[SI-LIST] Re: Differential signals - skew - and EMC
- From: "Hany Fahmy" <hanymhfahmy@xxxxxxxxxxxxxxxxxxx>
- To: <scott@xxxxxxxxxxxxx>, <Ravinder.Ajmani@xxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 4 Nov 2013 21:29:47 +0100
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Hi Guys,
Want to share w u an EMI-webcast:
https://dl.dropboxusercontent.com/u/16537887/Introduction%20to%20EMIEMC%20Ch
allenges%20and%20Their%20Solution.wmv,
It talks about radiated emission from memory channel data-traces on a
package due to return-path-discontinuity (without enough GND stitching when
data- traces change layers) and also HDMI example for (trace+connector+cable
combination) and the critical impact of the shielding or gnding of the
connector to reduce the radiated emission.
Return-path-discontinuity and trace to connector to cable transition do
radiate and need special attention to alleviate the
return-path-discontinuity and also effective gnding of the connector to the
PCB to filter the common-mode currents from going to the cable-antennas.
Have a wonderful day,
Hany Fahmy
CEO & Chief Consultant Officer
Intelligent Solutions BVBA
hanymhfahmy@xxxxxxxxxxxxxxxxxxx
http://www.intelligentsolutionsbvba.com/
http://www.linkedin.com/pub/hany-fahmy/66/852/b11
Phone: +32471650724
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Scott McMorrow
Sent: Monday, November 4, 2013 8:41 PM
To: Ravinder.Ajmani@xxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Differential signals - skew - and EMC
Rav
True, but I find it extremely difficult to transition from PCB traces to a
cable without using a connector, or direct attach method, which totally
messes up the common mode return paths. As a result, the problem is usually
much worse than Eric discusses. Unfortunately, we do not get to use common
mode chokes for 10G, 14G and 25G signalling.
Scott
On Mon, Nov 4, 2013 at 2:35 PM, < <mailto:Ravinder.Ajmani@xxxxxxxx>
Ravinder.Ajmani@xxxxxxxx> wrote:
>
> Hi Scott,
>
> Eric's reference was to PCB traces, particularly Microstrip traces.
> Connectors are always the key cause of common currents.
>
> Regards
> Ravinder Ajmani
> HGST, a Western Digital Company
> 5601 Great Oaks Pkwy
> San Jose, CA 95119-1003
> <mailto:ravinder.ajmani@xxxxxxxx> ravinder.ajmani@xxxxxxxx
>
>
>
> *Scott McMorrow < <mailto:scott@xxxxxxxxxxxxx%20%3cscott@xxxxxxxxxxxxx>
scott@xxxxxxxxxxxxx <scott@xxxxxxxxxxxxx>>* Sent by:
> <mailto:si-list-bounce@xxxxxxxxxxxxx> si-list-bounce@xxxxxxxxxxxxx
>
> 11/04/2013 11:14 AM
> Please respond to
> <mailto:scott@xxxxxxxxxxxxx> scott@xxxxxxxxxxxxx
>
> To
> <mailto:Ravinder.Ajmani@xxxxxxxx> Ravinder.Ajmani@xxxxxxxx
> cc
> " <mailto:si-list@xxxxxxxxxxxxx> si-list@xxxxxxxxxxxxx" <
<mailto:si-list@xxxxxxxxxxxxx> si-list@xxxxxxxxxxxxx> Subject [SI-LIST] Re:
> Differential signals - skew - and EMC
>
>
>
>
> I have yet to see a trace to connector to cable interface that does
> not have what is effectively a 3D slot.
>
> On Mon, Nov 4, 2013 at 2:07 PM, < <mailto:Ravinder.Ajmani@xxxxxxxx>
Ravinder.Ajmani@xxxxxxxx> wrote:
>
> > Hi Steve,
> > I agree. However I believe when Eric talks of tight return path, he
> > assumes that there are no return path discontinuities like slots.
> >
> > Regards
> > Ravinder Ajmani
> > HGST, a Western Digital Company
> > 5601 Great Oaks Pkwy
> > San Jose, CA 95119-1003
> > <mailto:ravinder.ajmani@xxxxxxxx> ravinder.ajmani@xxxxxxxx
> >
> >
> >
> >
> > steve weir < <mailto:weirsi@xxxxxxxxxx> weirsi@xxxxxxxxxx>
> > Sent by: <mailto:si-list-bounce@xxxxxxxxxxxxx>
si-list-bounce@xxxxxxxxxxxxx
> > 11/04/2013 10:45 AM
> > Please respond to
> > <mailto:weirsi@xxxxxxxxxx> weirsi@xxxxxxxxxx
> >
> >
> > To
> > <mailto:si-list@xxxxxxxxxxxxx> si-list@xxxxxxxxxxxxx
> > cc
> >
> > Subject
> > [SI-LIST] Re: Differential signals - skew - and EMC
> >
> >
> >
> >
> >
> >
> > Eric, those are all good points. The only exception that I take is
> > to statements about signals with tight return paths not radiating.
> > They do radiate. They don't radiate excessively. Doing foolish
> > things like passing signals, including differential signals, over
> > slots increases radiated power.
