[SI-LIST] Re: Damping of Cavity-Mode Resonances in PCB
- From: steve weir <weirsi@xxxxxxxxxx>
- To: bradb@xxxxxxxxxxx
- Date: Mon, 21 Nov 2011 12:27:37 -0800
Brad, we may have a definitions problem here. A resonance does its
damage by returning stored memories of wavefronts past at inconvenient
times via reflections. A damped resonance reduces the magnitude of the
returned energy. Once the amount of reflected energy is so low as to
be inconsequential I argue that it would be pedantic to distinguish that
from zero reflected energy.
Until he elaborates more, I don't know what Chris is after. I can only
speak to the physics.
Best Regards,
Steve.
On 11/21/2011 12:14 PM, Brad Brim wrote:
> hi Steve,
>
> I did not say it failed to dampen a resonance, I only said it does not serve
> to eliminate the resonance. I am familiar with Chris's applications and in
> many cases he may not be able to apply the commercially available thin
> dielectrics, let alone one less than 5um thickness, as cited from Istvan's
> paper. Our comments are each equally correct and equally applicable to
> Chris.
>
> cheers,
> -Brad
>
>> -----Original Message-----
>> From: si-list-bounce@xxxxxxxxxxxxx
>> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of steve weir
>> Sent: Monday, November 21, 2011 12:03 PM
>> To: si-list@xxxxxxxxxxxxx
>> Subject: [SI-LIST] Re: Damping of Cavity-Mode Resonances in PCB
>>
>> Brad, thin dielectrics most definitely do suppress cavity
>> modal resonances. They do so by reducing the Q. Inductance
>> goes down, but skin resistance remains fixed, and for any dK
>> and tangent loss coefficient dielectric loss increases.
>> There are a number of papers out there that show these
>> characteristics. Istvan published a paper circa
>> 2004 that showed for dK of 4 and tangent loss of 0.02 that
>> around 0.1 -
>> 0.2 mils thickness, cavity Q falls to one. Already having
>> served its
>> primary purpose, an empty beer can still makes a terrific
>> resonant cavity, very thin cavities are quite another matter.
>>
>> Best Regards,
>>
>>
>> Steve.
>> On 11/21/2011 11:42 AM, Brad Brim wrote:
>>> hi Chris,
>>>
>>> By "cavity modes" do you mean power plane resonances? If
>> so, is your
>>> objective to (a) drive down device impedance, (b) drive down
>>> emissions, (c) both? Is your goal to only change plane shapes,
>>> stackup, etc. or are you also able to modify decap/emicap
>> placement and selection?
>>> You cannot eliminate all resonance from a plane cavity of
>> electrically
>>> large size with only shape or stackup changes. The modes will have
>>> different spatial distribution as you change the shape but
>> you cannot eliminate them.
>>> As you remove plane cavity area you will reduce spectral
>> mode density.
>>> The big question is how much plane cavity are you able to
>> eliminate?
>>> Using thinner dielectrics does nothing to eliminate resonances. It
>>> certainly raises capacitance but its most significant
>> contribution is
>>> to reduce inductance (not just for spreading but also for
>> mounting caps).
>>> If you are trying to eliminate resonances by adding caps it is
>>> interesting to note that the best solution is not a
>> sequential process
>>> such as: (1) simulate to find the first resonance peak, (2)
>> add a cap
>>> near the resonance peak, repeat (1-2). This process yields reduced
>>> resonances. But if you are able to add multiple caps
>> simultaneously by
>>> considering broadband impedance across the entire PDN, then
>> you find
>>> better solutions with a smaller set of caps.
>>>
>>> cheers,
>>> -Brad
>>> Sigrity
>>>
>>>> -----Original Message-----
>>>> From: si-list-bounce@xxxxxxxxxxxxx
>>>> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Cristian Gozzi
>>>> Sent: Monday, November 21, 2011 7:52 AM
>>>> To: si-list@xxxxxxxxxxxxx
>>>> Subject: [SI-LIST] Damping of Cavity-Mode Resonances in PCB
>>>>
>>>> Hi SI expert
>>>> I'm looking for a good article/paper that describe techniques to
>>>> reduce or suppress cavity modes in PCB, just by working on plane
>>>> shape dimension, pwr to gnd dilectric thickness etc.
>>>>
>>>> Thanks in advance for your suggestion
>>>>
>>>> regards
>>>> Cris
>>>>
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>>
>> --
>> Steve Weir
>> IPBLOX, LLC
>> 150 N. Center St. #211
>> Reno, NV 89501
>> www.ipblox.com
>>
>> (775) 299-4236 Business
>> (866) 675-4630 Toll-free
>> (707) 780-1951 Fax
>>
>>
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>
--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV 89501
www.ipblox.com
(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax
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