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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: Fri, 25 Oct 2002 14:30:20 +1300
NNTP-Posting-Date: Fri, 25 Oct 2002 14:30:21 NZDT
"Robert Baer" wrote in message
> Hemon Dey wrote:
> > So I take it then nobody knows much about ferrites??? only about
> > :(
> > Hemon
> > "Robert Baer" wrote in message
> > news:3DB78E6F.F4E0BA72@earthlink.net...
> > > Hemon Dey wrote:
> > > >
> > > > You got me there, what's Tempest? do you have a link?
> > > >
> > > > "Robert Baer" wrote in message
> > > > news:3DB611D8.C62D54F9@earthlink.net...
> > > > > Hemon Dey wrote:
> > > > > >
> > > > > > Hello post,
> > > > > >
> > > > > > I'm wondering if anybody out there knows much about ferrite
> > and
> > > > chip
> > > > > > ferrites. I'm making a custom module that has USB and RS232
> > in
> > > > them.
> > > > > > The USB application note specifies the BLM18BB121SN from Murata
> > which
> > > > has
> > > > > > 120 ohm impedance at 100Mhz. How important is this impedance
> > I
> > > > was
> > > > > > looking at getting a 75 ohm equivalent due to availability -
> > > > anybody
> > > > > > see any problems doing this?
> > > > > >
> > > > > > I can't find an app note anywhere on what ferrite values to
> > to
> > > > the
> > > > > > RS232 lines (Rx,Tx, CTS, RTS) Any advice on the choice I should
> > > > making?
> > > > > > surely there must be a scientific way about it ...
> > > > > >
> > > > > > Thanks for your help.
> > > > > > Hemon
> > > > >
> > > > > The only app i know of that may use ferrites for RS-232 lines is
> > > > > Tempest.
> > >
> > > "Tempest" is a military EMI specification that virtually eliminates
> > > all forms of noise from the inside of a double shielded box (say with
> > > computer in it) to the outside.
> > > Have not tried to find links on that.
> > > I understand the method is highly classified, and the specs may also
> > > be classified.
> Well, you can buy DB25 connectors that have built-in ferrite and
> capacitive filtering.
> You can also buy feed-thru leadins of similar design.
> You did not say what kind of filtering/shielding you wanted; "tempest"
> might be a goal you wer reaching for.
> If you used controlled rise and fall times on all output signals to
> start with, less shielding and/or filtering is needed.
> Fairchild, before it got gobbled up buy National Semi, made an RS-323
> driver that was designed with these controls built into the IC; nobody
> else did that to my knowledge.
> They also made a companion receiver.
> Both devices adhered to (and exceeded) the RS-232 specifications with
> elegant simplicity, and no other known manufacturer ever did that.
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?
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