>
Yeah, from the description, like Richard said, it sounds like the
scope ground is not set properly, at least in one case, and sounds
like the scope is set to AC coupling in the other. Scope gnd needs to
be hooked directly to the board being probed.
> --- In oopic@yahoogroups.com, "Nick Mulder" <nick.mulder@> wrote:
> >
> > Hiya,
> >
> > I've set up two XBee pro units to do digital line passing - so
the pin
> > states are mirrored between the remote TX unit and the base RX
unit..
> >
> > In setting up the units I had them on the same power supply and
ground
> > potential and had them working fine passing a %50 cycle 25Hz
digital
> > pulse (3.3v) between themselves - Well, the purpose of having
wireless
> > is so you don't have to use the same power and ground potential
isn't
> > it...
> >
> > So I set the RX unit up on its own battery power supply and sent
it on
> > its merry way to the other end of the desk all by its
lonesome ... Now
> > on the scope its giving a sine-ish wave output on its output pin
(the
> > mirrored one) at about 20Hz-ish when idle and when I send my 25Hz
> > pulse its the same 20Hz-ish wave with little spikes where there
should
> > be a digital transition from high to low or vice-versa ...
> >
> > However ... this glitch disappears if I connect the units to the
same
> > ground potential.
> >
> > If I connect the two grounds by a 10MOhm resistor I get a kind of
> > 'half-problem' where I get my digital pulse at 25Hz but modulated
by
> > the 20Hz-or-so pulse ...
> >
> >
> > What am i doing wrong ? I have used the XBee's for line passing
> > successfully beforehand - no idea where this glitch is coming from
> >
> > prob something really beginners ?
> >
> > nick
> >
>
> It's nearly impossible to help with something like this without
being
> able to actually see what is going on. But, it sounds like a scope
> probe grounding issue to me.
>
> The scope chassis and probe ground lead are connected to safety
> ground. If you probe the signal without connecting the ground lead
> you can get strange results. If you do connect the lead, you are
> changing the ground reference for the device but that's the nature
of
> using a scope.
>
> That you are getting spikes at the rising and falling edge of the
> pulses makes me believe you are seeing the derivative of the pulse.
> This occurs when you have a capacitor between the signal and the
scope
> input or when the scope is set to AC coupling. Not connecting the
> probe ground can give the same results.
>
> Richard
>
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