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

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From: e (evillaf@home.com)
Date: Mon Jul 31 2000 - 20:53:41 PDT


Vinu,

As far as antennae go, no doubt the 5" long output trace is more
susceptible to noise (of a wider frequency spectrum). I believe the
discussion is about noise on power supplies, more specifically how to
keep that noise out of the power supply, and in that sense maybe you can
clarify for me how the noise picked up by the output trace can be a
bigger problem to the power supply pins than the noise on the power
leads/traces themselves. I don't really see your point. Please educate
me.

Ellis

Vinu Arumugham wrote:

> If a 5 mil wide, 500 mil long power trace of an oscillator picks up
> enough radiated noise to disrupt the system, I would expect the 5 in.
> long clock trace at the output of the oscillator to be a bigger
> problem.
>
> Vinu
>
>
> e wrote:
>
>> 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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