[SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator
- From: "Istvan NOVAK" <istvan.novak@xxxxxxxxxxxxxxxx>
- To: <vishrampandit@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 13 Aug 2003 09:04:59 -0400
Vishram,
There is no strict limit. The effectiveness of discrete capacitors
gradually go down with increasing frequency. In real life, all
three parameters of the usual C-R-L equivalent circuit are frequency
dependent. Moreover, the frequency dependency of R and L are
also linked to the external part of the current loop, which is determined
by the user and not necessarily by the part itself. The only thing
that helps is that if you intentionally and selectively want to target
higher frequencies with the series resonance frequency, it is done
with lower capacitance values (because there are practical limits
to reduce the inductance) and lower capacitance values tend to be
more stable: the capacitance versus frequency change is less.
But the loop inductance becomes more uncertain: low value
capacitors need just a few capacitor plates inside, but making the
part so thin would make them hard to handle. To keep the
capacitor body size at the specified brick geometry, the cover
layer is increased. There can be a large variation from part to
part and from vendor to vendor, making it increasingly hard
to precisely tune the capacitor to higher frequencies in a repeatable
way. Also, because we cannot reduce inductance proportionally
for the low capacitance parts, it means that Q gradually goes up,
making it double hard to tune the parts properly.
As your experience also suggests, under BGA packages, where
the via field may perforate the planes heavily, capacitors on the
bottom side in the via field still could prove to be a better solution.
Regards,
Istvan
----- Original Message -----
From: "Vishram Pandit" <vishrampandit@xxxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Tuesday, August 12, 2003 11:26 PM
Subject: [SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator
>
> The example I was referring to had a ground plane. However, with
simulations
> and with experiments, we determined that the capacitors directly under
> (bottom side of the board) the balls of the BGA performed better than on
the
> top side. If we put the capacitors on the top side instead of at the
bottom,
> EMI was worse at high frequencies (600 MHz- 800MHz). The harmonics of the
> BGAinternal frequencies were stronger. This was observed on multiple
designs
> with the same BGA.
>
> I have seen some comments on the SI list that the capacitors are
ineffective
> say above 500MHz. Is this really true?? I have seen improvement in EMI at
> 600MHz-800MHz if I tune the
>
> * value of the capacitors
>
> * ESR / ESL of the capacitors
>
> * location of the capacitors
>
> Any comments/advice will be appreciated.
>
> Thanks,
>
> Vishram
>
>
>
>
>
> >From: "Lee Ritchey" >Reply-To: si-list@xxxxxxxxxxxxx >To:
> si-list@xxxxxxxxxxxxx,"Vishram Pandit" >Subject: [SI-LIST] Re: UltraCAD
ESR
> and Bypass Capacitor Caculator >Date: Tue, 12 Aug 2003 09:25:16 -0700
>>Ah!
> The key here is double sided PCBs. That means no plane capacitor, >which a
> part of the UMR test. Problem with double sided PCBs is the >absence of
> planes to tie things together. This does make EMI with fast >edge parts
very
> difficult, if not impossible to control. In this case, >capacitor
placement
> does matter because there is a great deal of inductance >in the power path
> due to the lack of planes. As part of this, it is very >tough to make the
> circuit stable at high speeds, as well. >>When we tried to make NICs using
> double sided PCBs at 3COM for fast >Ethernet, we had this very problem.
Just
> barely worked. When we added >encryption, didn't work at all. Too many
data
> lines switching >simultaneously. >>Lee >>Lee Ritchey
> >leeritchey@xxxxxxxxxxxxx >Why Wait? Move to EarthLink. >>>>[Original
> Message] >>From: Vishram Pandit >>To: >>Date: 8/11/2003 3:21:05 PM
> >>Subject:[SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator
>>>>>>I
> agree with Ray. I have found in my designs that the EMI is very much
> >>dependent on the decoupling capacitor placement. Typically I can place
the
> >>decoupling capacitors with the PDS analysis. Also, I can find
> >>experimentally,the hot-spots and place the capacitors. With simulations
> >and >>with measurements, I have found that the decoupling capacitor
> placement is >>very critical. It may be in a particular section of the
> board,it may be >at >>the bottom (for double sided boards) .... it all
> depends on the nature of >>theproblem. >>>>>>Thanks, >>>>Vishram >>>From:
> RayAnderson >Reply-To: si-list@xxxxxxxxxxxxx >To: >>si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Re: UltraCAD ESR and Bypass >>Capacitor Caculator
>Date:
> Mon, 11 Aug 2003 09:36:25 -0700 >>>>Lee Ritchey >>wrote: >>If the location
> ofdecoupling capacitors matters, perhaps some >>technical >>demonstration
> would prove that. Short of such a demonstration, >>this is >>speculation
and
> not the sort of thing that should be used to >make >>design >>choices.
> >>>>It's time to do some good engineering on this >subject >>and do away
> with>>opinion. The UMR paper is good engineering. Anyone who >>chooses to
> >>disagree with it has the burden of showing where it is wrong >by >>using
> some >>good science. >>>>Lee >>>>Lee Ritchey
> >>>>leeritchey@xxxxxxxxxxxxx>>Why Wait? Move to EarthLink. >>>Or
conversely,
> >>show that it doesn't matter. >>Take a large system board (say about 24"
> >>square) put a bunch of high current >processors and ASICS on the left
side
> >>ofthe board. Put all your decaps >on the right side of the board (since
> >you >>maintain position doesn't matter). >>In this hypothetical case I
can
> just >>about guarantee the board won't function >properly or pass EMI.
> >Admittedly, >>it is a contrived case, but I think it >illustrates the
> point.>>As far as >>the technical demonstration goes, we have indeed
> demonstrated to >>ourselves >>that position does matter. >>I'm not
> attemptingto further an argument, >but >>Ido feel those who have
>responded
> with viewpoints other than yours >>shouldn'tbe chastised and accused >of
> spreading unfounded opinion. Don't >>discount the chance that an opinion
> that>differs from yours just might be >>correct. >>-Ray Anderson >Sun
> Microsystems Inc.
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