FreeLists / si-list / [SI-LIST] Re: How to Measure Ground Noise

[Doug Smith <doug@xxxxxxxxxx>]

Hi Gary and the group,

The scope chassis is large enough to be "grounded" at frequencies over 
a few MHz even if it were battery powered and completely floating. It 
is not possible to isolate the scope except at DC and power line 
frequencies, it it were no antenna would work and we all would not be 
in this line of work!

I agree that the current Pyng is seeing is probably environmental. If 
you leave the probe shorted and lift is 1 mm from the circuit and the 
scope display does not change much, then the loop of the probe itself 
is picking up a magnetic field. I have only seen this be the source of 
the problem once. Usually ground current on the probe is the problem.

By the way, the equipment to which the probe is attached does not need 
to be grounded for this to happen. A few pF of capacitance back to the 
scope chassis will allow tens of milliamperes to flow if driven by a 
few Volts at tens of MHz. A series resonant circuit is formed by the 
probe shield inductance back to the scope and the capacitance between 
the scope and the EUT. I demonstrate this live using a small battery 
powered oscillator powered by a few AA batteries in my seminar. Even 
this generates 2 to 4 Volts drop across a 6 inch ground lead of the 
scope probe.

Doug

Pratt, Gary wrote:

> I suspect you are actually measuring the common mode ground current
> coupling of the scope (plus the radiated energy passing through the loop
> of the probe ground wire).  See if your results change if you float the
> scope power with a good medical isolation transformer (at your own
> risk), or wrap the power cord many times through a good torroid, or wrap
> the ground cable around the tip of the scope probe (to reduce the loop
> area).  =20
> 
> If you see a dramatic change, you will know your measurement reflects
> more of whats going on in your measurement environment than what is
> going on in your PCB. =20
> 
> Gary
> 
> 
> 
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of ospyng
> Sent: Wednesday, April 06, 2005 1:23 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] How to Measure Ground Noise
> 
> 
> hi,
> I was taught to measure ground noise, by having the ground of the probe
> connect to ground TP of the board and the tip of the probe measuring the
> same TP.
> 
> is this the correct where? and Why?=20
> is it because of the ground cable (3 inch) cause a loop, therefore the
> reference and the tip of the probe has a different voltage?
> 
> thanks
> pyng
> 
> 
> 
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