[SI-LIST] Re: AW: Reg. GND plane requirement
- From: olaney@xxxxxxxx
- To: leeritchey@xxxxxxxxxxxxx
- Date: Fri, 30 Oct 2009 13:23:01 -0400
An excellent point. The essence isn't whether there will be
discontinuities - there will be - but how bad they are. For UTP, the CM
impedance varies all over the place (Cat5 cable droops through open air
then runs over a heating duct...) but hardly affects the DM impedance
because that is determined almost entirely by +/- coupling in the signal
pair. One function of the overall dielectric jacket is to guarantee a
minimum spacing from nearby objects. If a printed circuit design
likewise emphasizes line to line coupling to define diff Z and maintains
maximum clearance from planes and other objects, the same relative
immunity to plane cuts and similar disturbances is obtained. The
immunity obtained is, as always, is a matter of degree.
Orin Laney
On Thu, 29 Oct 2009 15:09:26 -0700 "Lee Ritchey"
<leeritchey@xxxxxxxxxxxxx> writes:
> Differential pairs work without planes all of the time. Think of the
> billions of miles of UTP in the Internet! And, these paths are
> pretty bad.
>
>
> Lee
>
>
> > [Original Message]
> > From: Havermann, Gert <Gert.Havermann@xxxxxxxxxxx>
> > To: <si-list@xxxxxxxxxxxxx>
> > Date: 10/29/2009 4:09:34 AM
> > Subject: [SI-LIST] AW: Reg. GND plane requirement
> >
> > in theory, for ideal differential transmissions, a differential
> pair will
> work without GND-planes. Reality is different though.
> > Think of it this way: For a good signal transmission you need
> impedance
> controlled traces. You don't want discontinuities on your line
> (especially
> at 3GHz speeds). Looking at one single stand alone differential pair
> on a
> PCB (there is nothing else on and around the PCB. Thats the theory
> I
> mentioned. in reality you have other traces, components copper pads
> heat
> sinks.... Every peace of conductive material in close proximity to
> your
> diff. Pair will affect its impedance. You will have dozens of
> discontinuities, doesn't sound like a transmission line, does it?
> Metal
> planes above and underneath the pair helps to control the impedance.
> You
> May think using coplanar coupled waveguide routing can do the job
> too, but
> thats not true. You will still have the impact of all conductive
> materials
> of the other layer, the impact is just a little less than without
> any
> GND-plane.
> >
> > Another Reality thing is, that there is no 100% symmetrie of a
> differential pait, thus there will be common mode currents
> generating
> common mode return currents. If you don't provide a GND-return path
> for
> these signals, you easily run into SI-trouble.
> >
> > My short answer is: Yes, GND/VCC planes are required above and
> below the
> routing layer.
> >
> > PS: si-list doesn't support attachments. You can draw things in
> ASCII, or
> upload it to some sharing place.
> >
> > BR
> > Gert
> >
> >
> >
>
-------------------------------------------------------------------------
-
> > Absender ist HARTING Electronics GmbH & Co. KG; Sitz der
> Gesellschaft:
> Espelkamp; Registergericht: Bad Oeynhausen; Register-Nr.: HRA 5596;
> pers??nlich haftende Gesellschafterin: HARTING Electronics
> Management GmbH;
> Sitz der Komplement??r-GmbH: Espelkamp; Registergericht der
> Komplement??r-GmbH: Bad Oeynhausen; Register-Nr. der
> Komplement??r-GmbH:
> HRB 8808; Gesch??ftsf??hrer: Torsten Ratzmann
> > -----Urspr??ngliche Nachricht-----
> >
> > Von: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]
> Im Auftrag von chundi srikanth
> > Gesendet: Donnerstag, 29. Oktober 2009 09:06
> > An: si-list@xxxxxxxxxxxxx
> > Betreff: [SI-LIST] Reg. GND plane requirement
> >
> > Hi Techies,
> > I have a query for you. We are designing a 12-layer mixed signal
> board.
> In which High-speed, ADC's are connected to FPGA through LVDS
> differential
> pairs. In PCB layout is it really required to have a GND/VCCV plane
> above
> and below the plane in which the differential are been routed? If it
> is not
> required then What about the cross talk effect? How can we over come
> that?
> >
> > The LVDS signals are running at 122.88MHz, 245.76MHz and 983.04Mhz
> and
> 3GHz speed. And there are few main clocks available i these signals
> too.
> >
> > Please let me know your views on this. Please see the attachment
> for my
> proposed stack up details.
> >
> >
> > Thanks & Regards
> > Srikanth Chundi
> >
> >
> >
> > ------------------------------------------------------------------
> > 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 technical documents are available at:
> > http://www.si-list.net
> >
> > List archives are viewable at:
> > //www.freelists.org/archives/si-list
> > or at our remote archives:
> > http://groups.yahoo.com/group/si-list/messages
> > 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 technical documents are available at:
> > http://www.si-list.net
> >
> > List archives are viewable at:
> > //www.freelists.org/archives/si-list
> > or at our remote archives:
> > http://groups.yahoo.com/group/si-list/messages
> > 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 technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> 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 technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
