[SI-LIST] Re: Ferrite bead question
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "steve weir" <weirsi@xxxxxxxxxx>, "Hassan O. Ali" <hassan@xxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Wed, 12 Apr 2006 08:33:15 -0700
Steve,
I don't think I said that the use of ferrite beads was globally bad, or at
least I did not intend to. There are cases where they have value and, as
you point out, they have been carefully engineered to do so. I have yet to
see an applications note that contains this careful engineering. I'm still
looking.
> [Original Message]
> From: steve weir <weirsi@xxxxxxxxxx>
> To: <leeritchey@xxxxxxxxxxxxx>; Hassan O. Ali <hassan@xxxxxxxx>;
<si-list@xxxxxxxxxxxxx>
> Date: 4/11/2006 11:50:46 PM
> Subject: [SI-LIST] Re: Ferrite bead question
>
> Lee,
>
> I agree that application notes that seek to solve a non-specific
> problem with a non-specific set of assumptions are weak at best, and
> dangerous at worst. But I am similarly uncomfortable with a global
> declaration that ferrite beads harm all designs, because that is
> demonstrably false. They harm designs where they are applied
> inappropriately. Given a specific noise profile and impedance
> profile ferrite PROPERLY APPLIED can be a useful answer. It just
> depends. For example, I can readily demonstrate for certain
> situations where by using beads PROPERLY to break up a PDS cleaner
> I/O with less EMI results because by breaking up the cavity we push
> the.resonance out to where there is far less energy exciting it and
> it is at a place that is easier to detune with other methods. But to
> work, it first has to be applicable to the circumstances and it has
> to be implemented properly. And to your point, it would be a good
> idea if it is cheaper and / or technically better than
> alternatives. Misapplication of anything is bad. Just as I can
> improve some situations, I can just as easily break a working design
> by throwing beads at it, either raising the supply impedance to the
> point that the load starves to death or where I move the resonance
> right on top of a big energy peak like a clock fundamental.
>
> I would hope that most people are smart enough to realize that if
> they stick a 10 Ohm ferrite in series with a power feed that needs 1
> or 5 or 20 mOhms that they need capacitance at sufficiently low
> inductance on the load side of the ferrite. If they don't, then they
> don't understand the most basic issues of power delivery and are
> likely in very deep trouble all over their design. The bigger
> problem that I see with ferrite applications is that people don't
> bother to understand them and get burned by resonance at low
> frequency between the high Q inductor and their capacitor
> network. Ferrites are typically only resistive from about 1/10th the
> peak impedance frequency on up. Ray has posted on that issue
> repeatedly. Those problems can be solved by any of a number of well
> documented methods to insert needed damping, which have also been
> discussed on this forum at length.
>
> As Istvan has noted, if the system as a whole can tolerate higher
> noise amplitude than a small subsection, we have various
> implementation alternatives including: separate voltage regulators,
> filters with resistors, filters with ferrites, feed through filters
> with X2Y(r)s, or costly brute force scaling of the entire system. A
> proper understanding of the requirements, circumstances and skills in
> the art will lead to effective solutions.
>
> Regards,
>
>
> Steve.
> At 01:15 PM 4/11/2006, Lee Ritchey wrote:
> >Hassan,
> >
> >You have the right idea. The problem is, the loads downstream from the
> >ferrite bead need a low impedance source of current over a wider range of
> >frequencies and ferrite beads prevent this.
> >
> >It is true that the usual reason given for using a ferrite bead is there
is
> >noise on the power rails due to less than optimum design of the power
> >distribution system, which is thedesign engineer's job, not the IC
> >manufacturer. IC manufacturers need to have the power supply design
skill
> >the same as the rest of us, else how can the demonstrate and test their
own
> >parts?
> >
> >
> > > [Original Message]
> > > From: Hassan O. Ali <hassan@xxxxxxxx>
> > > To: <si-list@xxxxxxxxxxxxx>
> > > Date: 4/11/2006 11:08:30 AM
> > > Subject: [SI-LIST] Re: Ferrite bead question
> > >
> > >
> > > I always thought that ferrite beads were primarily used to surpress
noise
> >FROM (rather
> > > than INTO) high-speed and clock-like devices. I thought ferrite-beads
> >were used as a
> > > measure against power-supply corruption and also as an EMI
prevention. I
> >thought ferrite-
> > > beads were harmless at lower (switching) frequencies over which
voltage
> >regulator
> > > modules are supposed to operate as long as their impedances were low
> >enough not to cause
> > > excessive voltage ripples. If the above is correct, is it then fair to
> >blame
> > > semiconductor folks for something that board designers have to make a
> >decision on?
