Cross-talk square-wave?

[Noel Chiappa]

Hi, a question about generic analog stuff.

In the process of getting SD cards to work, Dave is seeing square-wave noise
on a line. (1V of square wave, with pulses about 400ns long, running at
375kHz.) The line runs through a flat cable of modest length, along with
other signal-carrying lines. (No, we were not smart, and didn't put ground
lines between each pair of signal lines!)

Could cross-talk cause this kind of noise? We would have thought that you'd
only get spikes, associated with the rising and trailing edges of a signal in
a parallel wire, not a whole square-wave. During the constant-current period
in the middle of the pulse, there shouldn't be any cross-talk? Is there some
mechanism I/we don't understand that could do that?

(My guess is there's a leakage path in the circuitry on one end or the other,
not cross-talk in the cable, but...)

Thanks!

	Noel