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From: "Hemon Dey"
References: <3DB611D8.C62D54F9@earthlink.net> <3DB78E6F.F4E0BA72@earthlink.net> <3DB8928B.BF3D425C@earthlink.net>
Subject: Re: Chip Ferrites
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Tue, 29 Oct 2002 11:58:40 +1300
NNTP-Posting-Date: Tue, 29 Oct 2002 11:58:39 NZDT
"qrk" wrote in message
> 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
> >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
> >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
> >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
> >not having the ferrite beads would have on the USB chip ... the chip sits
> >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.
Thanks QRK ... that's very helpful information. Infact thanks to all for the
input. Ferrite beads I have found to be a little dear for what you need them
for, however high freq chip ferrites only cost 2 cents or cheaper. I think
we will have to put them in if it will help with passing the EMI tests,
which probably doesn't exclude the RS232 lines. Alowances have already been
made to include them at a later stage on the RTS, CTS, RX and TX lines, I've
found it esp helpful to design with redundancy in mind. It's good to know
however that the system will still work regardless of the ferrites, unless
in highly stressed RF areas. At the end of the day we are not making an RF
product, but it may have RF stuff in the future with Bluetooth on the
horizon and 802.11 becoming more useful everyday.
Once again, appreciate the input.
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