Cross-talk square-wave?

[Parent Allison]

On Mar 29, 2017, at 9:40 AM, Noel Chiappa via cctalk <cctalk at classiccmp.org> wrote:

> 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!)

Oops!  

> 
> 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?
> 
Transmission line theory applies.  Adjacent lines see the electric and magnetic fields 
nominally seen along transmission lines.  Some would say it this way, you get induction
from one line to another based on how those wires are routed and terminated.

Its only 375khz… No, its pulses with rise and fall times in the Nanosecond region
with bandwidth of hundreds of Mhz.


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

Nooooo.  You have to treat those wires as transmission lines ( like coaxial cable or parallel pair)
for signals.  Its not DC leakage.  You send a pulse (or a train of them) down a transmission line 
and if the line is not terminated the pulse energy will be reflected rather than absorbed.  Is there is 
a signal line next to it it will see the resulting fields from the currents flowing.

Add to that your ground for the SD card is remote so there will be a current flowing on that lead as well 
from circuit ground and the actual ground pin.

This is why people do not remote SD cards (unless someone is forcing it).  Its input looks like a capacitor 
at the end of a transmission line and incorrectly handled you get reflections and ringing.  Just like backplanes 
and all sorts of other media.


Allison

> Thanks!
> 
> 	Noel