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 effects( 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 frequencies( 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 > > > > ----------------------------------------------------------- > > > > > > > > --------- > > > > This message is coming from the emc-pstc discussion list. > > > > To cancel your subscription, send mail to majord...@ieee.org > > > > with the single line: "unsubscribe emc-pstc" (without the > > > > quotes). For help, send mail to ed.pr...@cubic.com, > > > > jim_bac...@monarch.com, ri...@sdd.hp.com, or > > > > roger.volgst...@compaq.com (the list administrators). > > > > > > > > > > > > > > > > > > --------- > > > This message is coming from the emc-pstc discussion list. > > > To cancel your subscription, send mail to majord...@ieee.org > > > with the single line: "unsubscribe emc-pstc" (without the > > > quotes). For help, send mail to ed.pr...@cubic.com, > > > jim_bac...@monarch.com, ri...@sdd.hp.com, or > > > roger.volgst...@compaq.com (the list administrators). > > > > -- > > ----------------------------------------------------------- > > ___ _ 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 :-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-) --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
