[SI-LIST] Re: Damping of Cavity-Mode Resonances in PCB
- From: "Oprea, Dorin (Dorin)" <dorin.oprea@xxxxxxxxxxxxxxxxxx>
- To: Cristian Gozzi <cristian.gozzi@xxxxxxxxx>, Brad Brim <bradb@xxxxxxxxxxx>
- Date: Wed, 23 Nov 2011 08:39:51 -0600
Hi Cristian,
I think this is a system grounding problem. Follow up the current return path
between the digital and analog circuitry; check for differential mode current
path and also for the common mode current path, the later is supposed to close
through the frame ground of the system. Does the backplane have frame ground
shielding?
Good luck,
Dorin
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Cristian Gozzi
Sent: Tuesday, November 22, 2011 12:51 PM
To: Brad Brim
Cc: steve weir; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Damping of Cavity-Mode Resonances in PCB
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
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