[SI-LIST] Benefits of twisted pair vs. flat cable for single ended signals

  • From: "Stefan Milnor" <stefan.milnor@xxxxxxxxxxxxxx>
  • To: <alexh1@xxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Wed, 7 Mar 2012 11:51:26 -0800

You might want to check out the flat cables that were popular a few years ago 
for Ultra-DMA PATA disk drive use. These have a GND plane embedded into the 
cable somehow, and were used for fast single ended signals before SATA became 
prevalent.

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Alex Horvath
Sent: Wednesday, March 07, 2012 11:39 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Benefits of twisted pair vs. flat cable for single ended 
signals

hi all,

I'm helping someone with a problem transmitting single ended, 5 ns rise time 
signals over flat cable. Not surprisingly, this only works to a length of 2' or 
so and a quick check on the scope revealed massive xtalk. This is happening 
even 
though the aggressor is surrounded by grounds. So I immediately suggested 
substituting twist and flat style cable so they could still use the IDC 
connectors and minimize downtime or custom cables.

As I checked the twisted pair cabling specs I started to question the benefit 
of 
UTP for single ended signal transmission but I think I have convinced myself 
that it will still result in a big improvement. The reason for this is that the 
majority of xtalk in UTP is from capacitive coupling, as opposed to inductive 
coupling. The capacitive coupling, since it alternates from positive to 
negative 
based on the twists, largely cancels out as compared to the flat cable which 
enjoys no such advantage. This of course is independent of the receiver type 
being single ended or differential.

Of course there will still be some inductive coupling since the wires do have 
space between them (in fact I don't think the spacing between wire pairs is 
much 
different between flat and twisted flat) and I think this results in common 
mode 
noise which of course would be canceled by a differential receiver.

Is my reasoning reasonably correct?
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