Re: electrical noise

Tom,

Does it seem likely that the noise could be coming from the PC 5v
powersupply? The BOB board is isolated but the step/dir signal comes
to the Pixie then the Pixie is powered by the PC 5v and grounded to
the PC ground. It seems like this would bypass the isolation provided
by the BOB. Is sure seems like a ground loop though.

Thanks

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "caudlet" <thom@...> wrote:
>
> --- 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.
>