[SI-LIST] Re: Ferrite bead question
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Mark Alexander <mark.alexander@xxxxxxxxxx>, silist <si-list@xxxxxxxxxxxxx>
- Date: Mon, 10 Apr 2006 18:49:42 -0400
Mark
You're right. Thanks for the correction I pressed the buttons on the
calculator right, but my brain failed to transcribe them correctly.
Power system Ztarget = 100mV/1.28Amps = 78 mOhms
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
Mark Alexander wrote:
> Scott,
>
> I think you've got a typo:
>
> Power system Ztarget = 100mV/20mA = 78 mOhms.
>
> Shouldn't it be divided by the total current transient of 1.28A? That
> gives the correct answer...
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Scott McMorrow
> Sent: Monday, April 10, 2006 2:31 PM
> To: ericg@xxxxxxxxxxx
> Cc: kfrobinson@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Ferrite bead question
>
> Hmmm ...
> Lets take an example of a 533 MT/s DDR bus, with a 266 MHz frequency.
>
> One bit time = 1.875 ns
> One period = 3.75 ns
> Let risetime = 400 ps (10%-90%)
> Fknee = .35/400ps = 875 MHz
> Assume that driver current is 20 mA/driver
> Assume that 64 drivers switch simultaneously
> Total current transient = 1.28 Amps in 400 ps.
> Assume total voltage swing = 2V pp for SSTL-2.5
> Assume a 100 mV power noise allowance (5%)
> Power system Ztarget = 100mV/20 mA = 78 mOhms
>
> Computing the Fourier Coefficients for a trapezoid, using a cute little
> calculator I found here:
> http://www.eecircle.com/applets/001/001.html, with 400 ps rise time
> we get:
>
> Harmonic component
> DC - 0.500
> 1st - 0.312 (266 MHz)
> 3rd - 0.089 (800 MHz)
> 5th - 0.037 (1.33 GHz)
> 7th - 0.013 (1.87 GHz)
> 9th - 0.001 (2.4 GHz)
>
> For this particular driver, 62% of the AC energy is contained at the
> fundamental frequency, 17.8% is contained at the 3rd harmonic, 7.4% is
> contained at the 5th harmonic, and 2.6% is contained at the 7th
> harmonic. If we use our one pole filter bandwidth formula of BW =
> 0.35/risetime, then we are covered up to the 3rd harmonic of the signal
> fundamental. But, 20% of the signal energy is still carried in harmonics
>
> above the 3rd. Our total power system noise in the time domain is
> approximately:
>
> Vnoise = 1.28 Amps x ( Z@266 MHz x 0.62 + Z@800MHz x 0.178 + Z@xxxx GHz
> x 0.074 + Z@xxxx GHz x 0.026 + Z@xxx GHz x 0.02)
>
> It is not unusual for power system impedance at high frequency
> resonances to exhibit a 100:1 increase over the Ztarget at lower
> frequencies. What if our power system had flat impedance of 78 mOhms
> out to 1.5 GHz and then climbed to a resonance peak of 7.8 ohms at 1.87
> GHz.
>
> Ignoring the 9th harmonic, the noise would be:
>
> Vnoise = 1.28 Amps x ( 0.078 x 0.62 + 0.078 x 0.178 + 0.078 x 0.074 +
> 7.8 x 0.026)
> Vnoise = 1.28 Amps x ( 0.078 Ohms x .872 = 7.8 x 0.026)
> Vnoise = 1.28 Amps x ( 0.068 + .2028)
> Vnoise = 87 mV below 1.5 GHz + 260 mV @ 1.87 GHz
>
> With a reactive network with a resonance such as this, the resonance
> peak would generally be out of phase with the lower frequency noise, so
> the result would not be generally additive. However, that 1.87 GHz peak
>
> would dominate, and show up as ringing in the power system. I suspect
> it would also show up as a healthy signature in the EMI profile,
> although I am not expert in these matters. Which is why those silly EMI
>
> engineers are often concerned about harmonics of the fundamental like
> the 7th, 9th and even the 11th.
>
> This particular example is contrived and not quite exact. But, it is
> not unusual at all in designs with planes fragmented like a map of the
> Balkans, as Dr Johnson would say. Or in large planes, where planar
> resonances are right in the middle of all the switching harmonics.
> We've measured power systems where there are some pretty interesting
> high frequency resonance peaks, which show up quite well in both the
> power system and time domain signal measurements, and are above Fknee.
> Remember, according to Dr Johnson, amplitude at the knee frequency is
> 6.8 dB below the 20 dB/decade rolloff seen in a signal's spectral
> energy. At the 9th harmonic (which can be approximated as one decade
> down) we have a total spectral attenuation of 26.8dB, or 21.9 times
> lower the the fundamental. It does not take much resonance "gain" over
> the power system target impedance to dwarf noise at the fundamental with
>
> noise at a higher harmonic ... even one beyond the knee. What's quite
> interesting about this, is that this sort of power system problem would
> show up as high frequency ringing on a DDR memory bus signal. But, it
> would not have been diagnosed with standard signal integrity tools, and
> would only show up when a complete simulation of simultaneous switching
> of the bus was performed that included modeling of the device, package,
> power planes, bypass capacitors, mounting pads and vias.