> >
> > Olive oil entrepreneur and sometimes EMC engineer Don Corleone to
> > his
> > son: "Keep your signal paths close. Keep your return paths closer."
> >
> > Best Regards,
> >
> >
> > Steve
> > On 11/4/2013 8:32 AM, Eric Bogatin wrote:
> > > Hi folks-
> > >
> > >
> > > Here is an EDN blog I wrote a few weeks ago related to common
> > > currents
> > and
> > > Charles Grasso's question about how much is too much.
> > >
> > >
> > >
> > >
> >
> >
> <http://www.edn.com/electronics-blogs/test-voices/4420453/Not-all-commo>
http://www.edn.com/electronics-blogs/test-voices/4420453/Not-all-commo
> n-curr
> >
> > > ents-are-bad
> > >
> > >
> > >
> > >
> > >
> > > In my Advanced GigaBit Channel Design class, we go through the
numbers.
> > Here
> > > is the brief analysis:
> > >
> > >
> > >
> > > It takes about 3 uA of common current on Unshielded Twisted Pair
> > > (UTP)
> > cable
> > > to fail FCC part 15 class B at about 100 MHz on a 1 m long
> > > external
> > cable.
> > >
> > >
> > >
> > > It takes about 1 mV of common signal driving the cable to drive 3
> > > uA
> and
> > > fail the test.
> > >
> > >
> > >
> > > If the diff signal is 600 mV (like PCIe II), this is -55 dB of
> > > mode conversion from the source to the UTP front end to fail a
> > > radiated
> > emissions
> > > test.
> > >
> > >
> > >
> > > How can any product ship with an RJ45 connector for UTW cables if
> > > this
> > is
> > > the limit to mode conversion, at the ~ 100 MHz point?
> > >
> > >
> > >
> > > The solution is common mode chokes. With a 40 dB isolation, this
> > > allows
> > as
> > > much as -15 dB mode conversion before the common currents that get
> > through
> > > cause EMI problems.
> > >
> > >
> > >
> > > However, even with shielded twisted pair, a bad connector negates
> > > the
> > value
> > > of the shield proving the return for the common signal.
> > >
> > >
> > >
> > > How much radiated emissions you get is a function of how 360
> > > degrees
> the
> > > connector is, hard to predict.
> > >
> > >
> > >
> > > This is why, as a "habit", do everything that is free to minimize
> > > asymmetries. When it cost more, use analysis to calculate the bang
> > > for
> > the
> > > buck.
> > >
> > >
> > >
> > > This is just one example of how common currents on external cables
> > > can
> > cause
> > > radiated emissions.
> > >
> > >
> > >
> > > --eric
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > > *******************************************************
> > > Dr. Eric Bogatin, Signal Integrity Evangelist
> > >
> > > Bogatin Enterprises
> > >
> > > Setting the Standard for Signal Integrity Training web site:
> > > < <http://www.bethesignal.com/> http://www.bethesignal.com/>
<http://www.beTheSignal.com> www.beTheSignal.com
> > >
> > > Blog: < <http://www.bethesignal.com/blog>
http://www.bethesignal.com/blog> <http://www.beTheSignal.com/blog>
www.beTheSignal.com/blog
> > >
> > > Twitter @beTheSignal
> > > e: < <mailto:eric@xxxxxxxxxxxxxxx> mailto:eric@xxxxxxxxxxxxxxx>
<mailto:eric@xxxxxxxxxxxxxxx> eric@xxxxxxxxxxxxxxx
> > >
> > > Laboratory: Connected Community Networks
> > >
> > > 105 S Sunset St, Suite J
> > >
> > > Longmont, CO 80501 USA
> > >
> > > cell: 913-424-4333 skype: eric.bogatin
> > > ***********************************************
> > >
> > >
> > >
> > > From: <mailto:si-list-bounce@xxxxxxxxxxxxx>
si-list-bounce@xxxxxxxxxxxxx [mailto:
> <mailto:si-list-bounce@xxxxxxxxxxxxx> si-list-bounce@xxxxxxxxxxxxx]
> > On
> > > Behalf Of Grasso, Charles
> > >
> > > Sent: Friday, November 01, 2013 12:32 PM
> > >
> > > To: <mailto:si-list@xxxxxxxxxxxxx> si-list@xxxxxxxxxxxxx
> > >
> > > Subject: [SI-LIST] Differential signals - skew - and EMC
> > >
> > >
> > >
> > > Hello,
> > >
> > >
> > >
> > > In an ideal situation differential signals will have no skew
> > > through
> the
> > > transmission path and (as I understand it) the common-signal (emi)
> > > will
> > be
> > > very low as a result. Given that EMC is very system dependent -
> > > does
> > any
> > > one have a rule of thumb or anxiety(!) factor for how much is skew
> > > is tolerable before becoming an emissions (regulatory) Issue?
> > >
> > >
> > >
> > > Thanks in advance.
> > >
> > > Charles Grasso
> > >
> > >
> > >
> > >
> > >
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> > --
> > Steve Weir
> > IPBLOX, LLC
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> --
>
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 16 Stormy Brook Rd
> Falmouth, ME 04105
>
> (401) 284-1827 Business
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> <http://www.teraspeed.com> http://www.teraspeed.com
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--
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