[SI-LIST] Re: Power Plane resonance analysis

Istvan,
        If you suspected a Plane resonance in a unusual shape, and you wanted 
to experimentally fix it how would you do that ?

For Example, let's say you had a shape similar to an inductive pickup (square 
loop), and you wanted to experimental eliminate the resonance how could you 
accomplish that ?
If you fixture up a spectrum analyzer to the Plane and excited it, would you 
expect to see the resonances show up as spurs in the power spectrum at the 
resonant frequency(s)?

 
Regards
Bob Haller
Enterasys Networks


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Istvan Novak
Sent: Thursday, August 27, 2009 8:26 AM
To: Sandhya I. M.
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Power Plane resonance analysis

Hi San,

If you have the good luck to have rectangular plane shapes, you can 
simply calculate the
modal resonance peak frequencies from the propagation delay along each 
of the two sides
of the rectangle.  1/(2*tpd) gives you the lowest resonance peak, and 
you will get all of
the harmonics, so it becomes a double infinite series.  On typical PCB 
dimensions today,
the first five or ten resonance peaks matter in self impedance; the 
higher-order modes
will tend to blend together into an inductive slope.

For more complex shapes the modal resonance frequencies need to be 
calculated by
an appropriate software.  You can go from simple and low-cost tools like 
EZPlane
(http://www.ems-plus.com/ezpowerplane.html) to more sophisticated tools 
like
Sphinx (http://www.e-systemdesign.com/products.aspx) to high-end tools such
as Ansoft SiWave or HFSS.  For HFSS and correlation on power planes, you can
check out the Unofficial HFSS User forum (http://www.hfss-forum.com/).

To make sure that resonances do not create a problem, best and cheapest 
defense
is to make sure you dont excite them.  You can do this by creating a 
stackup to
minimize signal-return currents going through unrelated power-ground 
cavities.
To reduce the resonances themselves, you can either sprinkle the planes 
with many
capacitors, match the planes or use thin laminates, or a combination of 
these. If
 you sprinkle the planes in the old-fashioned brute-force manner, you 
could use the
same capacitance value, say 0.1uF or 1uF.  Key is minimizing the inductance
in the capacitor connections because with the cumulative inductance of 
all capacitors
on the plane you want to stay below a couple of percents of the plane's 
inductance.
You can read more on these design options at 
http://www.electrical-integrity.com/,
go to Paper download.  In the Tool download section you can also find 
simple
illustration tools for plane resonances.

For measurement options, you can look at the paper "Frequency Domain Power
Distribution Measurements - An Overview" from DesignCon 2003 East, Boston,
MA, June 23-25, 2003.  You will find it at the site above.  For more 
details and
tips, you can look at the book Frequency-Domain Characterization of Power
Distribution Networks, Artech House, 2007.

Regards,

Istvan Novak
SUN Microsystems



Sandhya I. M. wrote:
> Hi SI experts,
>
> If any one can help me in doing PCB Power Plane resonance analysis by
> sharing documents or by explaining how it occurs.
>
> What are the causes of PCB Power Plane resonance analysis 
> How can we measure resonant frequency
> How decoupling caps helps in suppressing resoinance in planes.
> What are the factors we should consider while doing resonance analysis.
>
>  Thank you 
>
> Regards
> san
>
>   

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