Not inside a transmitter, but at a transmitter site where you use
1/8" thick, 6" wide copper straps for grounds.shows a nice burn mark on a large copper feedline from a transmitter.
-- Anyone wanting to run for any political office in the US should have to have a DD214, and a honorable discharge.
You connect the shield at one end. The input. Otherwise you can have all kinds of problems.
-- Anyone wanting to run for any political office in the US should have to have a DD214, and a honorable discharge.
And so they do - "USB isolators". Spendy, too.
-- Les Cargill
It's connected at one end. That should be enough to drain off EMI. Yeah, you'll have residual emitted hash from the shield, but presumably that's less bad than massive hum because of case potential difference.
I think the term of art is "telescoping ground".
Remember, this only matters if there's enough potential between the two case grounds are a problem. Ideally, this is never the case. You can also put a filter in series on the ground line.
For USB, the frequencies may also be far enough apart that it doesn't matter.
Signal ground should(?) probably be isolated from case ground.
-- Les Cargill
Yes, I spent $400 (of the boss' money) on the isolated hub. They aren't commonly needed so, of course, command a premium. They're usually built a lot better than the ordinary consumer crap, though.
The shield is not there to serve as a conductor in the cable. It is a shield to keep EMI from radiating or from entering the cable. It works by being held at ground potential.
The problem with connecting the power ground and the shield ground at both ends of the cable is that this creates a ground loop. Whether or not the ground loop is a problem depends on the details of how they are connected. It is customary for the shield to be connected at just one end of the cable. I expect in 99.9% of installations it makes no effective difference.
-- Rick
Phil keeps asking about hallucinatory drugs. I guess he has run out and is looking for a new supply.
-- Rick
A conductor grounded on one end is also known as a "vertical antenna." It will resonate at multiple frequencies, and the voltage at the far end will be jammed directly onto the wires inside the shield/antenna.
A room nowadays is flooded with RF. You can often hear cell phone bursts getting into PA systems and such. Too many unconnected shields!
Case potential differences are *reduced* by a continuous shield. Opening the shield certainly doesn't help the signals inside. If the grounds are at seriously different voltages, go differential on the signals. Ground the shield at both ends to reduce the common-mode voltage.
What about ESD? The shield and connector shell discharge that... if the shield is connected!
-- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
USB is differential signals. So it's at least partially the same argument.
Thing about USB is that you're much less likely to have power from different service points. You are, however, likely to have wall warts, and wall warts emit noxious ground potentials sometimes.
Unfortunately, USB is usually cheap consumer stuff, so these things happen.
Had I never been bit by it, I wouldna said nothin'. :) In this case, I was able to sub in a different power supply.
Transformer coupled audio is pretty rare these days.
-- Les Cargill
Oh, I understand. Ideally, you never have to think about this but teh dread god Chaos sometimes plays jokes on us...
Any hi-Z input is susceptible to cell hash.
... but sometimes in tragic ways...
It's a Hobson's choice - if the problem present is a ground loop, you take your chances on all the other stuff you mention. Fortunately for USB, it's a small antenna.
And yes, I had a USB circuit ( not of our design, some cheap Chinese stuff being used as test equipment ) that had a ground loop on USB causing poor performance. We replaced the "power supply" for the offending device and it all worked.
I would not use USB for deployable systems unless I had no other choice.
-- Les Cargill
Maybe, but there ARE code exceptions (for isolated power - it seems odd, but this HAS bit me, and others, on occasion). If the USB device is bus-powered, usually there's no ground loop created. If the USB device has an AC power cord, the only safe connection will have some power limit element in the shield circuit.
RC and L have already been suggested: let me add "fuse" and PTC resistor to the list).
I've seen scorch marks on the USB shield wiring after a "safety test".
No, it's not. The goals/priorities are completely different.
The FCC still cares, though.
It? Which "it"?
Wrong.
