Author Topic: Is EMI 'line of sight'?  (Read 3027 times)

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Is EMI 'line of sight'?
« on: March 21, 2015, 08:25:09 pm »
Hi

Possibly a dumb question, but can EMI be blocked by a simple grounded 'wall', or does the EMI bounce around the interior of an enclosure and render that useless?

I ask because although you usually see sensitive components (or presumably sources of EMI) enclosed in a full can, I've seen teardowns where there's just a short vertical grounded conductor, with the explanation; "it's for shielding".

Thanks
John
 

Offline tautech

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Re: Is EMI 'line of sight'?
« Reply #1 on: March 21, 2015, 08:39:09 pm »
EMI is just a broad spectrum of radio waves.
Shielding protects sensitive circuits more so than controlling emissions although it does help.
Any circuit that amplifies signals are much more prone to EMI than say a digital signal path.
Hence the popularity of digital circuitry in our everyday lives.

No it is not LOS

It is a science, just look how EMI test chambers are built( forgotten their name :palm: )

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Offline Neilm

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Re: Is EMI 'line of sight'?
« Reply #2 on: March 21, 2015, 08:45:04 pm »
It is a science, just look how EMI test chambers are built( forgotten their name :palm: )
Anechoic chambers or Open air test sites?
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Offline tggzzz

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Re: Is EMI 'line of sight'?
« Reply #3 on: March 21, 2015, 08:48:14 pm »
Radio propagation is not line of sight, so EMI is not LOS. The details depend on the wavelength and materials.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline tautech

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Re: Is EMI 'line of sight'?
« Reply #4 on: March 21, 2015, 08:55:12 pm »
It is a science, just look how EMI test chambers are built( forgotten their name :palm: )
Anechoic chambers or Open air test sites?
Thanks Neilm

Have a look at this thread:
https://www.eevblog.com/forum/testgear/emc-chamber-build-log/msg423564/#msg423564
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Re: Is EMI 'line of sight'?
« Reply #5 on: March 21, 2015, 09:16:13 pm »
So what might be the function of a simple 'wall' fixed to a PCB, between two part of the circuit?

J
 

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Re: Is EMI 'line of sight'?
« Reply #6 on: March 21, 2015, 09:25:05 pm »
Depends on the nature.  Close up (less than a wavelength distance), impedances can differ strongly from the impedance of free space (377 ohms, or usually some fraction of that due to resonance and geometry -- hence 50 ohm coax and 120 ohm twisted pair, to name but a few examples).  In particular, the fields due to inductors are heavily magnetic (low impedance), while those of loose wires (not really capacitors, because their fields are internal) are largely electric (high impedance).

Traces of a given nature (high or low impedance) are susceptible to fields of the same nature.  Or for sensitive nodes, it doesn't much matter how it gets in, it just has to be quiet, period!  The main difference is, electric fields simply need to be shunted to ground, hence, a conductive plate between the areas.  It doesn't have to be closed, though it helps (electric field will still creep around the sides).  To block magnetic fields, you need a closed loop... which can also be just a plate, if the currents tend to circle around in the plate; but again, a closed box helps, too.

It's noteworthy that extreme magnetic fields are best shielded by ferrite plates rather than trying to crowd them out with copper sheets.  In one product I worked on, the fields were *overheating the enclosure itself*... some ferrite took nice care of that.

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Offline Neganur

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Re: Is EMI 'line of sight'?
« Reply #7 on: March 22, 2015, 03:17:49 am »
Radio propagation is not line of sight, so EMI is not LOS. The details depend on the wavelength and materials.

I think this is misleading, radio propagation is not limited to line of sight but a great portion of it does actually sit inside the line of sight region (and a lot of the rest in the first fresnel zone)

@icon:
You can prevent unwanted radiation from escaping your circuit by enclosing it in a metal can, as you say. It doesn't prevent sub-circuits from interacting with each other and in such a case a grounded metal wall can help. You can reduce reflections inside the can (cavity resonance) by placing RF-absorbing materials inside the can (e.g. glued to the lid).

Keep in mind, not all EMI is radiated.
« Last Edit: March 22, 2015, 03:29:49 am by Neganur »
 

Offline tggzzz

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Re: Is EMI 'line of sight'?
« Reply #8 on: March 22, 2015, 10:01:50 am »
Radio propagation is not line of sight, so EMI is not LOS. The details depend on the wavelength and materials.
I think this is misleading, radio propagation is not limited to line of sight but a great portion of it does actually sit inside the line of sight region (and a lot of the rest in the first fresnel zone)

... and elsewhere. Consider reflections. Bits of metal can act as aerials, absorb energy, conduct it elsewhere and re-radiate it. All those effects are especially important at higher frequencies.

As a simple familiar demonstration, without such non-OLS effects cellphones simply wouldn't work.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Re: Is EMI 'line of sight'?
« Reply #9 on: March 22, 2015, 09:14:54 pm »
Thanks for all the replies, everyone.

J
 

Offline Neganur

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Re: Is EMI 'line of sight'?
« Reply #10 on: March 22, 2015, 10:26:14 pm »
... and elsewhere. [...]
yes of course :) like I said, not limited to.
 

Offline w2aew

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Re: Is EMI 'line of sight'?
« Reply #11 on: March 23, 2015, 01:36:47 am »
So what might be the function of a simple 'wall' fixed to a PCB, between two part of the circuit?

J

One common purpose for a vertical "grounded" plane or wall between circuits is to reduce capacitive coupling between two parts of a circuit.  Without the wall, there can be parasitic capacitance between components, etc.  With a grounded wall between them, these same components will see a parasitic capacitance to ground instead of each other.
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