JW

Shield question. Long run. Shield break in the middle?
« on: May 06, 2018, 06:27:25 PM »
Hi folks,

As people who have used the older IZ Radar machines probably know, it's machine room or bust for these guys. They're loud.

Anyway, I'm in a position where I need to extend my analog i/o cables to the Radar. All these connections are balanced (not transformer)

I currently have the cables wired directly to the patch bay on one side, On the other side we have DB25 connectors for the Radar. Inside these connectors,  I have the shield lifted going into the Radar, which solved a ground loop issue I had.

So, question is, if I just add another set of DB25 to DB25 cables I could extend these, but can I leave that break in the shield  in the middle of the run? The ground (shield)  would still be broken going into the Radar, just half way there, not right at the end of the cables. Seems like it shouldn't be an issue, as the shield to the new extension would be then be connected to the Radar input ground.

I guess this is a try it and see sort of thing, but I'd like to hear what you guys think. . .
« Last Edit: May 06, 2018, 06:30:26 PM by JW »


scott2000

Re: Shield question. Long run. Shield break in the middle?
« Reply #1 on: May 06, 2018, 09:08:28 PM »

 I'd definitely try and see ... Sounds like the easiest route for now. ?. Not sure how the Radar is handling that shield connection or what kind of interference you're dealing with???....  Is that even an issue in this context?
...... I'm always feeling like it's luck half the time I figure out what works best for me...I really should keep notes to compare to the ways that are documented so I can compare and figure things out better in my scenarios.......

I'm sure you've seen this but, I'll leave it here
From Rane

Fully Balanced

Fully balanced systems  provide the best performance when both ends of the shield connect to units with chassis-grounded shields . When units with signal-grounded shields are encountered, disconnect the shield at the signal-grounded end . This keeps the induced shield currents out of the audio signal ground. If both units involved have signal-grounded shields, you have entered the twilight zone . This is perhaps the most common scheme. Most disconnect one end of the shield, specifically which end is disconnected creates strong political debates and is left for the individual user to decide . Never disconnect both ends of a shield.


Re: Shield question. Long run. Shield break in the middle?
« Reply #2 on: May 06, 2018, 11:15:17 PM »
If you have to lift a shield at one end to break a ground loop, one way to maintain RF shielding is to use what's known as hybrid grounding. This technique uses RF quality capacitors to maintain shield continuity at the cable end with the cut shields, with the shields directly connected to pin1 at the other (non-cut) cable end.

I used this technique, along with shielded star quad cable,  in an A/V studio in the late 80s that was a few thousand feet from a 50kW AM clear channel transmitter, a situation where any piece of wire over a few feet long would have a couple of volts of 1.03MHz on it. Allowing that to not be common mode to the shield, by cutting the shields, is a recipe for RF signal injection disaster. I used some glass encapsulated 0.1µF MLCC ceramics to connect the cut shield to pin 1 at the console end, with the shields attached to pin 1 at the remote equipment end, and it worked well. It also helped to prevent random LTC and video coupling into the analog audio paths, since the shields were low impedance well beyond the audio band.

So, a DB25 extension that connects shield at both ends would work well, but the original cable with the shields cut at one end should be replaced with one that connects the shields to pin 1 at the cut end with small (0.01-0.1µF) MLCC ceramic caps to maintain low impedance at RF. There's generally no need to avoid an RF ground as well as an LF ground, so add the caps and minimize your RF shielding hassles.

JW

Re: Shield question. Long run. Shield break in the middle?
« Reply #3 on: May 17, 2018, 12:07:27 PM »
Thanks guys. I'm out in the country away from RFI so I think I'll avoid the caps to start with.

JohnRoberts

Re: Shield question. Long run. Shield break in the middle?
« Reply #4 on: May 19, 2018, 10:02:13 AM »
Thanks guys. I'm out in the country away from RFI so I think I'll avoid the caps to start with.
I don't know if it's possible to completely escape RF with the pervasiveness of smart phones and wireless device control and communication. It is a little amusing to see old texts about shielding before the wireless bandwidth filled up.
=====
I am not a big fan of ground lifts, usually a band aid for some other problems (like pin 1 problem).  But it is a free country so lift away. I endorse the hybrid ground scheme to connect all floating grounds through HF capacitors.

To answer your actual question I would not break the shield in the middle, but at one end or the other (with caps). This will make it easier to troubleshoot at some future time.

JR 

Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.


 

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