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Author Topic: Shield grounded at one end only  (Read 7507 times)

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Online Chaoticone

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Re: Shield grounded at one end only
« Reply #20 on: February 07, 2014, 05:55:46 PM »
Thanks again guys - meanwhile another (related) question: When we talk about noise - we're talking about induced voltages right? Any ideas on what levels we could be talking about? i.e. what sort of voltages can be induced into a screen?

If you know what the source of the noise is you may able to get a good idea here.

http://edn.com/electronics-blogs/bakers-best/4371301/EMI-problems-Part-three-strength-of-EMI-radiated-signals

Brett
;D If you could see the things I have in my head, you would be laughing too. ;D

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Re: Shield grounded at one end only
« Reply #21 on: February 07, 2014, 06:01:55 PM »
Thanks again guys - meanwhile another (related) question: When we talk about noise - we're talking about induced voltages right? Any ideas on what levels we could be talking about? i.e. what sort of voltages can be induced into a screen?

It's not hardly that simple....  There are many types of noise, and many ways for it to cause problems.  One of the purposes of a shielded cable is to provide a constant impedance over the length of the cable.  A break in the shield causes a change in impedance, which can cause the signal to be reflected back to the source, even in the absence of any external noise sources.  Noise can couple capacitively, magnetically, or electrically.  It's a very complex business.  Working with 5V signals, it's not terribly diffcult to get induced noise of the same magnitude, or greater, than the signal itself.

That said, you really should not havve any problems it you follow some pretty basic good grounding, and routing, practices, and keep the signal impedances as low as possible, by using stiff pull-up resistors.  A good shield, connected at the source end only, should work just fine, except in unusual cases.  Connecting them at both ends *can* work better, but only if the entire system is well-understood.  In the typical hobby CNC case, it's not, beause you don't know how things are connected inside the power supplies, motor drivers, BOBs, etc.  Connecting at one end will avoid a lot of potential ground loops.

My machines are wired with shields connected at the source end only.  All my limit and home switches, and other signals are 5V only, but with 300 ohm pull-ups.  I even run the servo motor cables and their encoder cables side-by-side, actually Zip-tied to each other about every 12 inches, for almost 20 feet, and I've never had a single noise-related problem.

Regards,
Ray L.
Regards,
Ray L.

Offline Tweakie.CNC

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Re: Shield grounded at one end only
« Reply #22 on: February 08, 2014, 03:09:39 AM »
Quote
But after all this I'm back at my original question which was why commercial ready made up cables have the shield connected at both ends. I guess it's either that or just one end so the designer tossed a coin and said - two it is

Ian,

I just checked a couple of USB cables that I have here and the PC Earth is not carried through like it is with LPT cables - with my USB cables it is the signal GND which is  carried through.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

Offline stirling

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Re: Shield grounded at one end only
« Reply #23 on: February 08, 2014, 07:28:33 AM »
RICH - understood - but my question was about specifically trying to get a handle on the AMPLITUDE (Vp2p/Vrms/dB) of the noise we might typically see. After all if we're happy to talk about signal level and we're happy to talk about signal to noise ratio then we MUST be happy to QUANTIFY noise in the amplitude domain - yes? I know it's kinda like asking how long is a piece of string - but just trying to get a picture. We hear all the time about "noise" in "general terms" but no-one ever quantifies it. That's my interest in asking is all.

Ray - thanks - but see my answer to RICH.

Tweakie - Interesting you mention USB because I tried to find if the USB spec specified how the SCREEN is connected on those also. All I could find was one link that said the screen is NOT connected at the USB device end. So it looks (though not conclusively) like USB is in the one-end-only camp.

Brett - perfect - exactly what I was after - thankyou.

Offline RICH

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Re: Shield grounded at one end only
« Reply #24 on: February 08, 2014, 09:49:10 AM »
Don't have any numerical values. If i get a chance will blow the dust off the specturm analyzer and show / create an actual example.

But in the mean time, here is an example that was a  rather interesting experiment we did a loooooong time ago.
On transmision of low power say 1 watts output from a a transciever so about 6 watts out of the antenna, we would lock up an on line pc. So we put an isolation transformer in the ac line to it, completely enclosed the pc , wring etc, in a solid metal shield, EXCEPT for the face of the monitors glass tube, and provided a separate grounding system for the pc and downstream side of the transformer. The ground wire was actualy a 1" diameter piece of copper tubing and the inside of it was used as the conductor tied to a 10' driven ground rod.

If the transmitted freq was near (don't remember how much) the pc cp operating htz, fundamental overload occured
and the pc screen would first turn blank white and stay that way even after the transmitioin was stopped and pc unplugged
for some time. Placed a wire meshed screen over the monitor screen some distance away from it and tied it to the ground system.
Don't remember the extimated / calculated freq based on screen opening, and it did provide for interference rejection but was not satisfactory, ie; could move the interfering freq a little closer ......signal to noise ratio was down a little.

Here is another un quantified one.
Just do TIG welding near a PC . Say the pc is 15 away, shoot, it wiped the hard drive!

Another little tid bit for info:
Ever read that tag that's on an electronic device that says something like " this device complies with FCC rule ???15"
so the device cannot  radiate signals but must be able to accept them.

Another practical example:
House telephones, cheap ones had no filtering for noise or filtering that was about usless, but you would never have a problem with
a good phone made by AT&T. Not much of a problem anymore as the phones function in the giga htz range.

Then there are rules form a safety point of view that came out some 10? years ago. The field as measured in gauss? had to be within a certain level from a people location point of view......don't remember specifics.

So the EMI / noise,contol & elimination of it is not simply a some value that one can use as a quideline.
So one must measure the values to have a feel of relative intensity and that info  is tied to how sensitve the offended circuit / electrornics is. Also as distance increases from the source the field intensity drops off.

That's why you don't see any qauntification, all is specific to the situation at hand, and also most folks don't have the equipment to measure.
 
RICH