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

From: steve weir <weirsp@xxxxxxxxxx>
To: "Istvan NOVAK" <istvan.novak@xxxxxxxxxxxxxxxx>
Date: Fri, 13 Feb 2004 09:36:22 -0800
Istvan,  I have been able to show that when properly designed, the big "V" 
actuallly optimizes the total number of decoupling capacitors.

This is achievable at no penalty in the size of the bulk capacitor(s).

Damping is met at the low frequency transition, hence no adverse peaking in 
the high 100's of KHz to low MHz where that transition occurs, and a clean 
transition that is purely inductive through the target impedance occurs at 
the high-end.

The issue of resonance against the planes for EMI reduction remains 
challenging, and is open ended.  I am not certain that capacitors even with 
controlled ESR will be enough as there is a problem with what ESRs to take, 
although the Proadlizers were they cheap and plentiful are intriguing.  For 
widely spaced SRFs we need to satisfy:

zeta = 0.5 ( ESRmlcc + Rplanes ) / sqrt( Lmounted_mlcc / Cplane )

About the only thing even close to constant in this equation is Rplanes 
which at the frequencies of interest is all skin, unless we go to high K 
materials, where G becomes significant.

The most interesting parts out there are the Proadlizers from NEC which are 
for lack of a better term high frequency electrolytics.  Those devices have 
very low Q's out to 100MHz or so, and properly mounted don't exhibit the 
plane resonances of MLCCs.

Regards,


Steve.

At 08:27 AM 2/13/2004 -0500, Istvan NOVAK wrote:
>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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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
  - From: Istvan NOVAK
[SI-LIST] Re: Stack up for EMI reduction,plane resonance and u-str ip radiation etc etc

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