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

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From: e (evillaf@home.com)
Date: Fri Jul 28 2000 - 19:24:28 PDT


Vinu,

Long narrow traces are also very good antennae(s?) for picking up
radiated noise, which defeats the purpose of the bead.

Ellis

"Gaboian, Jerry" wrote:

> Vinu,The long narrow traces are inductive as you would expect. A
> problem with this is when you start adding high frequency bypass
> capacitors on the trace, the inductance of the trace can cancel the
> properties of the capacitor.Regards,Jerry -----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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