Circuit shown is a VCO that reliably creates a 200MHz sine wave at 3V pk-pk. I would like to use this VCO to create an FSK modulated signal. My bit 1 frequency = 200Mhz and my bit 0 frequency = 205Mhz optimally.
When I try to add a varactor or a diode and capacitor in parallel to my original circuit as shown below instead of getting the two frequencies I would expect I get a very garbled fft (using LTspice) that is not at all what I want. I have tried using V1 (data stream in) as a square wave,sine wave or DC 0V for one iteration and DC 5V for another iteration. I have tried numerous ways of connecting the varactor (D1) to the original VCO with different orientations of coupling capacitors etc with no luck.
The two equations which I am using to generate the colpitts oscillator values as well as the value of the varactor/(diode capacitor pair) are shown below. C1 is actually C3 and C2 is actually C4.
Any help or ideas would be greatly appreciated as I am very confused and very frustrated. Cheers.
Matlab script to calculate L and C values. Then reruns it through to check correct frequency. I know I mentioned that I wanted 200MHz and 205MHz but I found these would good values that I hand tuned to get the desired frequencies.
Matlab script output. C3 is the varactor capacitance.