Correct assesment of PM and ringing frequency

[diarmuid]

Hello All,

I have the following 2 questions:

1. My first question is best described via an example:

Suppose well below my GBW my open loop phase response is say -160deg.
Then between this freq and the GBW suppose a LHP zero occurs to add +90deg.
Due to the LHP zero, suppose my phase response at GBW is now say -100deg.

Is my PM -40deg or -80deg?

2. When I apply a step input to a system with low PM the output step
response shows ringing (as we all know).
What is the frequency of this ringing?
Is it the frequency where the open loop phase response is -180deg?

Thanks for you responses,

Diarmuid

 

[erikl]

1. Is my PM -40deg or -80deg?

+80deg, I'd say.

2. Is it the frequency where the open loop phase response is -180deg?

No. Not sure, but I think it's about the frequency of the non-dominant pole.

 

[diarmuid]

I agree with your 80deg.

Im in the process of setting up a sim and measuring the ringing. Its inbetween designs at the moment
but should have an answer next week. Will update this when I do.