[SI-LIST] Re: How to measure voltage drop on plane

Geoff, it depends on what one is interested in.  The voltage ( potential 
difference ) between two points in space generally isn't that interesting 
from a signaling standpoint, unless those two points happen to be the 
receiver inputs.  However, they can be of considerable significance from an 
EMC perspective.

Doug Smith is probably one of the better authorities around on this, and 
his web site www.emcesd.com is chock full of interesting tips on practical 
probing and measurement.  He has construction plans for a GHZ + passive 
probe that has very flat response ( although it takes adjustments with an 
Xacto knife to get there ).  Just putting a series resistor at the end of a 
piece of coax makes for a peaky response due to the effective capacitance 
of the resistor.  In Doug's version, compensating shunt capacitance to 
ground is trimmed with a knife to take away the peak.

Regards,


Steve.
At 12:53 PM 7/20/2004 +0100, Geoff Stokes wrote:
>Hi Steve
>
>It seems important to remember that voltage, between two points, is the
>integral of the field and dependent on the path of integration.  So it's not
>unique.  Another way of saying this is that the voltage can only be measured
>by a probe which is very small or which responds to a particular wave mode
>(e.g transverse electromagnetic, on a coaxial cable feeding a 50 ohm
>measuring instrument).  I would therefore question the value of ever
>discussing voltage between different points on a ground plane unless fairly
>close together.  But what you do next is a problem - there you are - I've
>tied myself in knots again!!
>
>This does seem to agree with Ray Anderson's point - he mentions time of
>flight.
>
>You can measure with a single-ended probe, but may need to put a ferrite
>ring or two around the collar to attenuate common mode current.  A 450 (or
>470) ohm chip resistor soldered to the end of a piece of 50 ohm coax, with a
>short adjacent ground tip, makes a good high frequency probe feeding a 50
>ohm scope or spectrum analyser (may need a dc block).
>
>Cheers
>Geoff
>
> > -----Original Message-----
> > From: steve weir [mailto:weirsp@xxxxxxxxxx]
> > Sent: 20 July 2004 01:10
> > To: zhang_kun@xxxxxxxxxx; reanderson@xxxxxxxxxxxxx;
> > si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Re: How to measure voltage drop on plane
> >
> >
> > Voltage is the instantaneous difference in electric potential
> > between two
> > points, no more and no less.
> >
> > Ground is a convention that is useful to refer to the time
> > averaged voltage
> > across an area.  At high speeds, or high currents, the
> > average potential
> > deviates substantially from the instantaneous potential, and
> > the usefulness
> > of this convention breaks down.  When people talk about
> > ground noise, they
> > are usually referring to the spatial variation in ground potential.
> >
> >  From a signaling standpoint, if that variation is
> > identically matched by a
> > change in potential of associated signals, then receivers are
> > unaffected.  From an EMC standpoint, we worry about large
> > antennae formed
> > from by those varying potentials.
> >
> > Steve.
> >
> > At 07:22 AM 7/20/2004 +0800, zhangkun 29902 wrote:
> > >Dear Ray
> > >
> > >In Paul's paper "Modeling Electromagnetic Interference Properties of
> > >Printed Circuit Boards", he proposed how to measure the
> > voltage drop on
> > >the ground net. I think this method is not suitable for the
> > complex PCB.
> > >
> > >At one time, we are check one PCB with Intel. The engineer
> > said that "the
> > >noise on the ground" was too large. I ask them how to
> > measure "the noise
> > >on the ground". There is no answer.
> > >
> > >Is the voltage drop on the ground plane useful?
> > >
> > >Best Regards
> > >
> > >Zhangkun
> > >2004.7.20
> > >
> > >----- Original Message -----
> > >From: Ray Anderson <reanderson@xxxxxxxxxxxxx>
> > >Date: Monday, July 19, 2004 11:54 pm
> > >Subject: [SI-LIST] Re: How to measure voltage drop on plane
> > >
> > > > Zhangkun-
> > > >
> > > > The measurement of power/ground noise is a differential
> > > > measurement
> > > > (even if you use a single-ended probe) of the power voltage (Vdd)
> > > > with
> > > > respect to the associated ground voltage (Gnd).
> > > >
> > > > When making these sort of measurements you need to measure Vdd
> > > > with
> > > > respect Gnd at the same XY spatial location for the measurement to
> > > > make
> > > > sense. (i.e. vertically coincident). If you measure Vdd with
> > > > respect to
> > > > Gnd where the Gnd node is physically offset some significant
> > > > distance
> > > > (wrt lambda) then the measurement just doesn't make sense.
> > > >
> > > > If the Gnd measurement point is offset from the Vdd measurement
> > > > point by
> > > > only a small fraction of an inch instead exactly coincident with
> > > > the
> > > > measured Vdd location that is probably an acceptably small error,
> > > > however if the offset is on the order of  inches or more then the
> > > > measured voltage doesn't make sense. Why? Because you are
> > > > measuring
> > > > voltages 'across time' where the time is the time-of-flight from
> > > > the Gnd
> > > > node coincident with the Vdd node (which you should be using as a