>
> This goes back to what I term the "Wack-a-Mole" phenomenon in power
> distributions systems. If you do not fix the fundamental problem of
> power system resonances, then all you're doing is moving energy around
> from one place to another, just to see it pop up out of another hole.
>
>
> regards,
>
> scott
>
>
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
> (401) 284-1827 Business
> (401) 284-1840 Fax
>
> http://www.teraspeed.com
>
> Teraspeed(r) is the registered service mark of
> Teraspeed Consulting Group LLC
>
>
>
> Eric Goodill wrote:
>
>> Ken,
>> Yeah, that's pretty much exactly what I meant by "pick a probable
>> maximum frequency," but I didn't say that in my post. Sorry.
>>
>> -Eric
>>
>> kfrobinson@xxxxxxxxxx wrote:
>>
>>
>>> Eric,
>>> The frequency components in a digital system can be estimated from DC
>>>
> to
>
>>> .35/(rise time). The IC manufactures do give 10-90 rise time. Pick
>>>
> the
>
>>> fastest rise time IC in your system.
>>> Ken
>>>
>>> -----Original Message-----
>>> From: si-list-bounce@xxxxxxxxxxxxx
>>>
> [mailto:si-list-bounce@xxxxxxxxxxxxx]
>
>>> On Behalf Of Eric Goodill
>>> Sent: Monday, April 10, 2006 12:16 PM
>>> To: si-list@xxxxxxxxxxxxx
>>> Subject: [SI-LIST] Re: Ferrite bead question
>>>
>>> Howdy,
>>> One thing I've never understood about designing the frequency
>>>
> response
>
>>> of a power delivery system is what are the frequency requirements?
>>>
> That
>
>>> is, where does the vendor give me the spectrum of the current demand
>>>
> for
>
>>> the part? They don't (I'm skipping over they it'd be tough). So the
>>>
> only
>
>>> solution I can see is to pick a probable maximum frequency and say
>>>
> from
>
>>> DC to that frequency we need less than X ohms impedance based on some
>>> assumptions about the peak current needs (related to lack of current
>>> spectrum). This is probably over designing, but I don't have any
>>>
> other
>
>>> data to use. Do others feel the same way?
>>>
>>> -Eric
>>>
>>> Scott McMorrow wrote:
>>>
>>>
>>>
>>>> Joel,
>>>> I think Lee is being a little bit black and white, but not without
>>>> good reason.. Whether or not ferrites "work" in a power filtering
>>>> design is a matter of whether the engineering was performed. What
>>>>
> I
>
>>>> think Lee often sees are systems where no engineering has gone into
>>>> the design of ferrite, and they have been thrown into a circuit
>>>> without thought. Often because "that's the way we've always done it"
>>>>
>
>
>>>> or "that's that way the competitors do it" or because "that's the
>>>>
> only
>
>>>>
>>>>
>>>
>>>
>>>> ferrite we have in our parts system." The inductance of a ferrite
>>>>
> can
>
>>>>
>>>>
>>>
>>>
>>>> interact with the power system and capacitors to form a pretty nasty
>>>>
>
>
>>>> resonance. This resonance often sits in the low frequency region,
>>>> around the VRM output transition region, and has a strong tendency
>>>>
> to
>
>>>> cause peaking in the 100 kHz to 1 MHz range. This peaking can cause
>>>>
>
>
>>>> noise modulation of the Serdes, which Lee has often observed. If
>>>>
> the
>
>>>> PCB power delivery network has a lower impedance in the frequencies
>>>> that affect the Serdes, than they ferrite filter does, then shorting
>>>>
>
>
>>>> the ferrite will help. But if a ferrite filter network is correctly
>>>>
>
>
>>>> designed, it is well matched and does not cause peaking.
>>>>
>>>> Whether or not noise injected into a Serdes affects it's output is a
>>>>
>
>
>>>> function of the internal PLL/DLL design. This is hardly ever
>>>> specified, but can be measured.
>>>>
>>>> Scott
>>>>
>>>> Scott McMorrow
>>>> Teraspeed Consulting Group LLC
>>>> 121 North River Drive
>>>> Narragansett, RI 02882
>>>> (401) 284-1827 Business
>>>> (401) 284-1840 Fax
>>>>
>>>> http://www.teraspeed.com
>>>>
>>>> Teraspeed(r) is the registered service mark of Teraspeed Consulting
>>>> Group LLC
>>>>
>>>>
>>> [snipped off the reset of the thread]
>>>
>>>
>>> -- Binary/unsupported file stripped by Ecartis --
>>> -- Type: text/x-vcard
>>> -- File: ericg.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
>>>
>>>
>>>
>>>
>>>
>> -- Binary/unsupported file stripped by Ecartis --
>> -- Type: text/x-vcard
>> -- File: ericg.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
Other related posts:
