[SI-LIST] Re: Am I missing something on CM Impedance vs Diff Impedance
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Erin.McPhalen@xxxxxxxxxxxxxxxxxxxxxxxxx
- Date: Fri, 18 Apr 2008 12:06:36 -0700
Erin, that is actually a pretty easy answer. You'll probably find a
Technical Tidbit about it somewhere on Doug Smith's site. Anyway, there
are a couple of reasons:
1. The external clamp on device acts on all of the signals as a whole.
This is important, because it is the whole of that dangling cable versus
the rest of the surroundings that results in radiation.
2. The clamp is on the outside of the box. An ideal filter exists right
at the chassis egress. Then there is no opportunity for antenna pick-up
of new noise between the filter and the outside world. The clamp on
filter comes close to that. Whether that is significant here depends on
how long the leads are from your circuit board to the egress.
Steve.
Erin.McPhalen@xxxxxxxxxxxxxxxxxxxxxxxxx wrote:
>
> Hi
> During testing at a EMC lab, the solution to a radiated emissions problem
> wasfound using a standard cable clamp-on type ferrite choke (MAT28) over a
> cable(no shield). In order to move the solution to the inside of the
> product, a series of bead-on-lead type ferrites (multi turn, same material)
> where used on each line.
>
> The overall reduction in emissions at the frequency in question was
> significantly less using the one per line bead-on-leads type compared to the
> clamp-on common mode choke, even though the impedance at the frequency in
> question was substantially higher(x4) with the bead-on-lead ferrite.
>
> My question is, why would a Common Mode choke with say 200 Ohms at 100 Mhz
> over all signal lines perform better than individual chokes on every signal
> line with an impedance of 800 Ohms at 100 MHz.
>
> Thanks in advance,
>
> Erin
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
>
--
Steve Weir
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
California office
(866) 675-4630 Business
(707) 780-1951 Fax
Main office
(401) 284-1827 Business
(401) 284-1840 Fax
Oregon office
(503) 430-1065 Business
(503) 430-1285 Fax
http://www.teraspeed.com
This e-mail contains proprietary and confidential intellectual property of
Teraspeed Consulting Group LLC
------------------------------------------------------------------------------------------------------
Teraspeed(R) is the registered service mark of Teraspeed Consulting Group LLC
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
