[SI-LIST] Re: AW: Re: Reference layers for high speed diff pairs
- From: Vinu Arumugham <vinu@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 11 Jan 2013 09:58:02 -0800
Jeff,
"For instance, I don't see that scenario (a) would ever be superior to (b)."
Did you mean that or is it flipped?
I expect (a) to be better than (b). VDDQ planes are noisy w.r.t GND. You can
also view it as GND planes being noisy w.r.t VDDQ.
So solely VDDQ referenced is no different than solely GND referenced. Switching
between VDDQ and GND reference can be a problem. Referencing both VDDQ and GND
planes in the presence of VDDQ/GND noise can also be a problem.
Thanks,
Vinu
On 01/11/2013 08:01 AM, Loyer, Jeff wrote:
> Shchif touches on one of the (to me) most troubling aspects of this issue
> when it is in the "system" realm. From my experience and the discussions
> we've had in this forum, I'm not sure any measurements except for margining
> would indicate whether a problem existed. Imagine two scenarios:
> (a) Transmitters and receivers are solely VDDQ referenced and the PCB is
> solely VDDQ referenced
> (b) Transmitters and receivers are solely VDDQ referenced but the PCB is
> solely "GND" referenced
> (you can flip the referencing scheme if you like such that the Tx and Rx
> are GND referenced and the PCB is GND or VDDQ referenced and have the same
> discussion)
> The key aspect is that the Tx and Rx are identical for both scenarios, but I
> want to distinguish between the two PCBs w/ passive measurements. One is
> "good", the other is "bad".
>
> >From my experience, the s-parameters of the 2 PCB's may be virtually
> >identical (even bare boards w/o any decoupling) - interplane capacitance
> >between VDDQ and GND make them indistinguishable. How do you predict which
> >scenario will perform better?
>
> My experience also agrees with Scott's, that noisy VDDQ planes (vias, balls)
> are bad things which should be avoided, but I don't see it being related to
> any Tx or Rx referencing scheme; it seems to be a separate issue (a noisy
> plane or trace adjacent to a victim is a bad thing). For instance, I don't
> see that scenario (a) would ever be superior to (b). If there is some case
> study which explains otherwise, please point me to it.
>
> Regarding the original question, I've had experience with differential pairs
> that are virtually immune to common-mode noise; I couldn't break them by
> injecting common-mode noise on the pair no matter how hard I tried. I've
> also had experience with differential pairs (a clock signal particularly) in
> which common-mode noise broke that link. Yes, theoretically they are much
> less susceptible to common-mode noise (than single-ended signals), but they
> are not completely immune. For instance, any mismatch between the pair turns
> common-mode noise into differential noise.
>
> Thanks,
> Jeff Loyer
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of Havermann, Gert
> Sent: Thursday, January 10, 2013 11:59 PM
> To: shchifwork@xxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] AW: Re: Reference layers for high speed diff pairs
>
> Shchif,
> The 3D Field solver calculates the behavior oft he complete structure
> including the ref-planes. If the return current is forced to take a "detour"
> across coupling caps or far away vias, then this will directly influence the
> s-parameters of the signal path. If multiple return currents all use a single
> via, then this will increase crosstalk.....
>
> Many effects are hard or impossible to predict just by looking at the design,
> and this is especially true for people without long experience in Simulation,
> testing and verifying high speed stuff. Scott is one of these highly
> experienced SI-Gurus, and he warns you to follow option #2 for good reason.
> And I second that.
>
> BR
> Gert
>
>
> ----------------------------------------
> Absender ist HARTING Electronics GmbH, Marienwerderstraße 3, D-32339
> Espelkamp; Registergericht: Amtsgericht Bad Oeynhausen; Register-Nr.: HRB
> 8808; Vertretungsberechtige Geschäftsführer: Dipl.-Kfm. Edgar-Peter Düning,
> Dipl.-Ing. Torsten Ratzmann, Dr.-Ing. Alexander Rost
>
> -----Ursprüngliche Nachricht-----
> Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im
> Auftrag von Ilan Wolff
> Gesendet: Donnerstag, 10. Januar 2013 21:16
> An: si-list@xxxxxxxxxxxxx
> Betreff: [SI-LIST] Re: Reference layers for high speed diff pairs
>
> Hi Scott,
>
> Thanks for your feedback.
> I must admit ther's point in your reply regarding the 3D solver that I don’t
> quite follow.
