Has Anyone Been Able to Simulate a Twisted Pair Cable? - RF
Post subject: Has anyone been able to simulate a Twisted
Pair Cable? Posted: Tue Sep 20, 2005 3:27 pm
Good day. Has anyone
been able to simulate a twisted pair cable with ADS, Microwave Office,
I am trying to examine the effect of different
twists and how they would affect coupling between the two wires.
Thank you for your time and help.
Post subject: simulation of twisted pairPosted:
Wed Sep 28, 2005 5:56 pm
Because you're dealing with the physical
configuration, you can't use a simulator which isn't physics-based
for your task.
Sounds like you need an 3-D electromagnetics
simulator, if you want to investigate this in detail.
- the largest effect will be the increase in wire length to cover
a given distance. You should be able to calculate this easily.
The second-order effects will be the increase in capacitance
It may be that you don't need anything other
than the largest effect.
At any rate,
Post subject: Re: Has anyone been
able to simulate a Twisted Pair Cable?Posted: Sun Oct 02, 2005 6:48
Joined: Sun May 09, 2004 1:23 am
Location: Morgan Hill, CA (Silicon Valley, Bay Area)
Good day. Has anyone been able to simulate a twisted pair
cable with ADS, Microwave Office, Eagleware, etc.?
trying to examine the effect of different twists and how they would
affect coupling between the two wires.
Thank you for your
time and help.
What exactly are you looking
for when comparing different twists in a twisted pair cable? A 3D
simulator might help, but I would think that you'd just get different
impedances for different twists and that would be about it. Will
the twisted pair be kept far away from metal objects? That would
make a difference too since a twisted pair isn't shielded like coax
Posted: Thu Oct 13, 2005 7:12 pm
Fri Sep 02, 2005 7:25 pm
Location: Hampshire UK
Twisted pair like telephone wire pairs is around 110 ohms.
The impedance of shielded twisted pair is not much different to
the shielded variety. Cat5 network cable amounts to a relatively
high loss, but useful cable for short range applications, is of
You can get individually shielded pairs cable.
In all these, the presence of the shield is to protect the cable
from external interference fields and to limit the amount of conducted
and radiated EMI from the cable. The shield does not primarily define
the impedance, which is a function of the conductor diameter, the
spacing, the dielectric constant of the plastic separating the conductors.
It would affect it a little.
The "shield" around coax should
not be thought of as a shield. It is one half of an unbalanced transmission
line. It is what happens if you think of one of the conductors of
a balanced pair getting bigger, and morphing its shape (in cross
section), to surround the other, to eventually assume the shape
of coax. The lines of electric field then become radial between
the centre conductor and what now surrounds it. This coax form of
outer very definitely has a profound effect on the impedance, by
its dimension and the dielectric within.
Grounding the outside
when there is a standing wave on a coax cable is a major sorce of
EMI trouble. Triax, which is a coax with a second outer braid, allows
the outermost braid to be grounded (usually one end only), and is
an (expensive!) shield - the coax equivalent of the shielded twisted
In theory, if one conductor of a twisted pair gets
up against a metal object more than its partner, you get some unbalanced
current. In practice, with a pair having a high rate of twist, it
is remarkably immune to coupling. I once tried out some HF cable
distribution stuff that had six pairs around a mandrel plastic,
in a jacket that was carbon loaded to make it "pre lossy" so that
it did not change its characteristic much as it aged in use. Provided
all the unused pairs were terminated, the balance between pairs
was better than -60dB. We could run a small electric drill on one
pair while sending a vestigial sideband TV signal up the cable the
other direction. Definitely a "don't try this at home" stunt!
Then again, it only takes ONE leg of ONE pair in a fat cable
to get disconnected to have all the rest get mangled with crosstalk!
Ask any telephone engineer.