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).
