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
