How to measure Phase Margin from this bode plot?

phase margin measurement

As I recalled, all the books measure PM (phase margin) with respect to -180 degrees (at low freq, phase begin at 0 degrees)

But for my case here, my phase begin with +180 degrees, should I measure phase margin with respect to 0 degrees or -180 degree? :?: What is the phase margin of this plot? 85.05' or 265.05'?

and why isn't my phase start at 0' but at 180'?

measure phase margin

The phase margin is usually computed with respect to -180° because we expect the phase to start at 0.
The meaning of the phase margin is "How much phase left before I get a phase shift of 180° with the input ?"
So in your case it is about 80°...

bode plot phase

The PM is about 80 degree. The phase doesn't begin at 0 degree because the negative input terminal is used in ac analysis.

bode plot + phase + starts + 0

in your case ,the phase margin is about 80 degree,compared to the input ..

how to measure -3db from bode

ur phase margin=180-85=95 degrees

phase margin bode plot

I agree with shpongle. Phase usually start from 0degree.

bode margin

It would be helpful to post your design here. Maybe the curve is not phase but a phase shift from -180degree

measuring phase margin

Hello all.
There are always lots of different definitions for phase margin.... it becomes sometimes quite confusiong.
The phase margin problem hapens when you have a feedback system.
I believe that the best definition is:

1- A feedback system becomes unstable if the total phase delay of the system is greater than or equal to 360 degrees (Barkhausen criterion).

2- This means that a positive feedback system will always be unstable since the phase delay will always be more than 360 degrees.

3- A negative feedback system can be stable under certain conditions. Due to the negative feedback the loop has already 180 degrees phase delay.

4- There are still left 180 degrees.

5- Normally, adding some more processing inside of the loop may improve the loop time or frequency response.

6- The maximum phase delay allowed for all the processing that can be added is 180 degrees.

7- After all the extra processing is added, the amount of phase delay that is left is then the phase margin of the system. 45 degrees is a good target value to have after all processing and compensation has beeing added.

Could you show us your design so that people can give a better opinion?

Greetings
S.