A question of PLL bode plots.

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holddreams

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In Razavi's book,
Figure 15.19,Bode plots of type I PLL.

"What happens if a higher KpdKvco is chosen so as to minimize φout-φin?Since the entire gain plot in Fig.15.19 is shifted up,the gain crossover moves to the right,thus degrading the phase margin."

I want to know why the phase margin degrade??

Thanks.
 

coz the gain of the PLL system is become higher , and u know the gain BW product is constant , so the BW will be lower this move the poles , and the phase margin degrads

khouly
 

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    holddreams

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khouly said:
coz the gain of the PLL system is become higher , and u know the gain BW product is constant , so the BW will be lower this move the poles , and the phase margin degrads

khouly
Click to expand...


Do you mean when Kpd*Kvco increase,the BW will decrease?
But if the BW decrease,the phase margin will be better.
I got confused.
 

On increasing Kvco*Kpd, the magnitude gets shifted upwards, while the phase remains the same because the poles locations weren't changed .So, GCO point is shifted to the right while the phase plot doesn't move .Thus the GCO will have higher -ve phase i.e smaller phase margin .
 

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    holddreams

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