RE: [SI-LIST] : Noise Voltage levels vs. EMI levels

Larry Smith ([email protected])
Tue, 27 Jul 1999 10:13:13 -0700 (PDT)

This is a very important and timely discussion. I think it would help
if we carefully defined our terms. We are certainly familiar the the
term "ground bounce" but those words mean a lot of different things to
a lot of different people.

Voltages are always with respect to something else. To me, ground
bounce means the voltage on a power (Vdd) node with respect to a nearby
ground node. In modern systems, it can be measured using a single
ended probe connected vertically between the Vdd plane and Gnd plane
using _very_ short leads. The loop area between the hot side of the
probe and the ground connection should be less than 1/16 of a square
inch, otherwise you will be measuring the magnetic flux in your probe
loop instead of the noise between Vdd and Gnd. Ground bounce and Vdd
bounce cannot be sepparated because they are referenced to each other.

I do not know of any way to directly measure the voltage across a
ground plane. It is a valid thoretical concept because the common mode
voltage across the ground plane can radiate, creating EMI noise. But
there is no way to measure it using a scope or spectrum analyzer
probe. There is a time delay across any significant distance (inches
or cm). What does it mean to measure a voltage across time? The noise
you measure will actually be from magnetic flux penetrating the
loop area of the probe. The only valid measuremnt technique that I
know of involves some kind of antenna. An antenna can sense the
magnetic or electric field that propagates from the plane and turn it
into a measureable voltage. This is exactly what is done in an EMI lab.