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

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From: Mayer, Mike (mikem@artesyncp.com)
Date: Fri Jul 28 2000 - 11:35:36 PDT


My initial question was about adding ferrites to the power connections of
oscillators on digital designs with power and ground planes. Why would
oscillators be treated differently than other parts? Is their something that
makes an oscillator worse than a modern CPU? I would think a modern CPU with
hundreds of high-speed I/Os would be much noisier.
 

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Mike Mayer Artesyn Communication Products, Inc
Senior Hardware Design Engineer Madison, WI
mikem@artesyncp.com http://www.artesyncp.com
<http://www.artesyncp.com/>
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-----Original Message-----
From: Vinu Arumugham [mailto:vinu@cisco.com]
Sent: Friday, July 28, 2000 11:17 AM
To: e
Cc: Mark Gill; 'Mayer, Mike'; 'si-list@silab.eng.sun.com'
Subject: Re: [SI-LIST] : Ferrites on power leads

Why is a long narrow trace in series with a bead a poor filter? I expect
long narrow traces, being more inductive, to help block high frequency
components.

Vinu

e wrote:

Mike,

In my designs where I use ferrites for power supply filtering, I always
include caps on both sides of the ferrites. Also, the filtered side is
always a generously wide trace, if not a an isolated "island".

I agree with Mark that an improperly executed ferrite filter actually
increases noise. For example, a bead between a plane and a power pin would
probably not provide much benefit if the power pin were connected to the
bead with a long, narrow trace. In such cases, it would probably be better
to connect the power pin directly to the power plane and then use a high
frequency cap next to the pin to help filter noise.

The impedance you are adding is meant to block high frequency noise, not the
desired DC power levels. Generally this technique is useful for
noise-sensitive designs, like the front end of a receiver before A/D
conversion, even if planes are used. Like any other general statements,
there are likely to be exceptions, as Mark noted, specific situations may
require specific actions.

Ellis
  

Mark Gill wrote:

  

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.

        
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=
Mike Mayer Artesyn Communication Products, Inc

Senior Hardware Design Engineer Madison, WI
mikem@artesyncp.com http://www.artesyncp.com
<http://www.artesyncp.com>
============================================================================

=

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