Step time for oscillator - why is it important to be small?

[dynamac]

Step time for oscillator

Hi
Does anybody know why it is important for step time to be small when simulating oscillators. I was simulatiing 19.4 Mhz crystal oscillator. When my step size is 100p it does not oscillate, when it is 1ps it oscillates.
Thank You
Roger

 

[E-design]

Re: Step time for oscillator

Step time should be much smaller (at least 10 times) than the period of oscillation for accurate simulation. This is what makes simulating crystal oscillator circuits difficult especially when the crystal has a very high Q. It can take very long to start up in the simulator. One way around this for initial test purposes is to lower the Q of the crystal model circuit. For example if your motional inductance is 50mH, lower this to 5mH and increase the Cs accordingly. This way you can verify that the circuit will start without wasting a lot of time.