29 Mar
2017
29 Mar
'17
7:55 p.m.
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