Cross-talk square-wave?

[Tony Duell]

On Thu, Mar 30, 2017 at 9:13 PM, Noel Chiappa via cctalk
<cctalk at classiccmp.org> wrote:
> I'm still not clear, from the discussion, how exactly that nice 'square-wave'
> interference is happening - could it be capacitative crosstalk? (I'd have
> thought capacitative cross-talk would be inverted - driving a positive voltage
> on one 'side' of the 'capacitor' would, I would think, induce an oppposing
> voltage on the other. But I'm clearly no EE! :-)


The fundamental rule is 'You can't change the voltage across a capacitor
instantly'. There is a related one 'You can't change the current through
an inductor instantly'. It (of course) doesn't matter if said capacitor
or inductor is an actual component or 'strays'.

So instantaneously, a capacitor acts like a constant voltage drop.
If the charge on the capacitor doesn't change much (fast signals,
high resistors so not much current flow), the 2 sides of the
capacitor will be much the same signal (possibly with a
constant voltage offset).

-tony