[SI-LIST] Re: Damping of Cavity-Mode Resonances in PCB
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 21 Nov 2011 12:03:22 -0800
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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