[SI-LIST] Re: Decoupling for PLL

  • From: zhangkun 29902 <zhang_kun@xxxxxxxxxx>
  • To: reanderson@xxxxxxxxxxxxx
  • Date: Fri, 27 Aug 2004 07:46:40 +0800

ray and steve

Thank you for your good advice. I have measured the power ground noise at the 
power pin of PLL. I do not see great difference between using inductor(bead) 
and resistor. This is my problem. The difference is about 3db, which could be 
ignored.

I do not use scope by spectrum analyzer.

It is obvious that the RC filter has lower Q than LC filter. Here I use 
resistor of 0 ohm. This should have some effect on power ground noise. I want 
to get some different measurement about power ground noise. 

Best Regards

Zhangkun
2004.8.27

----- Original Message -----
From: Ray Anderson <reanderson@xxxxxxxxxxxxx>
Date: Friday, August 27, 2004 3:21 am
Subject: [SI-LIST] Re: Decoupling for PLL

> steve weir wrote:
> 
> > Ray, zhang_kun, any linear filter book will show you your 
> options.  
> > Basically, you should start with the required impedance of the 
> filter 
> > at DC, use that to determine your resistor, and then derive L 
> and C 
> > based on the cut-off frequency required.  To contain the 
> ringing, set 
> > the damping coefficient to a reasonably high value.  Circuit 3A 
> used 
> > to be very popular with voltage mode switching power supplies.
> >
> > Steve.
> 
> 
> 
> Yes, you could design an impedance matched filter by untilizing 
> any of 
> the tried and true methods for synthesizing RF filters, BUT, I 
> think you 
> will find that when you specify the source resistance to be very 
> low 
> (say  on the order of milliohms as PS supply outputs are) and the 
> load 
> resistance to be that of a typical analog ckt like a PLL (in the 
> 100's 
> of ohms) and that your desired roll-off frequency to be in the kHz 
> to 
> 100's of kHz range you will find that the L and C values that fall 
> out 
> of the synthesis procedure are such that they are unrealizable on 
> most 
> PCB designs. 
> 
> -Ray
> 
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