Need a frequency controlled voltage source!

(posting here to get the most views and the most talent!) I want to run a theremin to my Moog synthesizer. It has an input where 0-5 VDC bypasses the keyboard and goes directly to all the tone shaping controls. One volt is 1 octave. I guess I want a frequency controlled voltage (opposite of a VCO). Can that be done fairly simply? Some kind of Op Amp? Only thing that shows up on the Internet is VCO stuff. Thanks! Jeff