Shielded wire question

Thread Starter

Videodrome

Joined Nov 12, 2009
64
may be a stupid question. I am working on a project for this weekend which is replicating a circuit and it seems the original makes use of shielded cable quite a bit. i don't have any on hand, but im assuming that i could get by by just using two normal wires heatshrunk together with one acting as the core wire and one acting as the shield correct?
 

AlexR

Joined Jan 16, 2008
732
Not really. What you are proposing would be an unshielded untwisted pair which would have absolutely no noise immunity at all. The idea of a shield or screen ( as a the name implies ) to shield the signal conductor from electrical noise by enclosing it in an earthed conductive sleeve, in effect in a Faraday cage. Running a single earth wire beside the signal conductor will not provide very much shielding at all.
 

t06afre

Joined May 11, 2009
5,934
You may twist two wires together and use it to test. This setting may pick up some extra noise, but this will to some extent depend on noise sources in your environment .
 

AlexR

Joined Jan 16, 2008
732
Twisted pairs work best if neither side is tied to earth so that the induced noise signals can cancel. A twisted pair in which one side is earthed is likely to pick up more noise than a straight untwisted connection.

You really should try to get shielded wire for your project. You can try your original idea of running pairs of wires (untwisted) and depending on the what your circuit does and how noisy you environment is it might work in the short term (or might not) but in any case you should plan to replace the wiring with shielded cable as soon as you can.
 

studiot

Joined Nov 9, 2007
4,998
I am working on a project for this weekend
Since you haven't explained the project it is hard to comment. Audio? video? RF?

The reason I say this is because the 'shielded cable' may actually be a carefully engineered transmission line, depending upon the application. In this case you will definitely not be able to substitute without other design changes.

Are you also aware of the rules for connecting the 'earthy' shield at one point only to avoid ground loops?
 

Thread Starter

Videodrome

Joined Nov 12, 2009
64
its an audio application, an effect pedal for strings. it looks like the original circuit used the shielded cable in leu of just using 2 wires, the core would go to one point and the shield would be wound at the end, tinned and sent to another point close by. theres also uses of the shielded wire where the main core and shielded wire go to different points on one end but on the other it only seems as if the core comes out and goes to the point, like the shield was tied back and heatshrunk leading to nowhere. ill take a picture if need be. one direct example is that theres one bus coming from the leg of a lf353ic and the main core connecting to that, and then another bus connected to the ic leg with the shielded section attached, and then both the main core and shield are sent to a 'clean' knob (3 band eq:-clean-level-dirt).
 

t06afre

Joined May 11, 2009
5,934
Using a unshielded twisted cable will reduce inductive coupling
I would recommend this book http://books.google.com/books?id=qePLhXyx9jIC I do not know if you can anything out of the experiment on page 72-73 but it is important.
From this book
A shielded twisted pair has characteristics similar to a triaxial cable and is not as expensive or awkward. The signal current flows in the two inner conductors, and any induced noise current Flows in the shield. Common-impedance coupling is therefore eliminated. In addition, any shield current present is equally coupled (ideally). by mutual inductance, to both inner conductors and the two equal noise voltages cancel.
An unshielded twisted pair, unless its terminations are balanced provides very little protection against capacitive (electric field) pickup, but is very good for protection against magnetic pickup. The shielded twisted pair provide the best shielding for low frequency signals, in which magnetic pickup is the major problem. The effectiveness of twisting increases as the number of twists per unit length increases
See fig 2-31 p 77
 

studiot

Joined Nov 9, 2007
4,998
it looks like the original circuit used the shielded cable in leu of just using 2 wires, the core would go to one point and the shield would be wound at the end, tinned and sent to another point close by. theres also uses of the shielded wire where the main core and shielded wire go to different points on one end but on the other it only seems as if the core comes out and goes to the point, like the shield was tied back and heatshrunk leading to nowhere.
So do you understand the difference between combined shielding & signal return and shielding only?

You will need to work out which cable does what - and why - and then find the main earthing point in the equipment..
 

Duane P Wetick

Joined Apr 23, 2009
435
A shield grounded at 1 point in a cable acts as a true shield for the contained wire as no current can travel in the shield. This is why the opposite shield end is tied back and covered with heat shrink tubing. Also, the contained wire cannot induce any of its energy to adjacent sensitive parts of your circuit. Thats the double edge sword, so to speak, of shielding a wire.

Cheers, DPW [Spent years making heaters out of op-amps.]
 
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