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complex pole or zero (Read 2795 times)
bharat
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complex pole or zero
May 17th, 2010, 1:43am
 
Looking for answer of fundamental question.

If there is complex pole at (say) -3+4j and -3-4j , which means for s plane ( sigma axis, jw axis) and a plane perpendicular to jw axis where the Transfer function will blow to infinity at (-3,4) & (-3,-4) corordinates.
At what corresponding freq in bode's plot will I see the complex pole behaviour. Is it at sqrt(3^2+4^2) ??
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buddypoor
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Re: complex pole or zero
Reply #1 - May 17th, 2010, 2:56am
 
Yes, thats right. The mentioned sqare root is the magnitude of the vector directed to the pole position in the s-plane - and it is called "pole frequency".
But be careful, in the BODE diagram this pole frequency is NOT identical to the frequency where the transfer function magnitude has a (possible) peak resp. maximum.
The exact formula is:  Fpeak=Fp*sqrt[1-(1/2Qp^2)] ;  Qp=pole Q.
As you can see, for large Qp values both frequencies are very close to each other.
I suppose, you know how Qp is defined in the s-plane, don't you?

Nevertheless, you can identify the pole frequency very easily: It is simply the frequency at which the 2nd order transfer function has a phase excursion of exactly 90 deg.   This follows directly from the 2nd order transfer function - if expressed using Qp and wp=2*Pi*Fp.
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« Last Edit: May 17th, 2010, 5:01am by buddypoor »  

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raja.cedt
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Re: complex pole or zero
Reply #2 - May 17th, 2010, 8:39am
 
hi buddy poor,
                   here Q is very less(around 5/6), so you don't see much peaking here. Max peaking here you can get is 20log(5/3).

hi bharath,
              bode plot is only defined for the signal frequencies where damping is zero (means s=0+jw). So blowing(large peak) is only occurs for complex poles. or simple question, for 1/(1+s) have you seen blow at s=1?

Thanks,
Rajasekhar.
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buddypoor
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Re: complex pole or zero
Reply #3 - May 17th, 2010, 9:08am
 
Re: complex pole or zero

hi buddy poor,
                  here Q is very less(around 5/6), so you don't see much peaking here. Max peaking here you can get is 20log(5/3).

I suppose the pole location as given by bharath was only an example.
Nevertheless, for Qp>0.5 there will be always a magnitude peaking !
(Correction: Qp>0.7071; I have mixed it with overshoot in the time domain which exists for Qp>0.5)

hi bharath,
             bode plot is only defined for the signal frequencies where damping is zero (means s=0+jw). So blowing(large peak) is only occurs for complex poles. or simple question, for 1/(1+s) have you seen blow at s=1?


I am not sure if bharat will understand this question. Me not!

Regards
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« Last Edit: May 17th, 2010, 1:45pm by buddypoor »  

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