29 Mar
2017
29 Mar
'17
1:06 p.m.
There are few things that come to mind here. The op seemed to indicate the lines are
terminated. If they are not terminated in the characteristic impedance of the source and
the transmission line, it is very unlikely he would be seeing nice square waves at either
end. The reflections would distort the square wave. Given the reported squareness and that
the op indicates terminated line, I do not think impedance mismatch is the issue here.
I also agree that an induced current in an adjacent line would not be square. So I agree
with the op's thoughts that this signal is getting on this line in some other fashion,
I don't believe this is an issue of cross talk. However, some pictures of some
waveforms would be interesting to see
Eugene W2HX
-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Parent Allison via
cctalk
Sent: Wednesday, March 29, 2017 11:54 AM
To: Noel Chiappa; General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Cross-talk square-wave?
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