[SI-LIST] Re: How to measure voltage drop on plane
- From: John Fisher <jmfisher@xxxxxxxxx>
- To: gstokes@xxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Thu, 22 Jul 2004 14:44:07 -0700
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]
>> 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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