[SI-LIST] Re: Differential signals - skew - and EMC
- From: Ravinder.Ajmani@xxxxxxxx
- To: scott@xxxxxxxxxxxxx
- Date: Mon, 4 Nov 2013 11:35:49 -0800
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>
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11/04/2013 11:14 AM
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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
http://www.teraspeed.com
Teraspeed® is the registered service mark of
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