I know for close loop OPAMP or PLL, we always need 60 degree for stability and settling time performance.
But I have in many paper, the phase margin of DSM loop filter is only 20 degree, or even worse!
Should we watch the phase margin for DSM's stability criteria, or just looking the root locus will be enough to assure enough stability?
Many Thanks!

hi moore,
as mayank said ringing is the main concern rather than PM, in continuous time domain even 45 aslo acceptable in many cases (it all depends on settling time and acceptable ringing). But Phase Margin is less prefered parameter in z domain, main concern is wether your poles are in sides or not. If you want to approximate discreate domain with continuos domain (like plls and over sample data converters) phase margin has to be grater than 60, because sampled system has inherent delay which was not considered.

Thanks,
Rajasekhar.

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The criterion is whether the poles are inside or outside the unit circle, this is both we know.
But I don't think you fully answer the question,
it can be changed like this,
how many margins should we save between the z domain pole and unit circle ?

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They are two different things, I mean, stability and discrete-continuous approximation.
For the stability criterion, it's not a "approximation" but just a "transformation" from z to laplace