> (I might be exposing my ignorance in the next few lines, but I guess this is
> the way to learn.)
>
> Let’s say I go for the 1 GND & 1 power plane option (#2), and that on the
> “system” level PCB the same reference plane scheme is used.
> 1. How would the solver be
> able to differentiate between the two types of planes?
> 2. Up to now I’ve seen 3D
> solvers produce sNp files & TDR type simulations, but the reference planes
> were always “muted”. Are these solvers capable of producing a sNp file that
> would include the non-GND plane as one of the ports? What meaning would it
> have, since this plane is clearly not 50ohm?
> 3. Is the noise on the power
> planes more “dangerous” than noise on the GND planes? (Isn’t that one of the
> main reasons we have the signal routed as a differential pair)
>
> Thanks,
> Shchif
>
> ________________________________
> From: Scott McMorrow <scott@xxxxxxxxxxxxx>
> To: shchifwork@xxxxxxxxx
> Cc: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
> Sent: Thursday, January 10, 2013 3:45 PM
> Subject: Re: [SI-LIST] Reference layers for high speed diff pairs
>
>
> Ilan
>
> Having modeled, analyzed, designed, measured, and correlated measurements to
> modeling for 16 and 25G packages, you most definitely want to use GND/GND
> referencing. In fact, the entire stackup should be encapsulated by Gnd
> layers above and below any power layers, so that the first and last thing
> that a signal via sees is a ground layer. Otherwise, noise injection into
> the power supplies and crosstalk peaking will occur at very inconvenient
> places that are not necessarily localized.
>
> Unless you want to do the 3D package analysis necessary to convince yourself
> that you might be able to use GND/Power referencing, don't do it.
>
> When it comes to the signal path in packages, ground layers, ground vias, and
> ground balls, are good ... and power layers, power vias, and power balls are
> bad.
>
> best regards,
>
> Scott
>
> --
>
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 16 Stormy Brook Road
> Falmouth, ME 04105
> (401) 284-1827 Business
> http://www.teraspeed.com/
>
> Teraspeed® is the registered service mark of Teraspeed Consulting Group LLC
>
> On Thu, Jan 10, 2013 at 5:56 AM, Ilan Wolff <shchifwork@xxxxxxxxx> wrote:
>
> Hi experts,
>> I'm workingon a package design. This chip will have multiple 10G (and up)
>> differential pairs running between the PCB balls & the silicon bumps.
>> We are able (in terms of ball-out, bump-out & package layer count) to
>> accommodate the following 2 configurations:
>> 1.Sandwiching the diff pairs between 2 GND (Analog Vss) layers.
>> 2. Sandwiching the diff pairs between1 GND (Analog Vss) layer& 1 SERDES
>> supply layer (Tx supply for Tx pairs & Rx supply for Rx pairs).
>>
>>
>> Additional information:
>> Both non-GND supplies will have AC decoupling caps underneath the chip, on
>> the PCB, near the supply's vias into the package.
>> If using option 2, on the PCB end of the package, each diff pair will have
>> two reference viasof the relevant non-GND supply & (at least) two reference
>> viasof GND.
>>
>> looking at our past designs we have packages using both options.All of them
>> seem to work well. But now that we're moving on in data rates I'd like to
>> make an informative decision.
>> I'm trying to figure out if there is any preference in terms of SI.
>>
>> Care to voice your opinion?
>>
>> Many thanks,
>> Shchif
>>
>>
>> ------------------------------------------------------------------
>> To unsubscribe from si-list:
>> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>>
>> or to administer your membership from a web page, go to:
>> //www.freelists.org/webpage/si-list
>>
>> For help:
>> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>>
>>
>> List forum is accessible at:
>> http://tech.groups.yahoo.com/group/si-list
>>
>> List archives are viewable at:
>> //www.freelists.org/archives/si-list
>>
>> Old (prior to June 6, 2001) list archives are viewable at:
>> http://www.qsl.net/wb6tpu
>>
>>
>>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List forum is accessible at:
> http://tech.groups.yahoo.com/group/si-list
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
>
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List forum is accessible at:
> http://tech.groups.yahoo.com/group/si-list
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
>
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List forum is accessible at:
> http://tech.groups.yahoo.com/group/si-list
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
>
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
> .
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List forum is accessible at:
http://tech.groups.yahoo.com/group/si-list
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
