 2013
5
0
Hi,

I would like to calculate the generated frequency from my voltage-to-frequency converter.

I need a mathematic formula.

Here is scheme:

As you can see, capicator 0.1u in negative op-amp input is being charged to 0V (positive input) by input resistor and voltage. This is the easy part.

After op-amp output changes to negative, capicator is discharged to negative voltage. And to calculate the formula, I need to know this voltage.

Just after the op-amp switch, we can simplify the circuit:
(C2 is polystyrene cap)

I tried to calculate this in Lamplace transform, but I don't know how to model the voltage reference.

Formula for 0.1u capicator voltage (in time domain) is all I need.

Thanks

2. ### WBahn Moderator

Mar 31, 2012
23,567
7,212
The schematic claims that the input is 0V to 10V and that the output frequency is 1kHz to 10kHz with 0.005% linearity.

Assuming they are correct, then you can figure out the nominal frequency as a function of input voltage from the two endpoints.

In any simplified circuit that you want to apply Laplace to, you need to make sure that there are NO nonlinear components since Laplace transforms only apply to linear systems.

3. ### rafaello Thread Starter New Member

Mar 10, 2013
5
0
So I can't fully apply Laplace to model Zener diode, but maybe there is a way to model one of the states of this voltage reference.

Just like I did with normal diode - I replaced it with 0.7V voltage source.

I can't put a voltage source there, because the capicator needs to "see" the 1.5 resistor (it's being charged through it).

4. ### WBahn Moderator

Mar 31, 2012
23,567
7,212
You need to replace the diodes with their linearized approximations. A constant voltage source is the most simple one, but you can also include a series resistor, which is probably what you need here. But also be sure that the circuit is supposed to normally turn on those diodes. I haven't looks at the circuit close enough to know whether they should or shouldn't.