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Subject: Re: Chip Ferrites
References: <3DB611D8.C62D54F9@earthlink.net> <3DB78E6F.F4E0BA72@earthlink.net> <3DB8928B.BF3D425C@earthlink.net>
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Date: Thu, 24 Oct 2002 19:44:46 -0700
NNTP-Posting-Date: Thu, 24 Oct 2002 19:43:16 PDT
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On Fri, 25 Oct 2002 14:30:20 +1300, "Hemon Dey"
>Well, TEMPEST is not what I'm looking at. EMI is more what I'm concerned
>about - interference with the rest of the circuit from high frequency
>signals, in particular the USB D+ and D- signals. The impedance rating is
>what I'm not sure about with these chip ferrites ... the ferrite I quoted
>above is good for 120ohms at 100Mhz. However I wanted to know how this value
>effects the EMI rating, and how you would design for different types of
>ferrites seeing that there are so many to choose from. In particular I was
>wondering if RS232 needed it, and how big an impedance would be required ..
>ie. is it a function of frequency? I'd imagine that a lower impedance rating
>will do less to minimise EMI - in which case I don't really see why they
>make them so low (some as low as 5 ohms). I also wonder what the effects of
>not having the ferrite beads would have on the USB chip ... the chip sits in
>close proximity to a dragonball processor and SDRAM on a 4 layer board -
>would taking the beads out stop reliable operation of the circuit?
The beads are primarily to reduce the radiated emissions from the USB
chip and other circuitry around it to the outside world. To figure out
what impedance bead you could use requires measuring EMI emanating
from your particular equipment. This will vary with design of the
system as a whole. A spectrum analyzer and sniffer probes are very
Beads will also help from having external radiation from entering your
box (susceptibility). I like to use a cell phone as a simple test
since they are very common interfering sources.
Removing the beads won't affect the reliability of your USB port
unless your in a hot RF area. Removing the beads will make it harder
to pass radiation EMI rules.
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