Hello.

I want to post some questions about shielded cables uesd either for control or for power connections.

1) why does a twisteed pair shielded cable have a better immunity to incoming noise, protects the transferred signal better?

2) why do shielded cables have bigger capacitance than unshielded cables?

Thanks in advance for your time.

 

#1: I think when you have twists, <don't know why but> for some reason you're more likely to evenly pick up the noise equally on both conductors. So as long as your circuit has a good Common Mode Noise Reduction, the noise has less effect. Sounds good to me. I think I got a C+ in electromagnetics. Anybody have a better explanation?

#2: The definition of a capacitor is two electrodes separated by a dielectric. Any insulator is a dielectric, so the insulation on wires in a cable becomes the dielectric. I think more surface area and better dielectric makes for more capacitance, so the shield provides a lot more surface area than two regular parallel wires would.

Of course you care about capacitance because it increases the impedance (AC resistance) hence degrading the signal quality.

Hope this helps!

 

> 1) why does a twisteed pair shielded cable have a better immunity to incoming noise, protects the transferred signal better?

The shield tends to reduce the electrical noise that can be impressed onto the signal. It is not magic though, and you still need to be aware of good practices to prevent interference with your signal, especially voltage signals. OTOH, T/C extensions are routinely wired w/o using shields and they measure in microvolts.

> 2) why do shielded cables have bigger capacitance than unshielded cables?

Because a capacitor is two conductors seperated by an insulator - which is just what a shielded cable is.



Bob Peterson

 

> 1) why does a twisteed pair shielded cable have a better immunity to
incoming noise, protects the transferred signal better? <

[DP]
If you consider capacitive coupling to a conductor causing interference, the voltage induced in one conductor in a twisted pair is also induced in the other, thus common mode rejection can work. In a non-twisted cable the noise
induced in one conductor is different to the noise induced in the other as the one conductor is closer to the source of the noise, common mode rejection cannot cancel this noise.

> 2) why do shielded cables have bigger capacitance than unshielded cables? <

[DP]
Do they really???

Donald P

 

Jim, any component of the noise induced in a wire is sourced from some physical location. The strength of the signal which is induced is reduced as distance from the source increases. Thus if the two wires in a pair were parallel, and at a differing distance from the source, one wire would pick up a little more noise than the other, creating a differential. With twisted pair, the wires take turns being closer to the source as you move down the length of the cable, and the difference is averaged out over a length of cable comprising at least one twist (thus the specs on maximum twist pitch).

 

What kind of shielded cable?

 

Good question. Let's go back to basic circuit theory. Three basic types of components to be think about, resistors, inductors and capacitors. Electrical wiring behaves like all three of these.

What do we do to prevent a wire from having too much resistance? Make sure the gauge of the wire is big enough to carry the current we need. What happens if the wire is too small? We get a big voltage drop on the cable, and the equipment at the end of the line fails.

What do we do to prevent a wire from capacitively coupling with ambient electric fields? Make sure there is a grounded metallic shield surrounding the active wiring. The ambient electric fields couple with the shield instead of with the wires inside it. What happens if the wire isn't shielded? We see harmonics and transients induced on the wires, and the equipment at the end of the line fails.
What do we do to prevent a wire from being induced with an EMF due to interaction with ambient magnetic fields? Make sure the wires are twisted, which lowers the effective loop area (ELA) of the wiring. Think of the wires as being the secondary of a transformer, where the ambient wiring is the primary. With no twist, the wires have a 1:1 step-down ratio. With a twist, they have a 1:10000000 (very large number) etc step-down ratio.