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Schematic of circuit

The pictured Colpitts oscillator circuit will not oscillate on a plug-in protoboard without C3 and its connection to ground in place. In LTSpice the circuit oscillates with or without C3 and its connection to ground.

The LTSpice circuit oscillates at about 928 kHz and the protoboard at about 900 kHz.

I am trying to understand why C3 is necessary on the protoboard. The protoboard has a lot of stray capacitance and inductance and I have a number of long leads on the components. Perhaps that has a lot to do with it.

From a theoretical point of view, the "tops" of R1 and L1 have a AC path to ground through the DC power supply, V2. The higher impedance path through C3 seems to me to be superfluous. The protoboard is connected to an HP dual power supply.

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    \$\begingroup\$ Make V2 a bit less ideal by adding some series resistance and inductance. If you degrade V2 enough, also in the simulator C3 will be needed. At this moment V2 is ideal so C3 does nothing. \$\endgroup\$ Mar 14, 2016 at 15:18
  • \$\begingroup\$ "lots of stray capacitance and inductance" - none of which are taken into consideration by the simulation. C3 simply ties the supply rails together from an AC perpective. \$\endgroup\$ Mar 14, 2016 at 15:19
  • \$\begingroup\$ Hint: It will also not osicllate if you remove the inductor. Or transistor. Or voltage source. \$\endgroup\$
    – PlasmaHH
    Mar 14, 2016 at 15:25

1 Answer 1

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The inductance of the power feed on your prototype board is the problem. You need to provide a low impedance AC path at the top of L1. To do this either use much shorter leads or put a capacitor where C3 is shown on your diagram.

The problematic inductance is partly down to the cable length and partly down to the power supply itself.

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