RE: [SI-LIST] : Ferrites on power leads

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From: Mark Gill (gillmf@nortelnetworks.com)
Date: Tue Jul 25 2000 - 08:40:11 PDT


Mike -

I have seen both effects, where it has filtered noise from loud power&ground
planes, but also, where it increased the noise at the oscillator due to how
the filtering was designed and implemented. Also, be careful about EMC
recommendations - they are not generic in nature, and can lead to a great
deal of both problems and costs if not applied to the correct situation.
Specific situations lead to specific treatments.

Regards,

Mark Gill, P.E.
EMC/Safety/NEBS Design
Nortel Networks - RTP, NC, USA

> -----Original Message-----
> From: Mayer, Mike [SMTP:mikem@artesyncp.com]
> Sent: Tuesday, July 25, 2000 11:14 AM
> To: 'si-list@silab.eng.sun.com'
> Subject: [SI-LIST] : Ferrites on power leads
>
> A couple of people here attended a seminar recently and were given advice
> on
> a design. Among the suggestions were placing ferrite beads in series with
> the power connection of all oscillators and in series with the power pins
> of
> the board connector (it is a daughter card). I'm assuming the reasoning is
> that this will "keep noise out of the power distribution system".
>
> Is this technique valid for designs that use power and ground planes?
>
> In every other case we try to minimize the impedance of power connections.
> It seems counter-intuitive to take oscillators and try to add impedance to
> their power connections. Are they really that much worse than modern CMOS
> devices with millions of transistors switching at multiples of the bus
> frequency and hundreds of sub-nanosecond I/O drivers? And does a board
> with
> power and ground planes that plugs onto a board with power and ground
> planes
> need ferrites in series with its power connections?
>
> This seems like it improperly applying rules that may have applied to
> boards
> without power and ground planes.
>
> ==========================================================================
> ==
> =
> Mike Mayer Artesyn Communication Products,
> Inc
> Senior Hardware Design Engineer Madison, WI
> mikem@artesyncp.com http://www.artesyncp.com
> ==========================================================================
> ==
> =
>
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