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

Ron Miller (rmiller@Brocade.COM)
Tue, 27 Jul 1999 12:26:05 -0700

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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.

TIME REFERENCE

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.

PRACTICAL GROUND RETURN

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.

PRACTICAL MEASUREMENTS:

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
>
> > 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
> > >
> > >
> > > **** To unsubscribe from si-list: send e-mail to
> > > majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
> > > si-list, for more help, put HELP. si-list archives are accessible at
> > > http://www.qsl.net/wb6tpu/si-list ****
> > >
> >
> > **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com.
> In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP.
> si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****
> >
>
> **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

--
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
     ==========0000-(_)0000===========
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net

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<!doctype html public "-//w3c//dtd html 4.0 transitional//en"> 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.

TIME REFERENCE

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.

PRACTICAL GROUND RETURN

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.

 PRACTICAL MEASUREMENTS:

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

> 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
> >
> >
> > **** To unsubscribe from si-list: send e-mail to
> > majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
> > si-list, for more help, put HELP.  si-list archives are accessible at
> > http://www.qsl.net/wb6tpu/si-list ****
> >
>
> **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com.
In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP.
si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****
>

**** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP.  si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

-- 
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017                 
     ==========0000-(_)0000=========== 
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net
  --------------C020D0C5911B96E4D9308396-- **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****