# Trouble with FSK VCO modulation circuit

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.

• R5 is far too low - try 10 kohm or above. Then show your LTSpice picture that you say is garbled. Aug 21, 2019 at 7:28
• Just added the fft for R5 = 10kohm. However R5 is the source resistance of the signal generator which was why it was set to 50ohm Aug 21, 2019 at 10:55
• Varactor diode's added capacitance need only be a fraction of a picofarad to modify frequency 205MHz to 200MHz. It is better to capacitively-couple the varactor to your oscillator loosely (with a very small pf-type capacitor). Or use it to modify C4's capacitance. Aug 21, 2019 at 12:30
• I'm thinking I might try another varactor maybe a BB215 as it is in the LTspice diode library and I read that it is good for FM applications. Unless you have a better recommendation? Also when you say capacitively couple are you meaning wire the varactor to C4 rather than ground? Aug 22, 2019 at 2:22