[SI-LIST] Benefits of twisted pair vs. flat cable for single ended signals
- From: Alex Horvath <alexh1@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 7 Mar 2012 11:38:36 -0800 (PST)
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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