[SI-LIST] Re: PDS analysis?
- From: steve weir <weirsi@xxxxxxxxxx>
- To: "Bowden, Ivor" <ibowden@xxxxxxxxxxxxxxxxx>
- Date: Sat, 29 Mar 2008 03:00:27 -0700
Ivor, you can find examples in papers by people like Larry, Istvan, and=20
books that are out there. But, I am not sure where you are going with=20
this. Since terms like "well fed" are ambiguous, I don't know what=20
example you are looking for or to what purpose. My sense is that you=20
have a belief that if one follows some ambiguous "industry practices"=20
that the PDN will usually "work fine". Undefined engineering leads to=20
undefined results. If you want predictable results then: Define the=20
requirements and engineer to them.=20
We can characterize components to determine their actual power delivery=20
requirements. Once known we can easily demonstrate failures that result =
when we don't meet those requirements. It is also easy to demonstrate=20
what different PDN practices do to the impedance profile under any=20
particular set of controlled circumstances we wish to create.
If you want to see resonance effects for yourself then: Design some=20
board following one of the popular ad-hoc rules. If for example you=20
design a board with one 0402 capacitor every two square inches and the=20
plane cavity is 4mil FR406 on layers 2/3, the bypass to cavity PRF will=20
land around 330MHz depending on how you do your vias. If the planes are=20
deeper in the board that frequency will just go down. Then get yourself =
a programmable clock generator and use it to drive some device like a=20
PLD or FPGA with a bunch of outputs simultaneously in a simple 1-0-1-0=20
sequence while you monitor the power supply planes with a decent scope. =
Just step frequency until the bit rate / 2 excites the lowest parallel=20
resonance in the power system. After that experiment see if your=20
feelings about what works fine are still the same.
Steve
Bowden, Ivor wrote:
> Hi Gil,
> =20
>
> Thanks for sharing.
>
> =20
>
> By "real world" I do not mean rules of thumb and shortcuts, I mean exam=
ples. I expect that most folks on this list, especially those for whom SI=
is primary activity, can demonstrate examples of operational failures du=
e to bad board layout. I also know that a large number of designs never g=
et PDS analysis, yet work fine. I'm interested in typical modern technolo=
gy designs that are laid out in standard industry practice: contiguous gr=
ound planes, tandem heavy split power planes not used as return reference=
, plenty of properly located bypass caps, etc that have operational failu=
res which could have been pre-determined by PDS analysis, and if so, how =
did that operational failure manifest? For example, it would be interesti=
ng to see a scope (or simulation) snap shot of the voltage measured direc=
tly across vias to contiguous well fed planes as a device current require=
ment approaches the planes resonant frequency. And I mean a real device, =
not a hypothetical entity tuned to instigat
> e the problem.
>
> =20
>
> -Ivor
>
> =20
>
> -----Original Message-----
> From: Gil Simsic [IEEE] [mailto:gsimsic.ieee@xxxxxxxxxxxxx]=20
> Sent: Friday, March 28, 2008 1:08 PM
> To: Bowden, Ivor; si-list@xxxxxxxxxxxxx
> Subject: Re: [SI-LIST] Re: PDS analysis?
>
> =20
>
> Ivor
>
> =20
>
> I am involved in design since 1993, and have numerous of successful des=
igns=20
>
> under my belt. (I know - not very humble...)
>
> =20
>
> In one way or another I worked and learned from SI leaders (aka - gurus=
)=20
>
> like Lee Ritchey, Istvan Novak, Eric Bogatin, Scott McMorrow and Howard=
=20
>
> Johnson (and others that due to a senior moment I did not mention here =
-=20
>
> sorry) I gratefully owe my knowledge to them.
>
> =20
>
> by the way - all I tried to do is to learn about "real world picture".
>
> =20
>
> My greatest lesson is - there are no short cuts and no rule of thumbs. =
I=20
>
> read daily emails on this list that prove this point over and over agai=
n.
>
> =20
>
> And than again I might misunderstand you...
>
> If the by 'real world' you refer to 'rules of thumbs' and 'short cuts',=
I=20
>
> will strongly recommend you to shy away from that 'real world'. I reall=
y=20
>
> think that any design as hypothetic or rhetorical as it is, needs analy=
sis=20
>
> (the subject of your email is PDS analysis...).
