FreeLists / si-list / [SI-LIST] Re: Am I missing something on CM Impedance vs Diff Impedance

["Todd Hubing" <HUBING@xxxxxxxxxxxxxxxxxxxx>]

Erin,
 

Here are two more things to consider when replacing a single ferrite
with individual ferrites on each wire. 

 

1. The multiple ferrites are essentially in parallel to the common-mode
current, so one ferrite on each of 4 wires would have to have 4 times
the impedance of a single ferrite over all wires to present the same
common-mode impedance.

 

2. If you have significant differential currents (e.g. ferrites on power
wires), it is easy to saturate the ferrites on the individual wires
causing them to have very little impedance at any frequency.

 

Todd

 

Clemson University Vehicular Electronics Laboratory

http://www.cvel.clemson.edu <http://www.cvel.clemson.edu/> 

 

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Erin.McPhalen@xxxxxxxxxxxxxxxxxxxxxxxxx
Sent: Friday, April 18, 2008 2:44 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Am I missing something on CM Impedance vs Diff
Impedance

 

 

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 

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