[SI-LIST] Re: Stack up for EMI reduction,plane resonance and u-str ip radiation etc etc

From: "Bart Bouma" <bart.bouma@xxxxxxxxx>
To: weirsp@xxxxxxxxxx
Date: Fri, 13 Feb 2004 10:34:20 +0100
Content-Type: text/plain; charset="us-ascii"
> Zhangkun, I am curious, why do you use capacitors as small as 1nF?  Do 
you 
> use capacitors spaced by decades, ie:  1uF 100nF, 10nF, 1nF?  If so, why 

> not just use 100nF in an 0603 package?  They have the same inductance as 

> any other value in that package, and with just one value they will not 
have 
> an antiresonant peak.

Steve, 
you're right. There will be no parallel resonances in that case.
But impedance will not be a 'flat' line over frequency. There will be one 
deep dip at the part's resonance frequency which typically will be 20 MHz.

Using 1nF, 10nF etc. is not a bad idea: it results in a low impedance over 
a broad frequency range, with dips at regular intervals.
This is a wellknown method that is used by many people I believe.
By using low-Q parts, the resonance peaks can be controlled.
The 1nF parts are most likely not the best wrt to low ESR values, so are a 
good choice I think.
More problematic are e.g. the 100nF 0603 parts, they have a large number 
of electrodes and hence a low ESR-figure.
See attached plot: showing three curves for 1nF, 10nF and 100nF 0603 
parts.
(sorry si-listers: attachment will be filtered out).

best regards, Bart
Yageo Europe








steve weir <weirsp@xxxxxxxxxx>

13-02-04 02:59
Sent by: si-list-bounce@xxxxxxxxxxxxx

Please respond to weirsp

 
        To:     zhang_kun@xxxxxxxxxx
si-list@xxxxxxxxxxxxx
        cc: 
        Subject:        [SI-LIST] Re: Stack up for EMI reduction,plane 
resonance and u-str ip 
radiation etc etc
    Category: 



Zhangkun, I am curious, why do you use capacitors as small as 1nF?  Do you 

use capacitors spaced by decades, ie:  1uF 100nF, 10nF, 1nF?  If so, why 
not just use 100nF in an 0603 package?  They have the same inductance as 
any other value in that package, and with just one value they will not 
have 
an antiresonant peak.

Steve.
At 09:42 AM 2/13/2004 +0800, Zhangkun wrote:
>Dear all:
>
>I have reviewed the mails in this thread. The following is my points.
>
>a)From my view, I am caring about the EMI of PCB. Very small common mode 
>noise will give rise to critical EMI problem. In my experience, the 
common 
>mode noise is proportional to the impedance of power delivery systems. 
>This has been verified by measurement and simualtion.
>
>b)I have done some measurement. No matter have many caps are placed on 
the 
>boards, the impedance of PDS beyond 200MHz will not get better. It should 

>be clarified that now I do not use cap less than 1000pF. When the caps 
>less than 1000pF is used, there will be a lot of antiresonance. This is 
>also verified by simualtion and measurement.
>
>c)I have not studied the interaction between signal in trace and noise in 

>plane. However, I have treated one case, in which the noise in plane 
>seriously affect the signal in trace. After we eliminate the noise in 
>plane, the signal become very good.
>
>Best Regards
>
>Zhangkun
>2004.2.13
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Follow-Ups:
- [SI-LIST] Re: Stack up for EMI reduction,plane resonance and u-str ip radiation etc etc
  - From: steve weir
- [SI-LIST] Re: Stack up for EMI reduction,plane resonance and u-str ip radiation etc etc
  - From: John Barnes
[SI-LIST] Re: Stack up for EMI reduction,plane resonance and u-str ip radiation etc etc

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