Electronics > RF, Microwave, Ham Radio

Coverage of Ferrite Tiles in EMC Chamber/Shielded Room

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TimNJ:
Hi all,

At work, we have a small shielded room (about 4m x 2m x 2m) that we use to run CISPR32 conducted emissions pre-compliance testing, and we also use as a controlled environment for radiated emissions debugging.

We are trying a new-setup for CISPR25 (automotive) pre-compliance conducted emissions testing. The main difference compared to what we were doing before, is that the DUT and its associated cables are placed above a conductive metal plane. Additionally, instead of a tightly bundled cable, the cables are spread out forming a relatively large loop area.

Compared to our previous CISPR32 CE results (150KHz - 30MHz, AC input), where there are no notable resonances or setup related issues, the CISPR25 (150KHz - 108MHz, DC input) results show some pretty severe resonances/peaking at 10MHz, 20MHz, and 60MHz. We have reference data from a real test lab (for this DUT), and no such resonances appear in their plots. