[SI-LIST] Re: Differential signals - skew - and EMC
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Ravinder.Ajmani@xxxxxxxx
- Date: Mon, 4 Nov 2013 14:40:30 -0500
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, <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
> ravinder.ajmani@xxxxxxxx
>
>
>
> *Scott McMorrow <scott@xxxxxxxxxxxxx <scott@xxxxxxxxxxxxx>>*
> Sent by: si-list-bounce@xxxxxxxxxxxxx
>
> 11/04/2013 11:14 AM
> Please respond to
> scott@xxxxxxxxxxxxx
>
> To
> Ravinder.Ajmani@xxxxxxxx
> cc
> "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, <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
> > ravinder.ajmani@xxxxxxxx
> >
> >
> >
> >
> > steve weir <weirsi@xxxxxxxxxx>
> > Sent by: si-list-bounce@xxxxxxxxxxxxx
> > 11/04/2013 10:45 AM
> > Please respond to
> > weirsi@xxxxxxxxxx
> >
> >
> > To
> > 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-common-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/> www.beTheSignal.com
> > >
> > > Blog: <http://www.bethesignal.com/blog> www.beTheSignal.com/blog
> > >
> > > Twitter @beTheSignal
> > > e: <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: si-list-bounce@xxxxxxxxxxxxx [mailto:
> si-list-bounce@xxxxxxxxxxxxx]
> > On
> > > Behalf Of Grasso, Charles
> > >
> > > Sent: Friday, November 01, 2013 12:32 PM
> > >
> > > To: 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
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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
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--
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
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