> > > Please someone tell me if this understanding is wrong.
> > >
> > >
> > > Best regards.
> > >
> > > Hassan.
> > >
> > >
> > >
> > > On Apr 11, <art_porter@xxxxxxxxxxx> wrote:
> > > >
> > > > Back in the good old days when I was actually designing stuff
instead
> >of =
> > > > trying to sell stuff, use of beads was considered a serious sign of
=
> > > > weakness.=20
> > > > But like anything else, if it works, fine, but you need to
understand =
> > > > why it works. If you understand why a bead works, often that will
lead =
> > > > to an insight as to how to fix the problem without using a bead.=20
> > > >
> > > > Art Porter
> > > > Agilent =20
> > > >
> > > > -----Original Message-----
> > > > From: si-list-bounce@xxxxxxxxxxxxx
> >[mailto:si-list-bounce@xxxxxxxxxxxxx] =
> > > > On Behalf Of Joel Brown
> > > > Sent: Tuesday, April 11, 2006 10:46 AM
> > > > To: leeritchey@xxxxxxxxxxxxx
> > > > Cc: Zhangkun; si-list@xxxxxxxxxxxxx
> > > > Subject: [SI-LIST] Re: Ferrite bead question
> > > >
> > > > Lee,
> > > > I can't say that I disagree with any of the points you made.
> > > > The problem is these days semiconductor companies have changed
their=20
> > > > support model.
> > > > Most have done way with FAEs and factory applications engineers
> >unless=20
> > > > maybe if you are a large customer.
> > > > More often than not support is provided by email and the responses
> >are=20
> > > > slow in coming and usually don't adequately address the question.
> > > > Now when I encounter an application note with a ferrite bead=20
> > > > recommended, the chances of the IC manufacturer providing
information=20
> > > > that meets all your criteria is slim and none. The suggestion that
> >the=20
> > > > IC manufacturers provided a current spectrum is a good one, but it
> >seems =
> > > >
> > > > they are more preoccupied with the inner workings of their chips
than=20
> > > > supplying information about how their parts effect the power system.
> >My=20
> > > > inclination at this point is to include a footprint for a ferrite
> >where=20
> > > > I think it might help, then make measurements with various ferrite
> >beads =
> > > >
> > > > and a 0 ohm resistor and see which part results in lowest noise=20
> > > > measurements. The only problem is that a 0 ohm resistor would have
> >more=20
> > > > inductance than vias connecting the bypass caps directly to the
power=20
> > > > plane. This has definitely been a learning experience, thanks to
all=20
> > > > that posted responses.
> > > >
> > > > Joel
> > > >
> > > >
> > > > Lee Ritchey wrote:
> > > > > Joel,
> > > > >
> > > > >
> > > > > As others have observed, when the solution using a ferrite bead
has =
> > > > been
> > > > > engineered to satisfy the following set of conditions it is
warranted
> >=
> > > > and
> > > > > helpful.
> > > > >
> > > > > 1. There is a problem that needs fixing.
> > > > > 2. The proposed solution fixes the problem.
> > > > > 3. The proposed solution does not cause new problems.
> > > > > 4. The proposed solution is the best solution to the problem.
> > > > >
> > > > > When you apply this set of tests to the usual applications note
> > > > > recommendation, it often fails test number 1. The proponent has
not
> > > > > acertained that there is a problem and what it actually looks
like.
> > > > >
> > > > > I put this question to the students in my classes at UC
Berkeley, =
> > > > nearly
> > > > > always practicing engineers. Can you show me an applications
note =
> > > > with
> > > > > analysis supporting the use of ferrite beads? So far, there have
> >been =
> > > > no
> > > > > such examples. I keep hoping, but know that most applications
notes =
> > > > are
> > > > > not prepared that way. Soon as I find one, I'll report it in
this =
> > > > forum
> > > > > and show it in my classes- and give its author high praise for
doing =
> > > > good
> > > > > engineering.
