In Reply to: i need circuit diagram about FSK posted by yvonne on 04/20/03 at 12:49 AM:
The phase lock loop can be used both as modulator and demodulator. National Semiconductor Application Note AN-46 is a good tutorial on PLL and has some practical demodulator circuits. To make a modulator, feed a fixed oscillator into the phase detector, use a summer to add the control signal to the VCO, and take the output from the VCO.
You will find this ap note plus others at this URL: http://www.national.com/apnotes/Analog-Comparators__v1.html