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

Larry Smith ([email protected])
Tue, 27 Jul 1999 15:12:47 -0700 (PDT)

Let me give another pertinant example. One time I wanted to know the
voltage on the Vdd plane at the edge of the board and at the center of
the board. We were using a spectrum analyzer and were interested in
noise at a frequency that was near the half wave resonance of the
board, 700 MHz. It was an EMI problem frequency. At first, the
measurements were taken with the probe not grounded. All points on the
board were about the same. Then we went back and made the same
measurements with the same single ended probe, but this time connected
the ground side of the probe to the ground plane and the hot side of
the probe to the Vdd plane at unpopulated decoupling capacitor sites.
The difference was astounding. The ends of the board had high voltage
at that frequency but the center of the board was quiet. This is
exactly what you would expect from a half wave resonator with open
ends. The ends have maximum voltage and the center has maximum current.
It confirmed our thinking on decoupling capacitor placement. We
would not have gotten that information if we had not used a tight
connection to circuit ground.

In the SI world, I think it is important to specifically measure every
signal WRT something else. That is the signal that circuits respond
to. When I speak of ground bounce in an SSN problem, it is always
ground bounce WRT another local node.

The 'groundless' probe measurement may have meaning in the EMI world
but my feeling is that the measurement is a little out of control. It
may not be repeatable or consistent with measurements in other

Larry Smith
Sun Microsystems

> Ron Miller wrote:
> Hi Larry
> I like your discussion on ground bounce, and I used to think the same
> way you do. However, over the many years I have modified my origional
> thoughts.
> Ground bounce may also be considered with your reference not being
> another physical point but with itself versus time. At first this seems
> frivilous because it seems impossible to measure.
> However, If you ever used an RF high impedance probe you may have
> noticed that the ground wire can often be removed and the signal is
> still there. I have seen this many times with spectrum analyzers when
> looking into the gigahertz range.
> In this setup the ground return need only be an electrical mass with
> storage capacity and lossiness, as in your hand and body, or it could be
> a chassis or cover above the board being measured. The ground path
> through the chassis is unimportant, but the stored charge in the cover is
> relatively stable compared to the trace on the board. The charge is
> averaged by the capacitance over that area and as long as the probe
> does not draw appreciable current, it does not disturb the chassis or body
> ground reference.
> So, a differential fet probe might seem to be best with one input connected
> to the local chassis. However, the ground on a single ended fet probe does not
> load the reference because the wavelength gets so short that and the fet
> ground path has no effect. So, a differential probe is unnecessary, and since
> single ended probes have a higher cutoff frequency they are preferrable.
> So, when you float the probe with your hand on the ground wire, the bounce you
> measure in either the ground or the power is real. Because you are also picking
> up lots of other interference from florescent lights and such you need to vary
> the sweep rate and triggering of your scope to minimize interference and maximize
> the ground bounce. An external trigger and/or a sampling scope may be needed.
> Ron Miller
> *****************************************************************
> Larry Smith wrote:
> > 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.
> >
> > From an SI perspective, the only thing that is important is Vdd-Gnd.
> > That is the voltage that all circuits respond to. There probably is a
> > noise voltage between the Gnd plane and the center of the earth, or
> > from the Vdd plane to the center of the earth, but I know of no way to
> > measure it, without an antenna. In spice, we report the Vdd voltage
> > and Gnd voltage with respect to spice node 0, but these voltages are
> > not important to circuits, only local Vdd-Gnd is important. Spice node
> > 0 is a lot like the center of the earth. It is very difficult to model
> > the impedances from our circuit to the center of the earth, and it is
> > very rarely done. In SI analysis, we commonly connect spice node 0 to
> > some "Gnd" node, but this is only for convenience. You can dump a lot
> > of current into spice node 0 or into the center of the earth and see
> > no change in voltage. There is no node in our system where we
> > can do this.
> >
> > When an EMI engineer talks about ground bounce, many times he is
> > discussing the voltage on a ground node with respect to the center of
> > the earth. Measurements and models for this voltage are important for
> > EMI purposes, but are beyond the scope of what an SI engineer usually
> > does. When we discuss EMI and SI topics, it is very important to
> > define our terms.
> >
> > I hope this helps the discussion. This is a very important topic for
> > those of us who are trying to make circuits that both work and comply
> > to EMI standards at the same time.
> >
> > regards,
> > Larry Smith
> > Sun Microsystems

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