I'm simulating an oscillator that I have designed for a 40kHz oscillation frequency. I simulated it in Orcad and I got a flat signal. After some research, I found that it's due to the program not calculating any noise or something to start the circuit.


  • I added an Ipulse to simulate noise - didn't work.
  • Skipped initial bias point calculation - didn't work.

Do you have any idea how I can kick start my circuit ?


In real life, transients and/or noise start oscillations in oscillators. In a simulation, everything is ideal and no noise is around. Furthermore, connecting dc Voltage source to your circuit and simulating, it most likely just goes to steady state and has no transient. Unfortunately that transient is what would kick start the oscillator if and only if the oscillator is properly designed and configured correct with proper values for gain. Too many times to say proper?

Fortunately there’s a way: replace your dc voltages with step voltage sources. They would be the same voltage value but have a step look to it with a rise time of 10ns should do it. Then view the output in transient mode. This usually works.

  • \$\begingroup\$ So i should put a Vpulse instead of my Vdc, going from 0V to 5V in 10ns at the start ? \$\endgroup\$ – Gragon Apr 25 '20 at 23:44
  • \$\begingroup\$ Not a pulse, a step input. You can probably just use whatever rise and fall time they use as default. It steps from 0 to 5V and stays at 5V. Also change the 2.5 V to a step input of same value. \$\endgroup\$ – Leoman12 Apr 25 '20 at 23:51
  • \$\begingroup\$ It doesn't work as it should. There is no step input in Orcad so I have to play with Vpulse to have something similar. I don't know if I'm doing it right. Do you have the software ? \$\endgroup\$ – Gragon Apr 26 '20 at 0:54
  • \$\begingroup\$ I never used PSpice before. If your software doesn’t have an actual step input then maybe you can use a switch between power and circuit shown in video here: m.youtube.com/watch?v=MwSseZy3nvM \$\endgroup\$ – Leoman12 Apr 26 '20 at 1:04
  • \$\begingroup\$ Also you can remove that pulse input. \$\endgroup\$ – Leoman12 Apr 26 '20 at 1:05

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