How to measure Phase Margin from this bode plot?

[wylee]

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'?

 

[shpongle]

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°...

 

[Hughes]

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.

 

[xdlg]

bode plot + phase + starts + 0

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

 

[amraldo]

how to measure -3db from bode

ur phase margin=180-85=95 degrees

 

[leo_o2]

phase margin bode plot

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

 

[leo_o2]

bode margin

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

 

[sinatra]

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.