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

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From: Vinu Arumugham (vinu@cisco.com)
Date: Mon Jul 31 2000 - 10:01:35 PDT


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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