Re: electrical noise

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "toolspindoctor"
<clarkfamily5@...> wrote:

>
> Can someone give me some insight on how to track down a significant
> noise problem. I have an oscilloscope and voltmeter. I am using the
> Pixie card with a servo drive. The Pixie seems to be picking up
> horrible noise. Nothing I do makes a difference other than making it
> worse.
>
> Thanks
> Keith Clark
>

Finding noise with a conventional analog scope is not easy. The
problem has to do with grounding and the scope itself. By default the
shield side of the scope probe is connected to the scope chassis which
is connected to safety ground. When you connect the probe Return line
(which you have to) you are grounding that point. If it is already
grounded it's no big deal. If the noise problem is from a grounding
issue it may get worse or better from the local scope ground. TO see
the real signal at the point you really need to use a differential
input probe or have a lot of experience looking at wafeforms and
understanding what is really there and whst is artifact from the scope
itself.

If the encoder inputs to the pixie are single ended (the signal is
referenced to ground) then it's a long antenna for noise. You could
put your scope on each channel and the waveform should be pretty clean
when the encoder is sending. Even sitting still the encoder will send
signals on at least one channel. If it's a differential signal the
encoder should not be the source of the noise.

THe other potential noise conduit is the Step & Dir in. Once again
the biggest potential is a shared ground. Just hooking everything up
to a nearby grounding spot some of which can be some distance apart is
great for safety but may be a poor conductor of HF noise. I don't
know enough about the pixie card to know if they have a built in
cicruit to clean up a funky signal from the PC port. The further
those two are from each other the more drive you need from the
computer port. A good buffered BOB might help if the distance is
greater than a few feet.

It has been my (painful) experience that it's not about a circuit
being ungrounded as much as HOW/WHERE it's grounded.

Grounding for noise reduction is part science, part voodoo. Ground
loops are invisible and great noise conduits.