Cross-talk square-wave?

[Jon Elson]

On 03/29/2017 08:48 PM, W2HX via cctalk wrote:
> I am still not convinced it is coupling at all. You would expect the affected line to show a signal like dV/dt , no? I just don't think you can get square waves from square waves.
Yes, you can.  The capacitance of typical cables is about 35 
pF per foot.  Given a couple feet of cable and essentially 
infinite resistive load, it would be quite likely to give a 
near square-wave result.  If you were to load down the line 
with 100 Ohms, then you'd see tiny, short pulses at the edges.
> That's not to say the input of some logic somewhere isn't getting triggered by unintended coupling and then getting "squared up" in some gate to produce the square we see.
>
> Oh, and 270K surely is a transmission line load if the source has a characteristic impedance of 270K. Granted, that seems unusual and I don't know what the circuit looks like,
Well, in fact, it is impossible to make a transmission line 
with such impedance.  The impedance of free space is 
supposed to be 277 Ohms, IIRC.

Jon