> > > > >
> > > > > More commonly, they are justified because "we have always done
it =
> > > > this
> > > > > way." Further, there has usually not been any exhaustive testing
to =
> > > > insure
> > > > > the solution is valid. That is in no way good engineering. =20
> > > > >
> > > > > I'll accept a recommendation that is accompanied by supporting =
> > > > engineering
> > > > > analysis. Less, seems to me to be guessing and that's a good way
to =
> > > > get in
> > > > > trouble with modern e lectronic components.
> > > > >
> > > > >
> > > > >
> > > > >
> > > > > =20
> > > > >> [Original Message]
> > > > >> From: Joel Brown <joel@xxxxxxxxxx>
> > > > >> To: <leeritchey@xxxxxxxxxxxxx>
> > > > >> Cc: Zhangkun <zhang_kun@xxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> > > > >> Date: 4/10/2006 10:17:06 AM
> > > > >> Subject: Re: [SI-LIST] Re: Ferrite bead question
> > > > >>
> > > > >> Lee,
> > > > >>
> > > > >> After reading the responses and giving some more thought to this,
> >I=20
> > > > >> agree that in some situations the use of ferrite beads can be =
> > > > detrimental.
> > > > >> In the design I am currently working on, the ferrite beads are
> >used=20
> > > > >> mostly on low speed I/O signals and DC power to ICs that are not
=
> > > > running=20
> > > > >> at very high speeds.
> > > > >> The highest speed part is a 100 MHz PCI Express clock buffer, and
> >the =
> > > >
> > > > >> manufacturer does recommend to use a ferrite bead.
> > > > >> While I agree your statements have some merit, I find it
> >problematic=20
> > > > >> that you seem to be saying ferrite beads only work by luck.
> > > > >> Certainly there must be situations in which they can predictably
be =
> > > > used=20
> > > > >> to reduce noise.
> > > > >>
> > > > >> Regards - Joel
> > > > >>
> > > > >>
> > > > >>
> > > > >> Lee Ritchey wrote:
> > > > >> =20
> > > > >>> Joel,
> > > > >>>
> > > > >>> I agree with all the respondents who have cautioned you on the
use =
> > > > of
> > > > >>> ferrite beads in the power leads of devices. In all of the
cases, I
> > > > >>> =20
> > > > > have
> > > > > =20
> > > > >>> investigated, ferrite beads have been used to treat a symptom
rather
> > > > >>> =20
> > > > > than
> > > > > =20
> > > > >>> the problem. The problem being that the power delivery system
had =
> > > > too
> > > > >>> =20
> > > > > much
> > > > > =20
> > > > >>> noinse or ripple on it.
> > > > >>>
> > > > >>> Lately, I have seen many cases where placing a ferrite bead in
the =
> > > > power
> > > > >>> lead of a device, very often the power lead of very high speed =
> > > > serdes,
> > > > >>> =20
> > > > > has
> > > > > =20
> > > > >>> made the part perform poorly or below spec.
> > > > >>>
> > > > >>> Yes, thousands of applications notes insist that ferrite beads
be
> > > > >>> =20
> > > > > placed in
> > > > > =20
> > > > >>> the power leads and the applications engineers will demad you
use =
> > > > them
> > > > >>> =20
> > > > > or
> > > > > =20
> > > > >>> they won't guarantee the circuit will work properly. The other
> >side =
> > > > of
> > > > >>> that same coin is that when you ask if they will guarantee that
the
> > > > >>> =20
> > > > > circuit
> > > > > =20
> > > > >>> will work properly if you follow the applications note exactly,
the
> > > > >>> =20
> > > > > answer
> > > > > =20
> > > > >>> is still no! The reason is that there has been no analysis to
prove
> > > > >>> =20
> > > > > that
> > > > > =20
> > > > >>> the use of a ferrite bead is a good idea.=20
> > > > >>>
> > > > >>> I have measured wavforms of a 3.12 GB/S serial link where the
eye =
> > > > just
> > > > >>> barely makes the minimum signal amplitude with the ferrite bead
in =
> > > > place
> > > > >>> and has large amounts of margin when the ferrite bead is
shorted =
> > > > out.=20
> > > > >>> This, even with the manufacturer's recommended capacitors
> >attached.=20
> > > > >>> =20
> > > > > This
> > > > > =20
> > > > >>> data was taken from a demo board supplied by the manufacturer
who =
> > > > was
> > > > >>> =20
> > > > > not
> > > > > =20
> > > > >>> aware of the degradation caused by the ferrite bead.
> > > > >>>
> > > > >>> The right solution is to design a power delivery system with
power
> > > > >>> =20
> > > > > supply
> > > > > =20
> > > > >>> noise within the limits of the circuits being supplied and this
is =
> > > > not
> > > > >>> =20
> > > > > very
> > > > > =20
> > > > >>> difficult to do.
> > > > >>>
> > > > >>> I have done the SI rule generation for three terabit routers
with
> > > > >>> thousands of 3.125 GB/S and 4.8 GB/S serial links in them and
used =
> > > > no
> > > > >>> ferrite beads in them any where and they worked to
specification the
> > > > >>> =20
> > > > > first
> > > > > =20
> > > > >>> time wit hproper margins. So maybe people who want you to use =
> > > > ferrite
> > > > >>> beads should be challenged with why they want you to add these
=
> > > > parts.
> > > > >>>
> > > > >>> My fellow SI engineers call ferrite beads "voodoo" beads and
"get =
> > > > lucky"
> > > > >>> beads for good reason.
> > > > >>>
> > > > >>> This is not likely to make the ferrite bead salesmen happy, I'm
=
> > > > sure.
> > > > >>>
> > > > >>>
> > > > >>> =20
> > > > >>> =20
> > > > >>>> [Original Message]
> > > > >>>> From: Zhangkun <zhang_kun@xxxxxxxxxx>
> > > > >>>> To: <joel@xxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> > > > >>>> Date: 4/10/2006 1:14:21 PM
> > > > >>>> Subject: [SI-LIST] Re: Ferrite bead question
> > > > >>>>
> > > > >>>> Dear Joel
> > > > >>>>
> > > > >>>> As to my experience, it is not enough to select bead based on
the =
> > > > P/N.
> > > > >>>> =20
> > > > >>>> =20
> > > > >>> The decoupling circuit should be considered very carefully.
> > > > >>> =20
> > > > >>> =20
> > > > >>>> In several cases, we use beads and the noise become larger. The
> > > > >>>> =20
> > > > > following
> > > > > =20
> > > > >>>> =20
> > > > >>>> =20
> > > > >>> element should be considered:
> > > > >>> =20
> > > > >>> =20
> > > > >>>> 1.The resonance between bead and capacitor.
> > > > >>>> 2.The frequency span of the noise source.
> > > > >>>>
> > > > >>>> A simulation is suggested. It is very simple.
> > > > >>>>
> > > > >>>> Hope this is helpful
> > > > >>>>
> > > > >>>> Best Regards
> > > > >>>>
> > > > >>>> Zhangkun
> > > > >>>> 2006.4.10
> > > > >>>> ----- Original Message -----=20
> > > > >>>> From: "Joel Brown" <joel@xxxxxxxxxx>
> > > > >>>> To: <si-list@xxxxxxxxxxxxx>
> > > > >>>> Sent: Saturday, April 08, 2006 2:33 AM
> > > > >>>> Subject: [SI-LIST] Ferrite bead question
> > > > >>>>
> > > > >>>>
> > > > >>>> =20
> > > > >>>> =20
> > > > >>>>> I often see recommendations in application notes for ferrite
> >beads =
> > > > on
> > > > >>>>> =20
> > > > >>>>> =20
> > > > >>> DC=20
> > > > >>> =20
> > > > >>> =20
> > > > >>>>> power lines. Sometimes a particular P/N is specified and
sometimes
> > > > >>>>> =20
> > > > > just=20
> > > > > =20
> > > > >>>>> an impedance is given.
> > > > >>>>> Wouldn't I want to choose a ferrite bead that has the highest
> > > > >>>>> =20
> > > > > impedance=20
> > > > > =20
> > > > >>>>> possible at the frequencies of interest, and that the current
=
> > > > rating
> > > > >>>>> =20
> > > > >>>>> =20
> > > > >>> and=20
> > > > >>> =20
> > > > >>> =20
> > > > >>>>> DC resistance are appropriate? Would the frequencies of
interest =
> > > > be
> > > > >>>>> =20
> > > > > the=20
> > > > > =20
> > > > >>>>> fundamental switching frequency of the part and possibly the
3rd =
> > > > and
> > > > >>>>> =20
> > > > >>>>> =20
> > > > >>> 5th=20
> > > > >>> =20
> > > > >>> =20
> > > > >>>>> harmonics?
