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

Steve Weir (weirsp@a.crl.com)
Tue, 27 Jul 1999 14:02:44 -0700

I think that Larry brings up very valid points on measurement. However, we
still have the issue for single-ended signals that as the local ground
potential at the transmitting device differs from the the ground potential
at the receiving device. This potential difference may be propagating from
near either end, or some point in-between, in which case, it is
differential to the signal for some time to either or both ends.

At the receiver that ground noise directly subtracts from the signal noise
margins. The overall effect is measureable as apparent noise on the signal
at the receiver compared to the receiver ground. The most aggregious
example I can think of is an ESD discharge into the ground plane near a
receiver. On a good day, the receive signal is corrupted. On a bad day,
all the smoke comes out.

Regards,

Steve.

At 10:13 AM 7/27/99 -0700, you 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
>
>> From: "Grasso, Charles (Chaz)" <GrassC@LOUISVILLE.STORTEK.COM>
>> To: "'si-list@silab.eng.sun.com'" <si-list@silab.eng.sun.com>
>> Subject: RE: [SI-LIST] : Noise Voltage levels vs. EMI levels
>> Date: Tue, 27 Jul 1999 07:24:35 -0600
>> MIME-Version: 1.0
>>
>> The most ubiquitous phrase used for noise - EMI levels translation
>> is ground bounce. A term the experienced SI folks are very familiar with
>> I'm sure. Indeed the EMC folk spend a lot of energy minimizing
>> the noise that is developed across the ground plane inductance.
>>
>> A general rule of thumb is that "noise" on the ground plane should
>> not exceed 300mV. (From Henry Ott - Noise Reduction
>> Techniques in Electronic Circuits.)
>> Thank you
>> Charles Grasso
>> StorageTek
>> 2270 Sth 88th Street
>> Louisville CO 80027
>> Tel: (303)673-2908
>> Fax(303)661-7115
>>
>>
>> > ----------
>> > From: Spencer, David H[SMTP:David.Spencer@usa.xerox.com]
>> > Sent: Monday, July 26, 1999 10:03 AM
>> > To: 'si-list@silab.eng.sun.com'
>> > Subject: [SI-LIST] : Noise Voltage levels vs. EMI levels
>> >
>> > I'm looking to correlate PCB ground noise voltage to an EMI levels.
>> >
>> > My understanding goes as far as: the voltage is measured from the PCB
>> > ground
>> > input to a point on the board differentially. That voltage drop is the
>> > noise voltage. The voltage drop tells us that there is common mode
>> > current. If we knew the impedance between the two points we could then
>> > calculate the common mode current.
>> > Since every PCB and has different geometry's and cables (power and I/O),
>> > the
>> > difficult unknown is the impedance.
>> >
>> > My questions:
>> > Is there is an absolute noise voltage level that should never be exceeded?
>> > Is there a way to "accurately" quantify the impedance's of the PCB ground
>> > plane and cables?
>> > Can noise voltage become a PCB design specification?
>> > Most importantly, when all is said and done, is there an "accurate"
>> > correlation between noise voltage and EMI levels?
>> >
>> > Thanks for the input
>> >
>> > David Spencer
>> > Xerox Corporation
>> >
>> >
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>
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