[SI-LIST] Am I missing something on CM Impedance vs Diff Impedance
- From: Erin.McPhalen@xxxxxxxxxxxxxxxxxxxxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 18 Apr 2008 11:44:09 -0700
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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