voltage to frequency converter ?

linear voltage to frequency converter is it even exist ?
i have a homework to design a linear resistance to frequency converter , it's okay that i can change the resistance to voltage but the problem is in converting the voltage to frequency it seems like no circuit can do that , there's but all circuit's are very complicated , i design a circuit to change the sensed change in voltage to a current via voltage control current source with I = V (unit gain) , then this current is connected in parallel with both a resistor and a capacitor (i think it's all wrong) , but the main problem is how do i create a change in frequency and what is the appropriate elements to detect it ??????
please simplify as much as possible ...

the circuit :
**broken link removed**

How about connecting the output voltage of your capacitor to one input of an additional comparator. Connect the other input of the added comparator to a fixed reference voltage. Arrange for the output of the added comparator to short your capacitor out (say through a relay, or voltage controlled switch) when the voltage across your capacitor exceeds the fixed reference voltage. The amount of time it takes for the voltage across the capacitor to exceed the fixed reference voltage is determined by the value of your current source (and thus your circuit's input voltage). That amount of time will be the period between the repeated trippings of the comparator.

What frequency range? What operating voltage? What I.C. components can you use?

Actually, there is a section for homework questions for students, and your query may be moved. In the mean time, have you Googled "V to F converter"?

Check out an XR4151 IC.
V to F or F to V Conversion

thnx man I'll try that immediatly

I don't know let's assume frequency is between 100Hz to 100KHz and all components is allowed ...

Also known as a VCO... Voltage Controlled Oscillator