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

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From: Bob Lewandowski (Bob.Lewandowski@Vixel.com)
Date: Fri Jul 28 2000 - 12:31:12 PDT


Oscillators as a class of gadgets have major SI issues. The problem is
Susceptibility with a really big S! The effect is modulation of the output
signal by contaminants on ground, and power, feed back into the ouput, garbage
on the input (if it is controllable), and on, and on, ....... If signal quality
is anything of an issue in the performance of your circuit, it will seem like
the entire universe has something to say to your oscillator. Power is
particularly troublesome. Think of it as a combination drinking water supply,
and sewage system for your ALL your circuits. Anything you can do to improve
it's quality by filtering or whatever will not go unappreciated. 8-)

---Bob Lewandowski
    Vixel Corp.

"Mayer, Mike" wrote:

> 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. =============================================================================
>
> Mike Mayer Artesyn Communication Products, Inc
> Senior Hardware Design Engineer Madison, WI
> mikem@artesyncp.com http://www.artesyncp.com
> =============================================================================
>
> -----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.
> > >
> > >
> > > ===========================================================================
> > >
> > > =
> > > Mike Mayer Artesyn
> > > Communication Products, Inc
> > > Senior Hardware Design Engineer Madison, WI
> > > mikem@artesyncp.com http://www.artesyncp.com
> > >
> > > ===========================================================================
> > >
> > > =
> > >
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