[SI-LIST] Re: Damping of Cavity-Mode Resonances in PCB
- From: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>
- To: Brad Brim <bradb@xxxxxxxxxxx>, 'Cristian Gozzi' <cristian.gozzi@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Mon, 21 Nov 2011 16:27:24 -0700
Hello Brad,
It seems that simulation is finally catching up with EMI design practice
(tongue in cheek).
For the longest time it was (is) common practice to sprinkle seemingly useless
caps in
barren areas of a pwb to take care of resonant modes!
Best Regards
Charles Grasso
Compliance Engineer
Echostar Communications
(w) 303-706-5467
(c) 303-204-2974
(t) 3032042974@xxxxxxxxx
(e) charles.grasso@xxxxxxxxxxxx
(e2) chasgrasso@xxxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Brad Brim
Sent: Monday, November 21, 2011 12:43 PM
To: 'Cristian Gozzi'; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Damping of Cavity-Mode Resonances in PCB
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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