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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.

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. [1]

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.

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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.

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Matlab script output. C3 is the varactor capacitance.

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    \$\begingroup\$ R5 is far too low - try 10 kohm or above. Then show your LTSpice picture that you say is garbled. \$\endgroup\$ – Andy aka Aug 21 at 7:28
  • \$\begingroup\$ 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 \$\endgroup\$ – thedamp Aug 21 at 10:55
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    \$\begingroup\$ 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. \$\endgroup\$ – glen_geek Aug 21 at 12:30
  • \$\begingroup\$ 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? \$\endgroup\$ – thedamp Aug 22 at 2:22
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You need to have a much higher impedance between your modulation source and the tank circuit of your VCO at the VCO frequency. Otherwise, the source loads down your tank to the point where it no longer operates.

You need a good low-pass filter where you currently have R5. This filter needs to pass the frequencies generated by V1 with low loss, but have very high impedance at your VCO frequency. If these two frequency ranges are separated by a wide margin, often a simple RF choke (inductor) is sufficient to do the job.

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  • \$\begingroup\$ Thanks Dave that really cleared up all the nonsense. Playing around with1uH inductor seems to have helped quite a bit. Now I'm just going to test to make sure. \$\endgroup\$ – thedamp Aug 21 at 12:15
  • \$\begingroup\$ So it appears I am getting two seperate frequencies.1 at 210.2MHz and the other at 210.7MHz. Do these seem too close to be separate? I was expecting a 5Mhz difference. \$\endgroup\$ – thedamp Aug 21 at 12:25
  • \$\begingroup\$ Either your calculation is in error or your measurement is. You haven't provided enough information to determine which it might be. \$\endgroup\$ – Dave Tweed Aug 21 at 12:30
  • \$\begingroup\$ Added the matlab script I am using to calculate the values. I believe that the two peaks were not the circuit providing two seperate frequencies and it seems that the circuit only provides one oscillation frequency. I'll attach a copy of the current circuit build at the bottom of my post. \$\endgroup\$ – thedamp Aug 22 at 2:36
  • \$\begingroup\$ You also updated your schematic. The choke is in the wrong place -- it should be between the generator and the varactor, not between the varactor and the tank. Also, I have no idea where you got those equations. The varactor capacitance appears in parallel with the rest of the tank, not in series with it. \$\endgroup\$ – Dave Tweed Aug 22 at 3:59

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