variable frequency oscillator with colpitts and varactor diode

[sweetlou257]

Hello,

I'm new to this forum and I was wondering if I could get some help on a project. I basically need to make an FSK modulator using discrete components. I've been looking into using the colpitts oscillator and have heard about using a varactor diode to change the capacitance using voltage. The problem I'm having is that I don't know how to implement the diode. Here's the circuit that I'm current testing with PSPICE and I'm not sure if the diode model I'm using is the correct one, but the output only oscillates at one frequency. Any help would be appreciated.

Thank you.

 

[RCinFLA]

A voltage source has zero AC impedance. Putting one directly across varactor will be like an AC short circuit. Feed varactor with an isolating resistor or a choke at the oscillator frequencies.

Depending how you couple the varactor into the tank circuit you will change the feedback ratio of the oscillator.

 

[betwixt]

That's a strange schematic and has even stranger values.
RCinFLA is on the right track, you need to isolate the tuned circuit from the voltage controlling it, usually a resistor will do.
Look on Edaboard for the thread about 1 W FM transmitters and you will see how to use a varactor for analog modulation. To adapt it for FSK, allow your logic level voltage to reach the varactor.

Brian.