Frequency to voltage conversion can be easily achieved using a phase locked loop chip. The PLL uses a simple VCO that you tune to oscillate in your frequency range by varying resistor and capacitor values. Then when you phase lock this oscillator to your incoming signal, the VCO control voltage becomes the DC output of your circuit. If frequency goes up, the control voltage changes to track this frequency change, and it does so with a nice linear characteristic. If the control voltage is going up with frequency while you want it to go the other way, you can add an inverting amplifier using a simple op amp which will do this for you.
I prefer this chip, the CD4046 for such an application:
http://www.fairchildsemi.com/ds/CD/CD4046BC.pdf#search='4046'
You can also use a tone decoder chip for this kind of thing, such as the LM567. Here's an example:
https://www.electro-tech-online.com/custompdfs/2006/07/LMC567-1.pdf