hi,
   i have a system with some phase margin and i am getting some ringing in the step response, now if i increase system PM, then i know that no of rings will decrease..but how about frequency of ringing?

thanks,
Rajasekhar.

Rajasekhar,
I am reading your question as "but how about frequency at the ringing"?

1) You sweep the AC freq at the input and (where you are giving step response) and see the output in freq domain. The freq at which max O/P is seen is the freq of trouble where you are seeing the ringing. Let's say that freq is 250MHz. Mind it, it won't be regular AC sims of breaking the loop rather colsed loop simulation.
To verify the same in time domain run the transient simulation
and sweep the freq in the neighborhood of 240Mhz-260Mhz ( what you got in the freq response) of sinusoidal input for small amplitude of say 100mV, you may see that O/P is ringing with larger amplitude than the I/P is given.

Alternatively; If you can sweep the freq of sinusoidal input for small amplitude of say 100mV, you may see that O/P is ringing with larger amplitude that the I/P is given. Though this is tedious and you have to sweep the frequency of much larger range.

Regards,
-Bharat

hi raja,

If we consider a second order sys the damping freq is given by  Wd = Wn*sqrt(1-zeta^2)

so the ringing freq reduces with increase in zeta. As one can observe that zeta prop. to PM the ringing freq also decreases with increase of PM.

Thanks
vamshi