[SI-LIST] Re: AC coupling capacitors (SATA, PCIe)

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: Tesla <emcesd@xxxxxxx>
  • Date: Mon, 24 Sep 2012 09:58:10 -0700

The foolish idea of OOB signaling imposes a requirement for much larger 
capacitance values than would be needed had the functions used for OOB 
signaling been done in-band, or even over a low-speed channel like an 
LVDS pair. This would have made a number of channel design aspects much 
easier and more scalable: ISI, and VCO stability to name but two. Alas, 
we have no time machine to go back and correct this. We can only hope 
that the next time a group gets together they learn lessons from the 
past. Digital switching transistors are cheap. It's easy to build 
multiplexors to divide out overhead bandwidth in digital. Analog 
circuits are expensive. Everything possible should be done to minimize 
burdening a high speed channel.
Steve.

On 9/23/2012 7:50 PM, Tesla wrote:
> Hi, Steve
> "forcing us into ridiculously large coupling caps, and VCO stability 
> requirements"
> Do you mean PCI-SIG defines too large capacitor?
> If so, can we use small coupling caps for better performance?
> Thanks.
> Regards.
>
>
>
>
>
> At 2012-09-18 05:23:46,"steve weir" <weirsi@xxxxxxxxxx> wrote:
> >For most current generation SERDES applications many ordinary MLCCs do
> >just fine provided that you optimize the transition, pad, and anti-pad
> >geometries for maximum transparency.  There are differing schools of
> >thought with regard to whether or not one is better off choosing a
> >capacitor that is filled-out resulting in a thin cover layer, or one
> >uses a capacitor that has a relatively thick cover layer.  Arguments for
> >the former concentrate on the electrical superiority of that choice,
> >while arguments for the latter concentrate on the supply source
> >stability of the latter.  Economics dictate that today's thin cover
> >layer capacitor, is tomorrow's thick cover layer capacitor.
> >
> >If I had a time machine I would go back and beat the engineering
> >ramifications of channel percentage bandwidth into the PC-SIG folks who
> >designed PCI-Express, forcing us into ridiculously large coupling caps,
> >and VCO stability requirements.  The time for that is far behind us, so
> >we have to live with these things.  0201s are now available in large
> >enough capacitances to meet the spec.
> >
> >Steve.
> >
> >On 9/17/2012 1:20 PM, Hermann Ruckerbauer wrote:
> >> Hello Rohit,
> >>
> >> thanks for the answer, and sorry for my bad description.
> >>
> >> I hope to clarify what I ment:
> >>   Any kind of Capacitor that is specially designed for AC-Coupling
> >> (DC-Blocking) for a specific application (PCIe, SATA, ...) e.g. :
> >> - low ESR/ESL caps / high Q caps / controlled ESR caps
> >> - Special material
> >> - size other than 0402 (e. g. 0201 or reverse geometry)
> >> - Special designed to have the impdeance minimum fitting to the
> >> application (I have no idea how to fit 10MHz from plot in the link below
> >> to e.g. 3GHz of a SATA interface).
> >> - ... ????
> >>
> >> I found a lot of documents dealing with Capacitors for PDN, but not too
> >> much for Capacitors used as AC-coupling in serial link applications.
> >> e. g.     www.murata.com/products/article/pdf/ta1082.pdf
> >>
> >> Currently I can optimize my board e. g. in order to avoid the
> >> Capacitance of the pads, but I have somehow no idea whether I can use
> >> some optimized capacitors as AC-Coupling of a SATA or PCIe interface ..
> >>
> >> Any feedback welcome ..
> >>
> >> Hermann
> >>
> >>
> >>
> >> Our next Events:
> >> ================
> >>
> >> "Open the Black Box of Memory"
> >> Seminar on 08/09. November 2012
> >>
> >> Check our website or contact us for details
> >>
> >> EKH - EyeKnowHow
> >> Hermann Ruckerbauer
> >> www.EyeKnowHow.de
> >> Hermann.Ruckerbauer@xxxxxxxxxxxxx
> >> Veilchenstrasse 1
> >> 94554 Moos
> >> Tel.:      +49 (0)9938 / 902 083
> >> Mobile:    +49 (0)176  / 787 787 77
> >> Fax:       +49 (0)3212 / 121 9008
> >>
> >> Am 13.09.2012 12:38, schrieb Rohit MISHRA:
> >>> Hello Hermann,
> >>>
> >>> I don't know what do you mean by "special AC coupling capacitors" but 
> >>> there was a paper from Yaochao Yang & his team in DesignCon 2012, Titled 
> >>> " Embedded DC Blocking Capacitors in Connectors - Study of Impacts on PCB 
> >>> Design"
> >>>
> >>> You can have a look, May be you can find what you are looking for.
> >>>
> >>> Rgds,
> >>> Rohit
> >>>
> >>>
> >>>
> >>> -----Original Message-----
> >>> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] 
> >>> On Behalf Of Hermann Ruckerbauer
> >>> Sent: Thursday, September 13, 2012 2:34 PM
> >>> Cc: SI LIST
> >>> Subject: [SI-LIST] AC coupling capacitors (SATA, PCIe)
> >>>
> >>> Hi,
> >>>
> >>> i have done a short search on special AC coupling capacitors for serial
> >>> interfaces  (PCIe, SATA, .. ) , but found nothing really special.
> >>> I have found Steves DesignCon 2012 presentation, but this deals more
> >>> with characterization than performance requirements.
> >>>
> >>> Is there something better available, than a standard C0G/X7R  capacitors ?
> >>>
> >>> Thanks for any feedback ..
> >>>
> >>> Hermann
> >>>
> >>> Our next Events:
> >>> ================
> >>>
> >>> "Open the Black Box of Memory"
> >>> Seminar on 08/09. November 2012
> >>>
> >>> Check our website or contact us for details
> >>>
> >>> EKH - EyeKnowHow
> >>> Hermann Ruckerbauer
> >>> www.EyeKnowHow.de
> >>> Hermann.Ruckerbauer@xxxxxxxxxxxxx
> >>> Veilchenstrasse 1
> >>> 94554 Moos
> >>> Tel.:     +49 (0)9938 / 902 083
> >>> Mobile:   +49 (0)176  / 787 787 77
> >>> Fax:      +49 (0)3212 / 121 9008
> >>>
> >>>
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> >
> >
> >--
> >Steve Weir
> >IPBLOX, LLC
> >150 N. Center St. #211
> >Reno, NV  89501
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> >
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-- 
Steve Weir
IPBLOX, LLC
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Reno, NV  89501
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