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 >>>>> >>>>> >>>>> ------------------------------------------------------------------ >>>>> 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: =20 >>>>> //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 >>>>> =20 >>>>> >>>>> =20 >>>>> =20 >>>> ------------------------------------------------------------------ >>>> 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: =20 >>>> //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 >>>> =20 >>>> =20 >>>> =20 >>> ------------------------------------------------------------------ >>> 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: =20 >>> //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 >>> =20 >>> >>> >>> =20 >>> =20 > > > ------------------------------------------------------------------ > 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: =20 > //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 > =20 > > > =20 -- 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: =20 //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 =20 ------------------------------------------------------------------ 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 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