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

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From: Vinu Arumugham (vinu@cisco.com)
Date: Tue Aug 01 2000 - 11:10:49 PDT


Ellis,

Noise on the power pin of a device usually perturbs the output signal.
Such a perturbation may be significant, especially if the output is a
clock signal. In this context, if radiated noise impinging on traces is
a concern, my point was that the clock quality is affected more by the
noise picked up by the clock trace , than by the noise picked up by the
power trace of the clock driver.

Thanks,
Vinu

e wrote:

> 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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