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Phase margin definition (Read 2801 times)
mohanps
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Phase margin definition
Mar 01st, 2006, 11:08pm  

Basic question on the Phase margin definition. Why the PM is only
looked at the gain cross over point? If the phase goes down
to 20 degrees and come back to 60 degrees (due to Zero) at 0 crossing, is it
not dangerous as the phase is close to 360 degrees when the
gain is still positive (leading to oscillations)

Thanks
Mohandas
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ACWWong
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Re: Phase margin definition
Reply #1 - Mar 2nd, 2006, 2:09am  
sounds dangerous to me... perhaps you can see the difference in the transient behaviour of your feedback system when compared to a classical (ie PM always > 60 deg for >0dB gain) 60deg PM system. you might find a tendency for ringing at a frequency of poistive gain and low PM... especially if you purposely stimulate at that frequnecy...
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vivkr
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Re: Phase margin definition
Reply #2 - Mar 2nd, 2006, 2:27am  
Hi Mohandas,

The above definition of PM implicitly assumes that the gain curve monotonically decreases.
The PM criteria is just a simplification of the more general Nyquist Stability Criterion.
Whenever in doubt, refer to the latter.

Regards
Vivek
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