can't simulate univibe LFO in multisim, any idea why?

[tss]

I captured the LFO part of the circuit and inserted a dummy 100 Ohm resistor instead of the bulb but all I could measure from the LFO was DC. Any idea why could this be?

[~arph]

Isn't that a Phase shift oscillator? those are pretty tricky circuits to get working in real life. So in a sim all factors are important to match to the original circuit. Especially the gain of the circuit. I remember fiddling around with PSO's in spice an that gave me headaches too.. When I had a working sim it wouldn't work in real life.
Oh and please note the PSO's need some time to start oscillating too... So if you only run your sim for a second or so it probably won't oscillate yet.

[R.G.]

Phase shift oscillators can be hard to start in a simulator. On the 'vibe LFO I've had to use a switch to ground something momentarily to insert a jolt to start it. I believe back when I tried Multisim it had a time delay switch that changed state a certain amount of time after the simulation started that worked. If your sim allows on-the-fly switch activation, that should work too.

The problem is that simulation is too perfect. The parts are usually exactly matched, as they exist only as numbers, and there is no noise, so the imbalances and noise of real life don't get there. Simulators also calculate initial conditions for the voltages that will exist once things charge up, to save the simulation time from the devices all having to ramp up from an all-zeros start, so there's not even the power supply turn on transient to kick start oscillators.

Finally, the vibe LFO has much less gain than other phase shift oscillators, and is very near the sine-wave condition, which is just barely enough gain to oscillate. At the lowest frequency settings and with lower gain devices, real-world vibe oscillators will fail to start. So you have to make sure all is correct in the schematic and values.