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Shielding a cable with aluminium foil.

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flote21

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Hello guys!

I am having some EMC problems in my equipment and I need to try a better shield of the cables. So in order to test it, I have been thinking to use aluminium foil. But I don't know exactly if it is necessary to connect it to GROUND or to EARTH? and if it is only mandatory to connect it in one side or in both sides of the cable?....Can anyone give a short overview about how to connect this aluminum foil in order to improve the EMC of the system?

Greetings

EP
 

Hi,

There is no general rule.

At first you should find out what's the problem.
How do you recognize it? What happens?

Shielding a cable may help, but often it's no universal solution. The problem may come back if there is another cable involved or the cables are moved.

Is it a wired or an air EMC problem?
If wired: are there EMC filters close to the connector.
If air and/or wired: do you have a rock solid GND plane?

Klaus
 

I don't know exactly if it is necessary to connect it to GROUND or to EARTH? and if it is only mandatory to connect it in one side or in both sides of the cable?
Click to expand...

In a sense, Earthing acts more on electrical protection, whereas Grounding acts more on signal integrity, so both shielding schemes are complementary.
 

For high frequency radiated EMI the shield should be connected to chassis ground at both ends and the chasses connected to earth ground.
 

flote21 said:
...I am having some EMC problems in my equipment and I need to try a better shield of the cables. So in order to test it, I have been thinking to use aluminium foil.
Click to expand...

It is more important to pay attention to detail: how to wrap the Al foil around the cables. Regular kitchen foils will be good for VHF and above but may not be good for SW and FM band. Connection to ground (local) may help but also may not. Connecting to earth is only symbolic in the sense it will prevent a high voltage shock.

If the problem persists, perhaps you may consider driving the Al foil.

EMC is black art par excellence!!
 

For most cases, the aluminum foil suffice, but it is worthly to mention that the copper as well as other metal alloys, as being ferrous, when compared to aluminum presents a better performance in the shielding of predominantly magnetic EMI, that is, from fields capable of inducing fluxes such as originated from highly inductive devices, so there is no general recipe, each case dictates a different requirement.
 

copper as well as other metal alloys, as being ferrous, when compared to aluminum...
Click to expand...

Copper is not ferrous (which mean bivalent iron). Some times Co and Ni are also considered as ferrous metals (iron group elements). Iron containing elements have excellent permeability and they are very useful in magnetic shielding.

All metals provide electrostatic shielding; the metal is always at an equipotential surface (all electrical lines of force terminate on the metal surface).

Radiofrequency shielding is more tricky.
 

Iron containing elements have excellent permeability and they are very useful in magnetic shielding
Click to expand...

Thank you for correcting me, a braided iron mesh it was the material to which I wanted refer.
 

c_mitra said:
I
EMC is black art par excellence!!
Click to expand...

Indeed, it is.
It also helps to know a little of the science behind it.

Many moons ago, on a video processor which contained digital circuitry and analog-TV circuits, we had a very annoying interference on the video signal.
We knew, via analysis with a Spectrum Analyzer, that it was the difference frequency of the microcontroller's clock heterodyning with a local oscillator.

We had tried everything, literally. No luck.

Then our most senior RF engineer looked at the board layout. He noticed a stub on the ground plane, he proceeded to cut about 5 cm with an Xacto knife, and reconnected the components to the new ground point.
The problem disappeared!
 

Hi,

nice story.
that it was the difference frequency of the microcontroller's clock heterodyning with a local oscillator.
Click to expand...
Because of this - when I design such a sensitive ADC circuit - I try to use only one clock source and divide/generate all necessary clocks from this one.

Usually the interference isn´t really gone, but in most cases it just generates a DC offset. --> Easy to calibrate out, because it is stable.
In worst case there are some known/calculable frequency residuals that can be filtered out with a digital filter.

Klaus
 

Hello guys!!

After breking my head during a complete day, finally I solve the problem without any Al foil. The probles was that there was a 0Ohm resistor wich connected the EARTH with the GND and it should be not mounted.....

Greetings!
 

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