From: Ron Miller ([email protected])
Date: Wed Nov 17 1999 - 10:08:22 PST
Jim & Chris
The ferrite bead is needed on the shield to reduce EMI. It will absorb
circulating currents flowing through the shield returning through the
system and rack grounds. Since it is absorptive rather than inductive
it actually terminates this energy. It should be designed to absorb power
at a frequency associated with the edge rates of the signal. And it shoud
be a reasonable short at the data rate. An open at one end will eliminate
low frequency (<1 Mhz) power chopper ground currents. Capacitance can
ensure connection at higher frequencys.
Ron Miller
"Knighten, Jim L" wrote:
> Chris,
>
> In my experience we have provided a direct short from the shield of the
> connector to the chassis. This works very well and provides a good
> continuous shield envelope around the differential pair and their associated
> transceivers.
>
> Transfer impedance is a measure of outer current to inner voltage, so the
> lower the transfer impedance is, the better the shield is. So, I'd suspect
> that the ferrite bead/ac short approach is not as good.
>
> Jim
> ________________________________________________________
> Dr. Jim Knighten e-mail: [email protected]
> <mailto:[email protected]>
> Senior Consulting Engineer
> NCR
> 17095 Via del Campo
> San Diego, CA 92127 http://www.ncr.com <http://www.ncr.com>
> Tel: 858-485-2537
> Fax: 858-485-3788
>
> ***** Notice the Area Code change from 619 *****
>
> -----Original Message-----
> From: Chris Cheng [mailto:[email protected]]
> Sent: Tuesday, November 16, 1999 6:02 PM
> To: [email protected]
> Subject: RE: [SI-LIST] : FCAL DB9 cable shield
>
> jim,
> excellent point. one follow up question, should the
> shield/connector
> housing be shorted to chasis/local ground through
> a) direct short
> b) ac short through ferrite beads that have more transfer
> impedance
> then both the shield and the connector ?
> chris
>
> > -----Original Message-----
> > From: [email protected]
> > [mailto:[email protected]]On Behalf Of
> Knighten, Jim L
> > Sent: Tuesday, November 16, 1999 5:35 PM
> > To: [email protected]
> > Subject: RE: [SI-LIST] : FCAL DB9 cable shield
> >
> >
> > Fibre Channel cables are used for relatively short runs (a
> few tens of
> > meters maximum). For short cable runs, ground potential
> > differences within
> > facilities, hence ground loop currents, are not an issue
> of concern.
> >
> > What is an issue of concern is EMI from these cables. In
> all of our
> > applications, EMI is generated by the signal within the
> cable.
> > It is caused
> > by differential signal imbalance, which results in a net
> > common-mode current
> > on the signal pair. This common-mode current returns to
> its
> > source via the
> > inner surface of the cable shield. It leaks out of the
> cable assembly
> > through the inherent leakage of the cable shield (i.e.,
> the
> > surface transfer
> > impedance of the cable shield) and the though the
> connector. In general,
> > the connector is the biggest emitter of EMI. In the case
> of a properly
> > shielded connector (conducting backshell and a 360 degree
> bond to
> > the cable
> > shield) then the leakage mechanism is the connector
> shield's transfer
> > impedance. This is usually bigger than the cable shield's
> transfer
> > impedance, but is manageable. In the case of a cable
> shield that is not
> > connected to the connector shield, then the leakage of
> common-mode EMI
> > radiation is very high at the connector. That's why these
> Fibre Channel
> > cable assemblies have continuous shields from cable to
> connector and then
> > onto to the chassis wall that contains the mating
> connector.
> >
> > As far as Ethernet is concerned, the specifications
> prohibit grounding the
> > shield of the cable to the chassis through the connector
> (except
> > at a single
> > location). This is a prime reason why Ethernet is such a
> horrible EMI
> > offender. It presents a highway for noise internal to a
> chassis
> > to ride out
> > on the ungrounded cable shield.
> >
> > Jim
> > ________________________________________________________
> > Dr. Jim Knighten e-mail:
> [email protected]
> > <mailto:[email protected]>
> > Senior Consulting Engineer
> > NCR
> > 17095 Via del Campo
> > San Diego, CA 92127 http://www.ncr.com
> <http://www.ncr.com>
> > Tel: 858-485-2537
> > Fax: 858-485-3788
> >
> > ***** Notice the Area Code change from 619 *****
> >
> >
> > -----Original Message-----
> > From: Christian S. Rode
> [mailto:[email protected]]
> > Sent: Tuesday, November 16, 1999 4:57 PM
> > To: [email protected]
> > Subject: Re: [SI-LIST] : FCAL DB9
> cable shield
> >
> > Certainly any difference in ground potential
> between
> > locations could
> > cause
> > current to flow preferentially through the
> shield. I don't
> > think being
> > on
> > different phases is as much the issue as
> unbalanced use. If
> > one phase
> > of
> > your home triplex service is used more
> heavily or a large
> > motor blows a
> > phase on three-phase industrial service I
> think the
> > center-ground taps
> > will create a voltage differential.
> >
> > Capacitively or inductively coupling shields
> and ground at
> > one end
> > solves the ground current problem but you'll
> have to choose
> > a
> > coding scheme without a DC component...
> >
> > Isn't this why optocouplers were invented?
> > Aren't there safety issues, too? Seems I
> remember that it's
> > not to
> > (current?) code to run Ethernet cabling
> between buildings.
> > Not that
> > people don't do it anyway...
> >
> > > >
> > > Wouldn't this cause nasty galvanic
> currents through the
> > shield if the two
> > > pieces of equipment, attached by the cable
> with the DB9s,
> > were, say, on two
> > > different phases of a three phase power
> feed to a
> > building.
> > >
> > > Am I missing something?
> > >
> > > thanks
> > >
> > > Tom Gandy
> > > Industrial Catalyst
> > >
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--
Ronald B. Miller _\\|//_ Signal Integrity Engineer
(408)487-8017 (' 0-0 ') fax(408)487-8017
==========0000-(_)0000===========
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA 95131
[email protected], [email protected]
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