[SI-LIST] Re: Ferrite bead question
- From: steve weir <weirsi@xxxxxxxxxx>
- To: leeritchey@xxxxxxxxxxxxx,"Hassan O. Ali" <hassan@xxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Tue, 11 Apr 2006 23:50:47 -0700
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 --
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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
Other related posts:
