Jessie wrote:
Hi everyone,

I have a small question about PSPICE. I'm using PSPICE to simulate a
colpitts oscillator, and want to know the resonant frequency of the
circuit. What should I do to set up the simulation condition? I used
the transient simulation and FFT hoping to find the spectrum figure,
but it seems if I change the time step the spectrum also changes???
What should I do now? I'll appreciate any of your help!

At resonance, the phase shift through the circuit is zero. Break the
loop where it connects to the input of the active device and put a low
level source at the input of the active device in place of the loop
connection. Do a frequency sweep, plotting phase, and find the frequency
where the phase angle at the top of the tank is equal to the phase angle
at the capacitor tap. You should be able to do this with a single plot
of the phase angle difference.

This is close to the frequency at which the circuit will oscillate.
It'll oscillate where the total phase shift around the loop, which is
the sum of the shift through the tank and the shift through the active
device, is zero.

Roy Lewallen, W7EL