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Twisted pair works bigtime in 'unbalanced' signal transfer. (long)

Posted by cheap-Jack (M) on April 30, 2007 at 09:08:12

In Reply to: Re: In a grounded metal chassis, no shielding is needed for TWISTED pair. posted by Dave Cigna on April 28, 2007 at 18:04:13:

Hi.

Using balanced line drivers, balanced twisted-pair cables, and balanced line receivers is a long historic establishment in professional audio applications, e.g. long distance broadcasting audio signal relays.

But we are now talking about very close range home audios.

Let's get back to the topic of shielding.

What's a shield used for?

it is used to stop:-

(1) capacitance coupling, e.g. our body is a ready capacitance coupled noise source, which emits the noise to generate unstable oscillations inside a system. In digital systems, it can cause CROSS-TALKS in mult-wire cables.

It can be killed by surrounding the system/parts with a metallic shield, the most common method.

(2) inductive coupling - the topic now we have been talking.

The magnetic field generated from an external source induces noise current in the victim system. One way to stop this inductive noises is to reduce the current loop area of the system, e.g. the signal wire in question to miniumum.

This can be done with a signal carrying conductor in two ways:-

(1) coaxial cable: - the RETURN (-flow) of the signal from the load via the shielding jacket cancels out the magnetic field of the incoming signal at the centre conductor of the cable - zero external magnetic field. ********

In this zerio external magnetic field ideal situation, the conductor is also immmune to any external inductive noise sources.

This is most effective provided this is a PERFECT coaxial cable. But it does not really exist in the realworld due to the extrusion errors always exist in making the coaxial cable, rendering the centre conductor not exactly coaxial.

Improper implementation of grounding can easily destroy of ideal zero flux situation of a coaxial cable as the ground loops will take away part of the return signal currents from the coaxial shielding, killing the ideal of complete flux cancellation.

This return current divertion from the coaxial shield is more serious with audio frequencies, from zero to a few KHz, where the inductive reactance is insignificant, & the return signal current will go via the short DC resistance path of other ground loops instead of the coaxial shield.

Standard signal conductor(s) withn overal braid/foil shield employs the same principle of a coaxial cable, but is a very very lousy one as the cable is not built physically like a coaxial cable.

(2) twisted pair of two equal gauge size & identical construction
conductors. By the same principle of a coaxial cable, the -flow of the return current, being identical to the incoming signal current, will generate a -ve flux to cancel the flux generated by the incoming
signal.

Being smaller in overal size than a coaxial cable, cheaper to buy, a twisted pair also provides an excellent advantage of cancelling out the noises picked up in one twist by the next twist downstream.

Tt can handle signal transfers up to & well beyong 1MHz as compared to good coaxial cables which can go up 1GHz.

Since you've brought up the link of UTP. All LAN cables, e.g. Cat5e or Cat6 are U are UTPs - Unshielded Twisted Pairs.

One major American LAN cable maker is now supplying a super fast Cat5e LAN cable, using its proprietary twisting method, can extend this unshielded twisted 4-pair cable to 350MHz at a super fast data speed of 155Mbps !!!!

Who said unshield twisted pair is no good for noise/cross-talking killing.?

That said, twsisted pair is not very good in capactance or electrostatic shielding. Better housed in a metal sheet chassis to get the best result. Hence my post above.

c-J

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