grounding/shielding question

[moritz]

While wiring up a fuzz face today, I was using shielded wire for connections between the perfboard and pots/jacks/etc, making sure to ground one end of the actual "shielding" to the back of the pot/jack/etc, when I ran into a bit of a conundrum... i'd soldered several shielded wires to the ground pad on my perfboard for use as ground connections to the various pots, jacks and switches, when it hit me: isn't shielded wire useless for ground conections?  :?

I mean, if both the internal wire and shielding are connecting to ground, doesn't the shielding render itself obsolete? As you can probably tell, i'm i bit of an electronics newb, so any help would by much appreciated.  :oops:

[moritz]

bump...  :|

[Mike Burgundy]

First let's look at shielded wire. This consists of a core (copper) strand, which is conductor1, and another strand which is conductor 2. The trick is that 2 is tube-shaped, and positioned so that 1 is inside it. This means that the entire cable works like Faraday's cage (have a look here http://en.wikipedia.org/wiki/Faraday_cage ) if you ground number 2, so it shields number 1 from electromagnetic interference. This is handy.

BUT, it's also "just" two conductors. So basically youre running two ground wires next to each other - which is a ground loop although this one won't be a problem.

Shielded wire is sometimes good, but not really necessary except for some occasions.
-the entire circuit is shielded by the grounded metal box you've put it in. So no worries about outside interference.
-*inside* interference is only a possible problem when or if:
*you've got a power transformer inside the box close to electronics
*you've got a maddeningly high gain circuit
*when you insist on having in-and output right next to each other
* when you for whatever reason need to tie-wrap all wiring together
*your wiring is close to LFO's
*anything with electromagnatic fields INSIDE the box

Keep in mind that all tracks on your pcb aren't shielded either - so if THAT doesn't give a problem...

Usually it is enough to use shielded wire for the input lead,  single strand for the rest, but keep wires nice and separated, if you need to cross wires from different parts of the circuit, do so at right angles, keep power supplies and LFO's well away from anything, preferably star-ground and your fine.

[petemoore]

I've been keeping the in/out wires against the metal box for ground plane.
 I build the perf's so the input or output is right next to the switch.

[moritz]

awesome, thanks for the help guys!