Terminating a cable with a metallic braid

[Matt]

Hi, I'm just wondering if any of you know why you must take caution when terminating a cable with a metallic braid.

Thanks

[audioguru]

Hi Matt,
Welcome to our forum. ;D
I don't understand what is a metallic braid. What is the metallic braid's function? Do you mean a shielded audio cable?

[Matt]

I believe a metallic braid is used to bind all the strands together in a multistrand cable.

Cheers

[audioguru]

I just twist the strands together before tinning them.

[windoze killa]

The metalic braid you are refering to is the shield of the cable and is generally connected to earth or ground. If you are talking about a cable designed for audio then you can comb the strands out and twist them together. This and the centre conductor form an un-balanced cable. You terminate it in what ever way is required in the circuit.

Now, if the cable is an RF cable then it is a slightly different matter. The metalic braid is still a shield and is always connected to ground and is also an unbalanced cable. BUT, because of the frequency of the signal the impedence of the cable and any termination will affect the amount of signal that is passed (this is also true about the audio cable but is less critical). As such any termination must be very good and the use of specific connectors is required.

Also as I have called these unbalanced cables I guess I should explain what this is. An unbalanced cable implies that the 2 conductors are at different potentials and the shield is nearly always at gound potential. A balanced cable implies that the 2 conductors are balanced and at the same potential. Although a balanced cable doesn't have a shield they still do not radiate as much as one would expect. This is because the current flow in each conductor (when terminated in the cables characteristic impedence) is equal and opposite and tends to cancel the radiation.

WOW. Now that hurts the memory cells. I could go a lot deeper but I will avoid that for now.

More than willing to continue if anyone would like.

[indulis]

And don't forget about a "driven guard" and don't connect both ends of the shield drain wires in certain app's where EMI/RFI could be an issue, only connect one end.