# Cross-talk square-wave?

W2HX w2hx at w2hx.com
Wed Mar 29 22:16:48 CDT 2017

```> The impedance of free space is supposed to be 277 Ohms
377 ohms but you are right about the max impedance. I forgot about that.

> 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.

Except for the fact that in the same cable, the edges of the square wave of the clock is creating short pulses on the CS line. Why would the CS line now behave totally differently if theoretically coupled to the QBUS line? In theory maybe you are correct, but in the practice of troubleshooting this specific issue, I would have to say it is unlikely that an adjacent square wave clock line would cause spikes and a similarly adjacent square wave QBUS signal would cause a square wave. They would both really be expected to have the same effect as long as the CS line hasn't changed change in characteristic.

My 2c
Eugene

-----Original Message-----
From: Jon Elson [mailto:elson at pico-systems.com]
Sent: Wednesday, March 29, 2017 10:55 PM
To: W2HX; On-Topic and Off-Topic Posts
Subject: Re: Cross-talk square-wave?

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
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