+ Post New Thread
Results 1 to 8 of 8

6th October 2004, 10:49 #1
 Join Date
 Feb 2004
 Location
 Malaysia
 Posts
 98
 Helped
 8 / 8
 Points
 2,907
 Level
 12
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'?

Advertisment

6th October 2004, 10:55 #2
 Join Date
 Oct 2004
 Posts
 15
 Helped
 3 / 3
 Points
 2,008
 Level
 10
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°...
1 members found this post helpful.

6th October 2004, 14:48 #3
 Join Date
 Jun 2003
 Posts
 717
 Helped
 112 / 112
 Points
 8,029
 Level
 21
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.
[size=2]Best Regards,[/size]
[size=2]Hughes[/size]

Advertisment

8th October 2004, 13:54 #4
 Join Date
 May 2003
 Posts
 1
 Helped
 0 / 0
 Points
 2,741
 Level
 12
bode plot + phase + starts + 0
in your case ,the phase margin is about 80 degree,compared to the input ..

9th October 2004, 01:22 #5
 Join Date
 Aug 2004
 Location
 Egypt
 Posts
 1,185
 Helped
 145 / 145
 Points
 9,782
 Level
 23
how to measure 3db from bode
ur phase margin=18085=95 degrees

Advertisment

9th October 2004, 13:33 #6
 Join Date
 Sep 2004
 Location
 China
 Posts
 1,324
 Helped
 278 / 278
 Points
 10,068
 Level
 24
phase margin bode plot
I agree with shpongle. Phase usually start from 0degree.

9th October 2004, 13:35 #7
 Join Date
 Sep 2004
 Location
 China
 Posts
 1,324
 Helped
 278 / 278
 Points
 10,068
 Level
 24
bode margin
It would be helpful to post your design here. Maybe the curve is not phase but a phase shift from 180degree

9th October 2004, 16:18 #8
 Join Date
 Mar 2002
 Location
 Neverland
 Posts
 214
 Helped
 19 / 19
 Points
 4,903
 Level
 16
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.
+ Post New Thread
Please login