>
> =20
>
> Most of the SI phenomena are frequency depended.
>
> =20
>
> You said - * I'm more interested in answers like "you might see a wavef=
orm=20
>
> of xxx characteristics across the bypass capacitor". *
>
> My answer - "you might see a waveform of xxx characteristics across the=
=20
>
> bypass capacitor". ;-)
>
> How can one attempt giving you any 'ball-park' numbers for a frequency =
>
> dependent component with out knowing what the frequency parts are?
>
> =20
>
> Good luck!
>
> =20
>
> Gil
>
> ----- Original Message -----=20
>
> From: "Bowden, Ivor" <ibowden@xxxxxxxxxxxxxxxxx>
>
> To: <si-list@xxxxxxxxxxxxx>
>
> Sent: Friday, March 28, 2008 12:11 PM
>
> Subject: [SI-LIST] Re: PDS analysis?
>
> =20
>
> =20
>
> =20
>> Hi,
>> =20
>
> =20
>
> =20
>
> =20
>> I'd like to thank everyone who replied so far, off and on the list. I =
>> =20
>
> =20
>> emphasize that this is a rhetorical question, it doesn't represent any=
=20
>> =20
>
> =20
>> specific product. I also emphasize that I'm interested more in the "re=
al=20
>> =20
>
> =20
>> world observation" part. Instead of answers like "your power plane may=
=20
>> =20
>
> =20
>> have a resonance at xxx frequency", I'm more interested in answers lik=
e=20
>> =20
>
> =20
>> "you might see a waveform of xxx characteristics across the bypass=20
>> =20
>
> =20
>> capacitor". Also, this question is more about power distribution than =
>> =20
>
> =20
>> signal return path. All answers appreciated.
>> =20
>
> =20
>
> =20
>
> =20
>
> =20
>> -Ivor
>> =20
>
> =20
>
> =20
>
> =20
>
> =20
>> ________________________________
>> =20
>
> =20
>
> =20
>> From: Bowden, Ivor
>> =20
>
> =20
>> Sent: Friday, March 28, 2008 7:24 AM
>> =20
>
> =20
>> To: si-list@xxxxxxxxxxxxx
>> =20
>
> =20
>> Subject: PDS analysis?
>> =20
>
> =20
>
> =20
>
> =20
>
> =20
>> Hi SI Experts,
>> =20
>
> =20
>
> =20
>
> =20
>
> =20
>> Say you have a typical PCB with modern technology mix of CPU, DSP, DDR=
,=20
>> =20
>
> =20
>> GIGE, PCIE, etc. Say it is a multi-layer stackup in the form of=20
>> =20
>
> =20
>> GND-SIG-SIG-GND sets, with the power distribution centered in the stac=
kup=20
>> =20
>
> =20
>> as solid ground plane - split power plane - split power plane - solid =
>> =20
>
> =20
>> ground plane, using 1oz copper and 3.5 mil dielectric. Assuming the sp=
lit=20
>> =20
>
> =20
>> power planes utilize sufficient area to keep the point to point induct=
ance=20
>> =20
>
> =20
>> and resistance to reasonable values, and 0.1uF ceramic bypass caps are=
=20
>> =20
>
> =20
>> evenly placed at device pins, and bulk capacitance is placed as needed=
,=20
>> =20
>
> =20
>> would there be reason to expect any problems, such as plane resonance,=
=20
>> =20
>
> =20
>> etc? If so, what would be the observable real world manifestations, in=
=20
>> =20
>
> =20
>> terms of circuit performance and power pins scope waveforms? Would the=
re=20
>> =20
>
> =20
>> be significant advantage to analyzing this PDS, or should following th=
is=20
>> =20
>
> =20
>> "industry standard practice" for PCB PDS be sufficient to expect robus=
t=20
>> =20
>
> =20
>> behavior?
>> =20
>
> =20
>
> =20
>
> =20
>
> =20
>> Thanks,
>> =20
>
> =20
>
> =20
>
> =20
>
> =20
>> Ivor Bowden
>> =20
>
> =20
>
> =20
>> Senior Hardware Engineer
>> =20
>
> =20
>
> =20
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