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

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 
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