[SI-LIST] Re: Measuring Power and Ground Plane Noise
- From: Ray Anderson <Raymond.Anderson@xxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 29 Aug 2002 14:12:37 -0700 (PDT)
The measurement of noise on power planes is a relative
measurement. A noise voltage is measured _relative_ to
some reference.
You want to measure the noise on plane "A" relative to a
reference point on plane "B" at the same XY coordinates.
(i.e., don't measure the voltage at a point on the left side
of the board with your ground connection on the right side
of the board) The ground connection should be short and direct.
You can't just measure a voltage on a plane relative to some
fictitious (spice node 0) ground. It should be measured relative
to a local ground. Also note that you might have 10 volts of
a 100 kHz Hz AC signal (hopefully not) between your planes and say
the chassis, but at the same time you might only have 100 mV noise
between the ground plane and the Vdd plane. It is that 100 mV of
differential noise that will effect the operation of your circuit
from an SI perspective. however the larger common mode noise certainly
may be an EMI/EMC concern. (The common mode noise would be the same
on both planes, hence a zero differential potential).
Typically I like to make power plane noise measurements using
small diameter flexible semirigid coax connected from an oscilloscope
or spectrum analyzer to soldered down connections on the board.
Select a decoupling capacitor site, remove the capacitor and
solder the shield and center conductor directly to the mounting
pads on the PCB. (In the case of the spectrum analyzer be sure
to observe the DC voltage limitations of your instrument).
-Ray Anderson
Sun Microsystems Inc.
>
>How does one properly measure power plane or ground plane noise? I have a
>case where I believe there is 66 MHz noise on the planes. I want to know
>how much noise is on each plane.
>
>Both measurements require a reference point that you must assume to be
>stable, or perhaps that is considered stable by the measurement. So I
>measure across a decoupling capacitor and that shows some noise on the power
>plane. But how do I get a good measurement of the ground noise? And if
>there is noise on the ground plane, how do I get that noise out of the power
>plane measurement?
>
>Thanks for any help.
>
> - Paul
>
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