What is creating the power for the equipment shelf, a switching power
supply? The 20Khz to 100Khz oscillations you mentioned sound about
right for a switcher.
When do you measure this noise? Do you see the noise after the board
is installed, powered up and running? Or do you see the noise only
during the instant the board is first plugged in? Hot swap transients
are normally in the Mhz range. Loaded power supply noise is in the Khz
When you connect the ground lead of a probe to the probe tip you
create a magnetic field probe (loop). Assuming no other measurement
errors for the moment, your scope is measuring the voltage induced
into the loop. If you open the loop you will be sensitive to electric
fields (i.e. florescent lighting, etc.)
Assuming measurement errors for the moment, I would look at the
sources that can cause a voltage differential between the ground of
the scope and the ground pin you measured. Draw yourself a little
diagram of the shelf power distribution. You might find that you have
current flowing through the scope probe ground back to the power
Thank you |
Don Abernathey |
On Mar 4, 9:42am, Bradley L. Mendenhall wrote:
> Subject: Ground Bounce?
> Any scope experts out there?
> I am getting some wierd scope readings and would like some insight into
> the scope is telling me. I have an ungrounded eqipment shelf with a scope
> probe connected to a ground pin. The local probe ground wire is connected
> the same ground pin. The scope is isolated from AC ground and is grounded
> the shelf's logic ground. The shelf has three solid ground planes in the
> backplane. When I plug a board into the shelf, I get lots of noise and
> oscillations showing up on the scope, reading up to +/-0.5 volts. Most
> oscillations are in the range of 20 to 100 kHz.
> Can anybody tell me what I am measuring here? Is this ground bounce? What
> can I do to accurately look at the ground plane while plugging in a board?
> Thanks for any help,
> Brad Mendenhall firstname.lastname@example.org
> BroadBand Technologies, Inc. Phone: (919)405-4682
> 4024 Stirrup Creek Drive FAX: (919)544-5356
> Research Triangle Park, NC 27709-3737
>-- End of excerpt from Bradley L. Mendenhall