# More than one gain crossover frequency

1. Sep 3, 2008

### varunag

Hi

I was plotting the bode plot for $$\frac{1}{s^2 + 1}$$.
I found the following bode plot,
http://home.iitk.ac.in/~varunag/prob2q1-uncomp.jpg"

Here we can see two gain crossover frequencies. Thus two phase margins.
On googling, I found this paragraph in a book:
Which of the phase margins should be considered? and why?

TIA,
varunag

Last edited by a moderator: Apr 23, 2017 at 3:24 PM
2. Sep 3, 2008

### Staff: Mentor

Your plot looks to be unstable -- do you see why? Is the low frequency or high frequency area unstable?

As for the quote, you would chose the smallest phase margin point, because as things change in your circuit (temperature effects, component tolerances, noise, etc.), that is the point where you have the most chance of tending toward zero phase margin, and thus oscillations. Does that make sense?