[SI-LIST] Re: [Bulk] Re: Decoupling for PLL

Zhangkun-

Can you tell us what values of L and C you are using in your filter ?

One other thought that I had, which may or may not bear on this 
particular problem is this:

Besides isolating the noise on the PDS from your PLL, the low pass 
filter also, to some extent, prevents noise generated by the PLL from 
getting back onto the main PDS planes or busses. A simple L section 
filter (N=2) operating in the backwards direction (CL versus LC) will 
exhibit a different impedance on the "L" side than it does on the "C" 
side. I'm wondering if it is possible that you might be developing some 
noise at the PLL pin across the impedance of the filter L that you don't 
develop across the zero ohm resistor? Perhaps your PLL likes seeing a 
very low Z at it's power pin.....

Just a thought, perhaps not relevant, but I thought I'd through it out 
for discussion.

-Ray


Zhangkun wrote:

>steve
>
>I have measured the power ground noise by spectrum analyzer. There is no 
>obvious difference.
>
>At some frequency, when the inductor(bead) is changed to resistor, the power 
>ground noise become larger. I am confusing.
>
>Best Regards
>
>Zhangkun
>2004.8.27
>  
>
.
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