I found a 950 MHz to 2200 MHz Colpitts VCO circuit from this web page.

I tried to design it in Multisim, but doesn't produce any signals. Why?

I used BB130 diodes instead of SMV1232 because Multisim has that varicap diode, but it doesn't produce any signal. Why?

schematic in Multisim

original schematic

  • \$\begingroup\$ Did you add a startup source? No you didn't. I usually inject a current pulse somewhere in the circuit (like a 1 ns pulse of 10 uA in parallel with L3) to get is started. In the real world, oscillators start due to noise. In a simulator, there is (usually) no noise so the oscillator doesn't start. \$\endgroup\$ Commented Oct 20, 2020 at 12:49
  • \$\begingroup\$ You may need to run a transient analysis for hundreds of microseconds before the oscillator starts. You might also try repositioning C2 to decouple the collector - I have a feeling that may be a circuit mistake. \$\endgroup\$
    – Andy aka
    Commented Oct 20, 2020 at 13:03
  • \$\begingroup\$ I wonder if the oscillator is not working because it is not really a noise source. So would it be right to use a small signal source to try this? Or touching another simple oscillator circuit \$\endgroup\$ Commented Oct 20, 2020 at 13:15
  • \$\begingroup\$ I will try it because in other circuits decoupler capasitor c2 connecting with one ground together but this circuit have two ground \$\endgroup\$ Commented Oct 20, 2020 at 13:19
  • \$\begingroup\$ You were right Bimpelrekkie. Really, the multisim simulator cannot simulate like the real world. I added a noise source to the circuit and it generated a 600 MHz signal. \$\endgroup\$ Commented Oct 20, 2020 at 14:13

1 Answer 1


Try giving it a step voltage of 0-5 V instead Vcc = 5V. In this way, you simulate the "closing" of the circuits (the stop-run.) It works but there is another problem. The behavior is not the one of common-collector BJT.


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