[SI-LIST] Re: Damping of Cavity-Mode Resonances in PCB
- From: Cristian Gozzi <cristian.gozzi@xxxxxxxxx>
- To: Brad Brim <bradb@xxxxxxxxxxx>
- Date: Tue, 22 Nov 2011 18:51:00 +0100
Hi folks
thanks to everybody for your feedback
Let me describe better my application!
I have a backplane with one Digital_GND over one Analog_GND
of course these 2 nets are separated each others, but due to big plane
shape dimension I got resonance modes starting from 220 MHz
I don't think I can use decoupling capacitor in this case, so I guess I can
only work with plane shape dimension and dielectric thickness
My idea is to move/shift at higher frequency these modes.
I don't want to couples these low frequency modes to digital signals that
operates at 400-800 Mbps
Thanks for any further comments
Regards
Cris
2011/11/21 Brad Brim <bradb@xxxxxxxxxxx>
> 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
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