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

  • From: boris.traa@xxxxxxxxxxx
  • To: gstokes@xxxxxxxxx
  • Date: Fri, 30 Jul 2004 12:08:28 +0200

The voltage between 2 points A and B on a conductive plane (or wire) 
carrying an RF-current can have every value. When losses are neglected 
even 0 V is possible. The measured value is the induced voltage in the 
loop formed by the plane (or wire) and the probe leads. So if the probe 
leads are almost on top of the plane (or wire) approaching an zero mm2 
loop area  then the measured voltage amounts aproximately 0 V. In this 
case, even if the assumed (calculated) self-inductance between A and B 
would result in a high impedance, the eventually measured voltage level 
would be 0 V.
With this in mind I still do not understand all the discussions and 
attempts to measure the voltage between A and B. 

Kind regards
Boris Traa

System design engineer EMC

PDSL/EMC3
Building WDV, room 1.01
Prof Holstlaan 4
5656AA Eindhoven, The Netherlands
Tel: ++ 31 40 27 43766
Fax: ++ 31 40 27 42224
E-mail:  boris.traa@xxxxxxxxxxxx Seri: btraa@nlsce1

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Geoff Stokes <gstokes@xxxxxxxxx>
Sent by: 
si-list-bounce@xxxxxxxxxxxxx
26-07-2004 17:37
Please respond to gstokes

 
        To:     si-list@xxxxxxxxxxxxx
        cc:     (bcc: Boris Traa/EHV/PDSL/PHILIPS)
        Subject:        [SI-LIST] Re: How to measure voltage drop on plane
        Classification: 




I repeat: why measure the voltage on the plane?
 
-----Original Message-----
From: John Fisher [mailto:jmfisher@xxxxxxxxx]
Sent: 22 July 2004 22:44
To: Geoff Stokes; si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: How to measure voltage drop on plane



Geoff,

I agree with reasonable care one can build a compact  high BW probe using
450 Ohm chip resistors and flexible semi rigid coax.  One could use a
differential probe for pwr plane drops but your limited physically how far
you can span  by the distance you can spread the tip adaptors that come 
with
the probe which is usually under 0.5 in.  Its also recommended that the
probe be perpendicular to your DUT to minimize extraneous field pick-up.

John

 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 <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
> > > >
> > > >
> > > >
> > > .
> > >
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