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[SI-LIST] Re: Ferrite bead question
- From: <art_porter@xxxxxxxxxxx>
- To: <joel@xxxxxxxxxx>, <leeritchey@xxxxxxxxxxxxx>
- Date: Tue, 11 Apr 2006 10:59:37 -0600
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
>>>>>
>>>>>
>>>>>
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