Grounding Shielded Cable

shiney · #1 02-26-2009, 09:30 AM

What is the best way to ground shielded cable ? I have had an email saying;

"All servo wires should be shielded and the shields grounded at both ends of the harness". Is there a particular way or technique involved?

Martin

JonJay · #2 02-26-2009, 10:00 AM

you can "tease" the shield off of the cable with a pick, then twist it into a braid, wick it with solder, and crimp a lug on it. You dont need to solder it but it helps keep the tiny wires from fraying out of the lug. Tease enough off so you can deal with it before you strip back the other insulation and wire.

noelf · #3 02-26-2009, 10:04 AM

web site has a good picture tutorial on how to deal with shielded wire terminations...

shiney · #4 02-26-2009, 10:10 AM

Thanks fellas, just checked out Bobs site, great pics.

Martin

Mike S · #5 02-26-2009, 10:25 AM

Quote:

Originally Posted by shiney

" and the shields grounded at both ends of the harness". Martin

Both???

IIRC, other than antenna coax, it is typical to just ground one end of shielded wires.

Is this normal for servo wires??

fl-mike · #6 02-26-2009, 10:41 AM

Quote:

Originally Posted by Mike S

Both???

IIRC, other than antenna coax, it is typical to just ground one end of shielded wires.

Is this normal for servo wires??

Normally shields should be grounded at both ends. This is especially important for higher frequency signals, with RF being in that group. Note that you could be trying to keep high frequency signals in or out, or both. (emissions and susceptabilty) Sometimes low level audio shields are only grounded at one end. But this is really more of an isolation of grounds thing to keep other signals from taking a shortcut on the shield for their (ground) return currents and thereby adding noise to your microphone signal (for example).

noelf · #7 02-26-2009, 01:32 PM

...TDR measurement equipment.

Just out of curiosity, I measured some RG142 / 50 ohm coax, and some RG59 C/U 50 ohm coax. Both showed 50 ohm z.

Then I "measured" some of our mil spec shielded wire. Here is what I measured:

Single conductor shielded (as used for P leads)...15 ohm, very flat, very consistant.

Two conductor shielded...one conductor to shield = 22 ohm. Differential, conductor-to-conductor,
single ended = 35 ohm. Very flat, very consistant.

Three conductor shielded (as in audio phone jacks, mic jacks)...just measured one conductor to shield = 28 ohm and extremely flat trace.

My conclusions for this shielded mil spec wire is that it is very high quality and exhibits a very uniform impedance. If we ever needed a "coax" cable with a low impedance (15, 22, 35, 28 ohm), then this shielded cable would work quite well.

Now, back to the subject of this thread...if the manufacturer is calling out that both ends of the braid of this shielded cable (don't call it coax) should be attached to gnd, then by all means follow the suggestions. The design point and implementation called out may have nothing to do with single ended driver / receiver circuits and the techniques used to minimize the potential for noise reduction. It may just be a requirement of the specific equipment that is being attached...

rv9av8tr · #8 02-26-2009, 02:01 PM

OK folks, I'm not an EE, but it's always been my understanding that the shield should only be grounded at one end. The shield is meet to bleed off induced RF noise TO ground, you don't want it carrying current as an actual conductor.

The only exception I've seen to this is LSE's power circuit where the shield is tied to the unit AND back to the battery ground.

EE's.... chime in here!!

Mike

SMO · #9 02-26-2009, 02:10 PM

I was looking at Bob N.'s book a couple days ago and noticed he recommends shielded cable for the mag P lead, AND recommends using the shield as the ground wiire, one end connected to the mag switch and the other end connected to the engine ground.

alpinelakespilot2000 · #10 02-26-2009, 02:51 PM

Quote:

Originally Posted by rv9av8tr

OK folks, I'm not an EE, but it's always been my understanding that the shield should only be grounded at one end.

As I understand it, the general rule of thumb is ground only one end of a shielded wire that you want to keep noise OUT of. Ground both ends of a shielded wire if you want to keep the noise INSIDE. Not sure which one of these would apply to autopilot servo wires.