A similar thread was on Time-Nuts a while ago with similar conclusions. The thread starts here:
Noise and non-linear behaviour of ferrite transformers
A relevant post was made by John Miles, KE5FX:
Date: Sun, 20 Jul 2014 From: John Miles Subject: Re: [time-nuts] Noise and non-linear behaviour of ferrite transformers
I often find that when I use coaxial baluns to cut down on ground loop noise, I end up with more noise and interference than I started with. Not always, but often enough that I'm leery of them.
Due to skin effect, most signal propagation in a coaxial cable takes place between the outer surface of the center conductor and the inner surface of the braid. Ideally, the outer surface of the braid just underneath the jacket will act like an equipotential shield to keep external EMI away from the signal path inside the cable.
But that's really only true when the cable connects two devices in well-shielded enclosures that are themselves at a similar ground potential.
When you "lift the ground" with a coaxial balun such as an FTB-1-1+, you can no longer pretend that the coax braid is at ground potential along its length. From an RF perspective the braid is floating at one end, which makes it an antenna.
Put another way, a balun will reject common-mode signals in favor of differential signals. That's fine if you're using it with a twisted pair or other balanced line (you're probably aware that this is how UTP Ethernet cables work). RF interference in such a line is picked up equally by both conductors and rejected by the balun. But a length of coax cable is as far from a balanced line as you can get. One conductor is well-shielded, the other has its outer surface flapping in the breeze. The balun can't tell the difference between desired signals on the inside surface of the braid and undesired signals on its outside surface. They both look like differential-mode signals, relative to the inner conductor.
The same thing happens with instruments that allow you to lift the ground at their input jacks. Apart from the unwanted-antenna effect, this is almost always a bad idea because it's very hard to properly ground the jack's outer shell to the chassis. Few things in EMC are more important than ground integrity at the point of entry to an enclosure.
When fighting ground loops, a good first step is to minimize the loop area if you can. Try plugging your DUT, reference, instrumentation, and computer into a single power strip. That will take care of most of your power-line interference problems. Baluns can help too, but don't be surprised if they don't.
- john, KE5FX Miles Design LLC
You can search the Time-Nuts archive using this url plus your search string:
There is an amazing amount of valuable information in the archive.
I can't believe how much misinformation there is, still persisting today.
Tim
-- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
** Only ones used with microphones where there is obviously no option.
In 40 years of working with professional audio, all XLR cables seen have pin 1 linked to the shield at both ends.
(The metal case of the plug itself may or may not be linked to pin 1.)
** Huh ?? Wot utter nonsense.RCA leads supply the *only* ground connection to most audio items that use them - cos for the last 30 years or more almost every thing made is Class II or "double insulated" so does not have a safety ground connection.
.... Phil
Who are you talking about? I've had to cut hundreds of connections to eliminate ground loop problems in studios. Everything run on a single phase, of 120/208 three phase. This was all balanced, transformer coupled audio lines with a transformer at each end and inside metal equipment cases.
-- Anyone wanting to run for any political office in the US should have to have a DD214, and a honorable discharge.
I guess you're too young to remember turn tables.
Try that with an EMC susceptibility test...
If it was humming even with isolation (and the shield made any difference despite the supposed isolation), that's their problem. Hey, just because it costs $10k doesn't mean it was made at all correctly...
...And it doesn't mean that cutting the shield is ever a good idea in general. Maybe it works in one special case, maybe it works in a few. It ain't a general rule, and there's good reason, whether you understand it or not.
Tim
-- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
What good is passing your test if the equipment doesn't function?
Nonsense.
General rules aren't universally useful.
e ** Pass with flying colours.
Got any clue what a ( high quality, audio line) transformer does to RF ?
** The problem is mainly historic and has to do with the internal signal gr ounding inside the equipment. Because makers often linked XLR shields to va rious points on PCBs, induced hum current flowing in the shields of cables could result in hum voltages appearing across PCB earth patterns. Breaking the shield at the send end fixed this.
However, linking all XLR pin 1s direct to metalwork at entry and exit point s eliminates the possibility. PCB ground can then link to the metalwork at a single point via say a 100ohm resistor and a 100nF cap in parallel.
... Phil
.... Phil
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.