[SI-LIST] Re: reference plane cutout under DC blocking capacitor pads
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Antonis Orphanou <orphanou@xxxxxxxxxxxx>
- Date: Tue, 19 Nov 2013 16:02:49 -0500
Antonis
You are correct, the original question concerned capacitor pads. From my
point of view, the pads and the capacitor body come together as a unit.
It is usually better to optimize the cutout for all pads and capacitors in
a differential DC block simultaneously. Extra points for modeling the
plates inside of the capacitor. Done right, the solution can have
extremely wide bandwidth and avoid sharp cutoff. I generally try to keep as
much energy out of the PCB cavity as possible, contain that energy which
does leak in, and keep slow wave capacitor resonance from being strongly
excited.
Scott
On Tue, Nov 19, 2013 at 2:45 PM, Antonis Orphanou <orphanou@xxxxxxxxxxxx>wrote:
>
> Hello scott,
> I was talking about the capacitor pcb-pads not the capacitor itself. I
> thought that what the discussion was about :) .. ?
>
> regards
> Antonis
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Scott McMorrow
> Sent: Tuesday, November 19, 2013 11:40 AM
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: reference plane cutout under DC blocking capacitor
> pads
>
> Antonis
> with all due respect, a capacitor is not an insignificant discontinuity.
> Given the pad, body, end cap, and the plate structure for a 0402 MLCC
> capacitor, it exhibits a low impedance discontinuity, with a cutoff
> frequency around 10-15 GHz, without some compensation structures built into
> the planes. In many designs that I've seen there are no ground stitching
> capacitors between a multitude of DC blocking capacitors, and as a result,
> excessive crosstalk exists. I've seen quite a few designs where the
> capacitors are arranged in a linear array at minimum spacing, which ends
> allowing coupling of the body sidewalls.
>
> Vias correctly placed will serve to minimize crosstalk and contain the
> common modes that propagate due to signal skew. Common mode conversion
> near the receiver can have some disastrous multi-aggressor crosstalk
> peaking implications.
>
> regards,
>
> Scott
>
>
>
> On Tue, Nov 19, 2013 at 2:02 PM, Antonis Orphanou <orphanou@xxxxxxxxxxxx
> >wrote:
>
> > For the most part the capacitor pad is rather short to be considered a
> > significant discontinuity. However when you try to make it a 50 ohm
> > transition you have to open up the planes underneath. Usually there are
> > plenty of ground vias around the cut-up area so the need for extra
> > stitching might be little bit excessive but a safe practice.
> >
> >
> >
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> > On Behalf Of Balaji G
> > Sent: Monday, November 18, 2013 6:26 PM
> > To: si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] reference plane cutout under DC blocking capacitor
> pads
> >
> > Hi all,
> > I believe reference plane cutouts under the DC blocking capacitor pads
> > for high speed signals help us minimizing the extra capacitance created
> > between pads and planes and reduces the impedance discontinuity.
> >
> > This means that the pad should refer the farther ground/power as
> > reference (20 mils away from signal layer). Is that means we need to
> > engineer the layers under the cutouts? Say we should move the traces away
> > which are going under the cutout region in the signal layer directly
> under
> > high speed reference plane.
> >
> > Also, in certain application notes, I got to look at a recommendation
> of
> > adding ground stitching vias near the pads to provide current return
> path.
> > If the signals are high speed (12Gbps), I believe the returns would
> prefer
> > to take a loop around the cutout region in the immediate reference plane
> > rather taking a loop through the ground stitching vias. Can you provide
> > your thoughts on this?
> >
> >
> > Regards,
> > Balaji
> >
> >
> > ------------------------------------------------------------------
> > 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
> >
> >
> >
>
>
> --
>
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 16 Stormy Brook Rd
> Falmouth, ME 04105
>
> (401) 284-1827 Business
>
> http://www.teraspeed.com
>
> Teraspeed(r) is the registered service mark of
> Teraspeed Consulting Group LLC
>
> ------------------------------------------------------------------
> 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
>
>
>
>
>
--
Scott McMorrow
Teraspeed Consulting Group LLC
16 Stormy Brook Rd
Falmouth, ME 04105
(401) 284-1827 Business
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
------------------------------------------------------------------
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:
