Joel, You make good points. Semiconductor manufacturers have cut back on engineering support just at the time their products have become so complex they need it badly. Trial and error is a tough way to get answers. Costs like the dickens as well! > [Original Message] > From: Joel Brown <joel@xxxxxxxxxx> > To: <leeritchey@xxxxxxxxxxxxx> > Cc: Zhangkun <zhang_kun@xxxxxxxxxx>; <si-list@xxxxxxxxxxxxx> > Date: 4/11/2006 9:43:43 AM > Subject: Re: [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 > support model. > Most have done way with FAEs and factory applications engineers unless > maybe if you are a large customer. > More often than not support is provided by email and the responses are > slow in coming and usually don't adequately address the question. > Now when I encounter an application note with a ferrite bead > recommended, the chances of the IC manufacturer providing information > that meets all your criteria is slim and none. The suggestion that the > 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 > supplying information about how their parts effect the power system. My > inclination at this point is to include a footprint for a ferrite where > 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 > measurements. The only problem is that a 0 ohm resistor would have more > inductance than vias connecting the bypass caps directly to the power > plane. This has definitely been a learning experience, thanks to all > 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. > > > > 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. > > > > > > > > > > > >> [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 > >> 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 > >> mostly on low speed I/O signals and DC power to ICs that are not running > >> 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 > >> that you seem to be saying ferrite beads only work by luck. > >> Certainly there must be situations in which they can predictably be used > >> to reduce noise. > >> > >> Regards - Joel > >> > >> > >> > >> Lee Ritchey wrote: > >> > >>> 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 > >>> > > have > > > >>> investigated, ferrite beads have been used to treat a symptom rather > >>> > > than > > > >>> the problem. The problem being that the power delivery system had too > >>> > > much > > > >>> 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, > >>> > > has > > > >>> made the part perform poorly or below spec. > >>> > >>> Yes, thousands of applications notes insist that ferrite beads be > >>> > > placed in > > > >>> the power leads and the applications engineers will demad you use them > >>> > > or > > > >>> 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 > >>> > > circuit > > > >>> will work properly if you follow the applications note exactly, the > >>> > > answer > > > >>> is still no! The reason is that there has been no analysis to prove > >>> > > that > > > >>> the use of a ferrite bead is a good idea. > >>> > >>> 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. > >>> This, even with the manufacturer's recommended capacitors attached. > >>> > > This > > > >>> data was taken from a demo board supplied by the manufacturer who was > >>> > > not > > > >>> aware of the degradation caused by the ferrite bead. > >>> > >>> The right solution is to design a power delivery system with power > >>> > > supply > > > >>> noise within the limits of the circuits being supplied and this is not > >>> > > very > > > >>> 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 > >>> > > first > > > >>> 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. > >>> > >>> > >>> > >>> > >>>> [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. > >>>> > >>>> > >>> The decoupling circuit should be considered very carefully. > >>> > >>> > >>>> In several cases, we use beads and the noise become larger. The > >>>> > > following > > > >>>> > >>>> > >>> element should be considered: > >>> > >>> > >>>> 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 ----- > >>>> From: "Joel Brown" <joel@xxxxxxxxxx> > >>>> To: <si-list@xxxxxxxxxxxxx> > >>>> Sent: Saturday, April 08, 2006 2:33 AM > >>>> Subject: [SI-LIST] Ferrite bead question > >>>> > >>>> > >>>> > >>>> > >>>>> I often see recommendations in application notes for ferrite beads on > >>>>> > >>>>> > >>> DC > >>> > >>> > >>>>> power lines. Sometimes a particular P/N is specified and sometimes > >>>>> > > just > > > >>>>> an impedance is given. > >>>>> Wouldn't I want to choose a ferrite bead that has the highest > >>>>> > > impedance > > > >>>>> possible at the frequencies of interest, and that the current rating > >>>>> > >>>>> > >>> and > >>> > >>> > >>>>> DC resistance are appropriate? Would the frequencies of interest be > >>>>> > > the > > > >>>>> fundamental switching frequency of the part and possibly the 3rd and > >>>>> > >>>>> > >>> 5th > >>> > >>> > >>>>> harmonics? > >>>>> Thanks - Joel > >>>>> > >>>>> > >>>>> > >>>>> -- Binary/unsupported file stripped by Ecartis -- > >>>>> -- Type: text/x-vcard > >>>>> -- File: joel.vcf > >>>>> > >>>>> > >>>>> ------------------------------------------------------------------ > >>>>> 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 > > > > > > > > ------------------------------------------------------------------ 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