> > > > reference) to the Gnd node which is distant (which you should NOT
> > > > be
> > > > using as a reference). Measurements of voltage across time are
> > > > undefined, at least with respect to the measurement of
> > > > power/ground
> > > > noise which you propose.
> > > >
> > > > -Ray Anderson
> > > >
> > > >
> > > >
> > > > Zhangkun wrote:
> > > >
> > > > >Dear all
> > > > >The power ground noise at one location is easy to measure. How to
> > > > measure voltage drop on plane at different location? Is it
> > > > impossible? There is one method proposed by C. R. Paul. However,
> > > > the structure of Paul's is too simple.
> > > > >
> > > > >Best Regards
> > > > >
> > > > >Zhangkun
> > > > >2004.7.19
> > > > >
> > > > >
> > > > >
> > > > .
> > > >
> > > > ------------------------------------------------------------------
> > > > To unsubscribe from si-list:
> > > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the
> > Subject field
> > > >
> > > > or to administer your membership from a web page, go to:
> > > > http://www.freelists.org/webpage/si-list
> > > >
> > > > For help:
> > > > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> > > >
> > > > List FAQ wiki page is located at:
> > > >                http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> > > >
> > > > List technical documents are available at:
> > > >                http://www.si-list.org
> > > >
> > > > List archives are viewable at:
> > > >               http://www.freelists.org/archives/si-list
> > > > or at our remote archives:
> > > >               http://groups.yahoo.com/group/si-list/messages
> > > > Old (prior to June 6, 2001) list archives are viewable at:
> > > >               http://www.qsl.net/wb6tpu
> > > >
> > > >
> > > >
> > >
> > >------------------------------------------------------------------
> > >To unsubscribe from si-list:
> > >si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> > >
> > >or to administer your membership from a web page, go to:
> > >http://www.freelists.org/webpage/si-list
> > >
> > >For help:
> > >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> > >
> > >List FAQ wiki page is located at:
> > >                 http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> > >
> > >List technical documents are available at:
> > >                 http://www.si-list.org
> > >
> > >List archives are viewable at:
> > >                 http://www.freelists.org/archives/si-list
> > >or at our remote archives:
> > >                 http://groups.yahoo.com/group/si-list/messages
> > >Old (prior to June 6, 2001) list archives are viewable at:
> > >                 http://www.qsl.net/wb6tpu
> > >
> >
> >
> > ------------------------------------------------------------------
> > To unsubscribe from si-list:
> > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> >
> > or to administer your membership from a web page, go to:
> > http://www.freelists.org/webpage/si-list
> >
> > For help:
> > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >
> > List FAQ wiki page is located at:
> >                 http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> >
> > List technical documents are available at:
> >                 http://www.si-list.org
> >
> > List archives are viewable at:
> >               http://www.freelists.org/archives/si-list
> > or at our remote archives:
> >               http://groups.yahoo.com/group/si-list/messages
> > Old (prior to June 6, 2001) list archives are viewable at:
> >               http://www.qsl.net/wb6tpu
> >
> >
>
>
>_________________________________________________________
>
>Zetex Semiconductors - Solutions for an analog world
>
>EID Award Winners for  'Best Use of Technology' 2003 for the
>AcoustarTM ZXCW8100 End-to-End Digital Audio Amplifier Controller
>
>http://www.zetex.com
>_________________________________________________________
>
>######################################################################
>E-MAILS are susceptible to interference. You should not assume that
>the contents originated from the sender or the Zetex Group or that they
>have been accurately reproduced from their original form.
>Zetex accepts no responsibility for information, errors or omissions in
>this e-mail nor for its use or misuse nor for any act committed or
>omitted in connection with this communication.
>If in doubt, please verify the authenticity with the sender.
>######################################################################
>
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>http://www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List FAQ wiki page is located at:
>                 http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
>List technical documents are available at:
>                 http://www.si-list.org
>
>List archives are viewable at:
>                 http://www.freelists.org/archives/si-list
>or at our remote archives:
>                 http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
>                 http://www.qsl.net/wb6tpu
>


------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List FAQ wiki page is located at:
                http://si-list.org/wiki/wiki.pl?Si-List_FAQ

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  

Other related posts: