In such a case did you place a ferrite device at both ends of the cable?  It
has worked for me.
Ralph

----- Original Message -----
From: Price, Ed <ed.pr...@cubic.com>
To: 'Ralph Cameron' <ral...@igs.net>; <d...@dsmith.org>
Cc: emc-pstc <emc-p...@majordomo.ieee.org>
Sent: Thursday, December 02, 1999 12:29 PM
Subject: RE: Ferrites can increase emissions?


> I have had the experience of putting ferrite chokes on a cable bundle
which
> connected two parts of a system. Putting the choke close to Box A, some
> radiated emissions went down and some went up. It doesn't seem reasonable
at
> first, until you remember that each box may be contributing some of the
> combined noise currents in the cable. The location of the choke affects
the
> size of the loops oppositely.
>
> Ed
>
> > -----Original Message-----
> > From: Ralph Cameron [SMTP:ral...@igs.net]
> > Sent: Thursday, December 02, 1999 12:10 PM
> > To: d...@dsmith.org
> > Cc: emc-pstc
> > Subject: Re: Ferrites can increase emissions?
> >
> >
> > Hi Doug:
> >
> > The term "ground loop" is misleading I agree. I meant to say coupling
and
> > by
> > placing the ferrite remotely from the source of the emissions only
serves
> > to
> > end load the conductors which will change the resonant length.
> >
> > In the case of placing the toroidal device on the power cord, right at
the
> > point of entry to the PCB, chassis, cabinet etc. the "coupling loop" as
> > opposed to ground loop is generally broken and the harmful
ffects( device
> > malfunction) disappear.
> >
> > I guess the point I'm trying to make is why defeat the purpose of a
> > suppression device by placing it on conductors remotely from the source
of
> > the emissions?
> >
> > Ralph
> >
> > ----- Original Message -----
> > From: Douglas C. Smith <d...@dsmith.org>
> > To: Ralph Cameron <ral...@igs.net>
> > Cc: emc-pstc <emc-p...@majordomo.ieee.org>
> > Sent: Thursday, December 02, 1999 8:43 AM
> > Subject: Re: Ferrites can increase emissions?
> >
> >
> > > Hi Ralph and all,
> > >
> > > Please define "ground loop" in your reply below. Normally, the term
> > > ground loop only has meaning at low frequencies (60 Hz and DC). At
> > > high frequencies an infinite number of loops exist and they do not
> > > require a conductor to complete them. You need to define exactly the
> > > effect for the particular case below.
> > >
> > > The special case in my article goes a level deeper than your
> > > discussion to show that ferrites at one end of a cable can either
> > > increase or decrease emissions from equipment at the opposite end by
> > > either causing an impedance match or mismatch. No "ground loops"
> > > needed to explain this phenomenon.
> > >
> > > Doug
> > >
> > >
> > > Ralph Cameron wrote:
> > > >
> > > > The purpose of a common mode choke whether it be of ferrite or
> > powdered
> > iron
> > > > is to isolate the connecting conductors from the rest of the
mainboard
> > or
> > > > chassis.   If the toridal core is correctly placed as close to the
> > source of
> > > > the emissions i.e. the PCB, the conductors which carry the emitted
> > noise
> > are
> > > > effectively isolated from high frequency noise currents to flow in
> > common
> > > > mode.  The attenutaion will vary acording to the efficiency of the
> > material
> > > > selected and a permeability of a nominal 850 is useful over the
range
> > 3-40
> > > > Mhz.
> > > >
> > > > Some of the telphone companies use common mode chokes to attempt to
> > suppress
> > > > induced RF energy on phone lines and sometimes it works.  They
alsmot
> > always
> > > > specify placement of the in line encapsulated choke (AT&T Z1000) at
> > the
> > wall
> > > > socket.  The amount of connecting cable from the phone to the wall
> > socket is
> > > > a good antenna too so picks up RF and bypasses any effect of the
> > common
> > mode
> > > > choke. Although the problem is removing the condcuted current before
> > it
> > > > becomes a problem , the same principle applies to emitted noise.
> > > >
> > > > In some cases of suppressing consumer equipment there is a dramatic
> > increase
> > > > in sensitvity to conducted currents at different
requencies( usually
> > > > higher) and this requires that the ground loop provided by the power
> > cord be
> > > > isolated from the device.   Inevitably this has cured the problem.
Be
> > aware
> > > > that any cabling connected to a device can radiate as well as
conduct
> > > > undesireable energy into the device. Ferrites provide a simple, non
> > > > intrusive, inexpensive solution to such problems.  You will see them
> > on
> > all
> > > > the better quality computer monitors and laptops.
> > > >
> > > > Ralph Cameron
> > > >
> > > > Independant EMC Consultant and suppresion of consumer electronics
> > > > (After sale)
> > > >
> > > > ----- Original Message -----
> > > > From: Douglas C. Smith <d...@dsmith.org>
> > > > To: emc-pstc <emc-p...@majordomo.ieee.org>
> > > > Sent: Wednesday, December 01, 1999 9:46 PM
> > > > Subject: Ferrites can increase emissions?
> > > >
> > > > >
> > > > > Hi All,
> > > > >
> > > > > I have noticed (like I expect many of you) that sometimes adding a
> > > > > ferrite on a cable to suppress common mode current caused
emissions
> > > > > actually increases emissions at some frequencies. After thinking
> > about
> > > > > this and trying an experiment to confirm one mechanism, I wrote up
> > an
> > > > > article describing that mechanism. I have posted the article on my
> > > > > website (emcesd.com or www.dsmith.org) as the "Technical Tidbit"
> > > > > article for December.
> > > > >
> > > > > For the case shown there, a ferrite added at the OPPOSITE end of
the
> > > > > cable from EUT2 would actually reduce emissions from EUT2 at
> > frequency
> > > > > F2. Whereas if added at EUT2, emissions from EUT2 go down but go
up
> > > > > from EUT1. Sort of an unusual case. Granted this is a special
case,
> > > > > but the result is interesting and suggests lots of other possible
> > > > > configurations with strange results.
> > > > >
> > > > > Doug
> > > > >
> > > > > --
> > > > > -----------------------------------------------------------
> > > > >     ___          _           Doug Smith
> > > > >      \          / )          P.O. Box 1457
> > > > >       =========              Los Gatos, CA 95031-1457
> > > > >    _ / \     / \ _           TEL/FAX: 408-356-4186/358-3799
> > > > >  /  /\  \ ] /  /\  \         Mobile:  408-858-4528
> > > > > |  q-----( )  |  o  |        Email:   d...@dsmith.org
> > > > >  \ _ /    ]    \ _ /         Website: http://www.dsmith.org
> > > > > -----------------------------------------------------------
> > > > >
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> > > >
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> > > --
> > > -----------------------------------------------------------
> > >     ___          _           Doug Smith
> > >      \          / )          P.O. Box 1457
> > >       =========              Los Gatos, CA 95031-1457
> > >    _ / \     / \ _           TEL/FAX: 408-356-4186/358-3799
> > >  /  /\  \ ] /  /\  \         Mobile:  408-858-4528
> > > |  q-----( )  |  o  |        Email:   d...@dsmith.org
> > >  \ _ /    ]    \ _ /         Website: http://www.dsmith.org
> > > -----------------------------------------------------------
> > >
> >
> >
> >
>
:-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-)
> Ed Price
> ed.pr...@cubic.com
> Electromagnetic Compatibility Lab
> Cubic Defense Systems
> San Diego, CA.  USA
> 619-505-2780 (Voice)
> 619-505-1502 (Fax)
> Military & Avionics EMC Services Is Our Specialty
> Shake-Bake-Shock - Metrology - Reliability Analysis
>
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>
>
>


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