> > > > >>>>> Thanks - Joel
> > > > >>>>>
> > > > >>>>>
> > > > >>>>>
> > > > >>>>> -- Binary/unsupported file stripped by Ecartis --
> > > > >>>>> -- Type: text/x-vcard
> > > > >>>>> -- File: joel.vcf
> > > > >>>>>
> > > > >>>>>
> > > > >>>>>
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> > > >
> > > >
> > > > -- Binary/unsupported file stripped by Ecartis --
> > > > -- Type: text/x-vcard
> > > > -- File: joel.vcf
> > > >
> > > >
> > > > ------------------------------------------------------------------
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> > > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject
field
> > > >
> > > > or to administer your membership from a web page, go to:
> > > > <a
>
>href='//www.freelists.org/webpage/si-list'>//www.freelists.org/we
b
> >page/si-
> > > list</a>
> > > >
> > > > For help:
> > > > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> > > >
> > > > List FAQ wiki page is located at:
> > > > <a
> >href='http://si-list.org/wiki/wiki.pl?Si-List_FAQ'>http://si-
> > > list.org/wiki/wiki.pl?Si-List_FAQ</a>
> > > >
> > > > List technical documents are available at:
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> >href='http://www.si-list.org'>http://www.si-list.org</a>
> > > >
> > > > List archives are viewable at: =20
> > > > <a href='//www.freelists.org/archives/si-
> > > list'>//www.freelists.org/archives/si-list</a>
> > > > or at our remote archives:
> > > > <a href='http://groups.yahoo.com/group/si-
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> > > > =20
> > > >
> > > > ------------------------------------------------------------------
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> > > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject
field
> > > >
> > > > or to administer your membership from a web page, go to:
> > > > <a
>
>href='//www.freelists.org/webpage/si-list'>//www.freelists.org/we
b
> >page/si-
> > > list</a>
> > > >
> > > > For help:
> > > > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> > > >
> > > > List FAQ wiki page is located at:
> > > > <a
> >href='http://si-list.org/wiki/wiki.pl?Si-List_FAQ'>http://si-
> > > list.org/wiki/wiki.pl?Si-List_FAQ</a>
> > > >
> > > > List technical documents are available at:
> > > > <a
> >href='http://www.si-list.org'>http://www.si-list.org</a>
> > > >
> > > > List archives are viewable at:
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> > > list'>//www.freelists.org/archives/si-list</a>
> > > > or at our remote archives:
> > > > <a href='http://groups.yahoo.com/group/si-
> > > list/messages'>http://groups.yahoo.com/group/si-list/messages</a>
> > > > Old (prior to June 6, 2001) list archives are viewable at:
> > > > <a
> > href='http://www.qsl.net/wb6tpu'>http://www.qsl.net/wb6tpu</a>
> > > >
> > > >
> > > >
> > > ------------------------------------------------------------------
> > > To unsubscribe from si-list:
> > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> > >
> > > or to administer your membership from a web page, go to:
> > > //www.freelists.org/webpage/si-list
> > >
> > > For help:
> > > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> > >
> > > List FAQ wiki page is located at:
> > > http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> > >
> > > List technical documents are available at:
> > > http://www.si-list.org
> > >
> > > List archives are viewable at:
> > > //www.freelists.org/archives/si-list
> > > or at our remote archives:
> > > http://groups.yahoo.com/group/si-list/messages
> > > Old (prior to June 6, 2001) list archives are viewable at:
> > > http://www.qsl.net/wb6tpu
> > >
> >
> >
> >------------------------------------------------------------------
> >To unsubscribe from si-list:
> >si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> >
> >or to administer your membership from a web page, go to:
> >//www.freelists.org/webpage/si-list
> >
> >For help:
> >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >
> >List FAQ wiki page is located at:
> > http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> >
> >List technical documents are available at:
> > http://www.si-list.org
> >
> >List archives are viewable at:
> > //www.freelists.org/archives/si-list
> >or at our remote archives:
> > http://groups.yahoo.com/group/si-list/messages
> >Old (prior to June 6, 2001) list archives are viewable at:
> > http://www.qsl.net/wb6tpu
> >
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
> List FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
> List technical documents are available at:
> http://www.si-list.org
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
