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

From: "Istvan NOVAK" <istvan.novak@xxxxxxxxxxxxxxxx>
To: <weirsp@xxxxxxxxxx>
Date: Fri, 13 Feb 2004 08:27:29 -0500
Steve,

Well said.  Given the fact that there is still hardly any bypass capacitor
on the market where the designer would have a known range for its ESR,
selecting the largest value cap in a ceramic case style, and creating a
single deep V seems to be a good working compromise.

There are two penalties associated with this solution.  At low frequencies,
where the V shape interfaces with the impedance of bigger capacitors,
we will have to pay a factor of two either in the inductance of the bigger
caps (need twice as many) or in the capacitance of the ceramic caps creating
the deep V (if we selected the biggest capacitance in the case style, this
also
means we need twice as many).  There is a similar but more severe penalty
at high frequencies, where these ceramic capacitors interface with the
planes,
let it be thin dielectric -:) or thick dielectric.  To sufficiently suppress
the
capacitor-plane resonance and the first few plane modal resonances,
the cumulative inductance of the parts has to be several times less,
which means correspondingly more parts.  But I agree that given the
circumstances this is a safe working solution.

I hope sooner than later the industry will demand bypass capacitors
with specified ESR values (with +- tolerance) where the nominal ESR
value can be selected from a list, similar to nominal voltage, material,
etc.

Regards,

Istvan



----- Original Message -----
From: "steve weir" <weirsp@xxxxxxxxxx>
To: "Bart Bouma" <bart.bouma@xxxxxxxxx>
Cc: <si-list@xxxxxxxxxxxxx>; <si-list-bounce@xxxxxxxxxxxxx>;
<zhang_kun@xxxxxxxxxx>
Sent: Friday, February 13, 2004 4:47 AM
Subject: [SI-LIST] Re: Stack up for EMI reduction,plane resonance and u-str
ip radiation etc etc


> 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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>---------------------------------------------------------------------------
-----------------
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be
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or
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you
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>
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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
References:
- [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

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