Can twisted wire replace shielded wire?

I had to make a communication cable for a customer today. I went to Orvac's and asked for 100' feet of 4 wire shielded cable. The salesman said he went to a 3 day wire seminar from Beldon Cable. They claim that twisted wire will work as good or better than shielded cable. I asked why and he said that any noise picked up by one wire gets cancelled out by being wrapped around the other wires. I figured that would probably be true for any electromagnetic noise but what about RF noise? He didn't know, so I bought the shielded because I don't want to experiment on a customer. Has anyone used twisted wire cable for communications?

Reply to
Bill Roberto
Loading thread data ...

Absolutely not

Unshielded Twisted Pairs cancel noise out, only because each line recieves equal amount of noise, and Hence at the *Balanced* line RECEIVERS there is noise cancellation the actual noise cancellation occurs AT the receivers, and ONLY if they are balanced on each end... like computer network cables. If your communication cable setup is not balanced at each end, then the magical noise cancellation disappears, and in fact you'll get quite a bit of noise.

btw.. RF noise IS electronmagnetic, so i'm not really sure what you mean by that

--
********** 
shu 
 Click to see the full signature
Reply to
shu

Oh, Goodie .

Delta V = ~0 at about any place on the twisted pairs as a result of EM noise from the environment, assuming that it's wavelength is at right angles to the lines and it's wavelengths far larger than the pair conductor to conductor distance (excluding speed of light changes.)

How did it get there?

??

You could add a rectifier and get free energy, right? Just from the thermal background noise ...

--
Cliff
Reply to
Cliff

I'd say it depends on the application - frequency, impedance and allowable attenuation.

I personally know of an example of a strip mine that wanted video cameras at each of their gates, all viewable from a central guard shack. The gates were about a mile apart and coax had too much loss so they used twisted pairs for the video. I saw the video at the guard shack and it was flawless.

Unfortunately I know very little of the characterists of twisted pairs at higher frequencies so I wouldn't risk it. However at audio and video frequencies I'd give it a try.

Cliff wrote:

Reply to
skeptic

Twisted pairs eliminate noise because the amplifier/reciever on both ends have matching impendence.. that is to say both ends of the cable are balanced in audio and video applications it's very rare that both ends will be balanced, a twisted pair is generally bad choice for wiring audio and video. In computer networks they work very well because the network cards/devices all have the exact same impendence, and the noise is canceled out. If the impendence is NOT the same, then the twisted pair does nothing to sheild away noise.

a normal coax cable consists of an inner conductor, a dielectric, and an outer conductor, and a jacket.. these are also known as "transmission" cables, if you dont' use the right coax cable (ie matching impendence) you can have a lot of loss... ie 50ohm coax on a 75 ohm system, or 75 ohm coax on a 50 ohm system if you had a lot of loss, you probably could have just as well used a normal Shielded cable and not get the loss.. a shielded cable consists of an innerconductor, an insulator, and an EM sheild

if your twisted pair video system worked really well, then it was because both ends were balanced (quite possible) , or you dont' have a lot of EM noise around,

********** shu
Reply to
shu

No that is not what eliminates noise and that is not what "balance" means.

Both ends are _balanced_ _to_ _ground_, and each receiver has a "differential input" circuit. Which is to say the input is the voltage *between* the two wires, rather than the voltage between ground and the wire. Almost all noise is between the wire and ground and if the two wires are really well balanced there will be no differential voltage due to noise, and hence no noise voltage will be seen by the input. That is called "common mode rejection". (Do a web search on it.)

The "balance" of the two wires is improved vastly by twisting them. That assures that any electrical field which induces a voltage into one of them will induce and equal voltage into the other one. (If the wires are not twisted the one nearest to a noise source will have a higher voltage.) The higher the frequency, the more twist required to be effective.

Coax is a single ended circuit, with the receiver looking for a voltage difference between the center conductor and ground. The outer shield attempts to prevent noise from being induced into the center conductor.

