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Design >> Analog Design >> could damping factor >>1
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Message started by casual on Jul 29^th, 2010, 10:35pm

Title: could damping factor >>1
Post by casual on Jul 29^th, 2010, 10:35pm

For a over-damped pll system, the zeta (damping factor) must be >> 1.

I checked the damping factor formula (Cartesian plane), it shows zeta=wn*cos(theta)/wn = cos(theta), which is bounded by +1,-1 from mathematics aspect.

could zeta/damping factor >>1?

Title: Re: could damping factor >>1
Post by raja.cedt on Jul 30^th, 2010, 10:51pm

hello,
when you are saying damping factor is cos function you are assuming that pole are on complex plane, so damping factor is between 0 to 1. If pole are on the real axis then damping factor will be more than one. So your cos formula don't catch this one.

Assume you have poles at A+JB and A-jB so total equation is S^2+2AS+A^2+B^2, so damping factor is A/(sqrt(a^2+B^2)..so geta is cos function.

when they are on the real axis this equation is not correct.

Hope you got the point.

Thanks,
Rajasekhar.

Title: Re: could damping factor >>1
Post by casual on Jul 30^th, 2010, 10:57pm

yup, it is same as what I assumed.
So the cos(theta) formula is only applicable until critical damped pll when zeta=1.

any other comments?

Title: Re: could damping factor >>1
Post by raja.cedt on Jul 31^st, 2010, 1:17am

yes...or you can say if poles are in complex plane only