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

  • From: steve weir <weirsp@xxxxxxxxxx>
  • To: "Bart Bouma" <bart.bouma@xxxxxxxxx>
  • Date: Fri, 13 Feb 2004 01:47:12 -0800

Bart, I don't know why people fear that big "V".  Capacitors by the decade 
are something that I oppose.  I have seen people, including respected 
consultants mess up capacitors by the decade and blow impedance targets by 
a factor of 3:1 or more.  In the meantime, no parts were saved.
There is nothing wrong with an impedance lower than target, and the 
capacitor count is driven by the requisite inductance to meet the HF 
intercept.  Take the same qty of capacitors using decade spacing, and just 
substitute the larger value for all of them and the impedance plot is still 
very well behaved, and the phase doesn't go all over creation.

The only argument that anyone could ever try and make for smaller value 
capacitors that makes any sense to me is the higher ESR of the small 
values, provided it is high enough to get close to Ztarget that will help 
damp anti resonance with the planes.  In that case, I can see clear to two 
values of ceramic caps properly chosen, but not by the decade.  But, I have 
yet to see any author who advocates multiple values of MLCCs advocate on 
the basis of bringing up the ESR.  It has always been based on this 
folklore surrounding some perceived need for a flat impedance curve, that 
many then blow due to antiresonance.

Regards,


Steve.




At 10:34 AM 2/13/2004 +0100, Bart Bouma wrote:

> > 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
>
>Re [SI-LIST] Re Stack up for .gif
>
>
>
>
>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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