Almost all audio uses unshielded twisted pair wiring. Think about the millions of miles of telephone cables!

Video is a mixed bag, as both twisted pair and coax are used in different circumstances. Most video that you'll notice is coax.

That isn't true in any way shape or form. (Actually there is a good bit of variation in the impedance of computer network devices.) The requirement is that each of the two wires has the same impedance to ground. It makes relatively little difference what that impedance is, or what the impedance of the line is from end to end. In fact twisted pair cable impedances are often listed, but for some specific set of parameters that in practice will not be what is experienced.

All of the above is equally true for twisted pair transmission lines. Note however that there will *not* be "a lot of loss" from mixing 50 and 75 ohm coax. It will actually be a very small loss. (There are other factors that are much more significant, but that gets more complicated than I'm going to get in this article. Certainly if someone wants to discuss it, we can and there will probably be a number of people who can describe it in detail.)

That describes coax.

There are such things as shielded twisted pair transmission lines. Shielding increases the loss (by significant amounts at higher frequencies) and is therefore not often used.

Generally it is the lack of noise sources that allow poor implementations to work. Unfortunately they then fail as soon as someone installs a noise source in close proximity.

--
Floyd L. Davidson             
Ukpeagvik (Barrow, Alaska)                         floyd@apaflo.com
Reply to
Floyd L. Davidson

Twisting wires helps reduce noise induced from ambient H fields. If you have a high ambient E field, you can still get common mode noise coupled to a bare twisted pair. Particularly on a high impedance circuit. This becomes a problem at low signal levels, where a CMRR sufficient to reject the noise may be impractical. Low signal level circuits are where one finds high impedances as well. In this case, carefully designed shielding can help.

--
Paul Hovnanian     mailto:Paul@Hovnanian.com 
------------------------------------------------------------------ 
 Click to see the full signature
Reply to
Paul Hovnanian P.E.

transformers typically have very a high CMRR. this may be whay they are used in network cards.

taken from an old 10Mb/s card.

1nF 2KV _______ || ______________\ _) || (_ / from _) || (____||___ to RJ12 driver _) || (_ || | socket chip_____) || (_________|____\ || | / ___L__ /////

I think the capacitor is to reduce the amount of CM noise transferred capacitatively through the transformer.

AIUI transformers are also often used for balanced microphone leads.

--
Bye. 
   Jasen
 Click to see the full signature
Reply to
Jasen Betts

Yes. But some circuits must be DC coupled.

--
Paul Hovnanian     mailto:Paul@Hovnanian.com 
------------------------------------------------------------------ 
 Click to see the full signature
Reply to
Paul Hovnanian P.E.

Wouldnt the earths magnetic field turn that wire into a long wire transformer and burn you and itself to a crisp?

Gunner

"Pax Americana is a philosophy. Hardly an empire. Making sure other people play nice and dont kill each other (and us) off in job lots is hardly empire building, particularly when you give them self determination under "play nice" rules.

Think of it as having your older brother knock the shit out of you for torturing the cat." Gunner

Reply to
Gunner Asch

Ok, but 24K miles isn't very close to infinite. ;-) A one foot copper t-line looks like a pretty good resistor for a 2E-9 seconds or so. ...and not too much further from "ideal" or "infinite". ;-)

--
  Keith
Reply to
Keith Williams

That depends entirely on how quickly the Earth's magnetic field changes, and how far the ends of the cable are apart.

OK, for safety issues, don't put the cable around the entire circumference but leave a ten-foot gap.

robert

Reply to
Robert Latest

24K miles is as close to infinite length as 0.25 seconds of steady current is to DC.

robert

Reply to
Robert Latest

Touche. However, one foot is as close to infinite as is 2E-9 seconds.

--
  Keith
Reply to
Keith Williams

That's a long time.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
 Click to see the full signature
Reply to
Fred Abse

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.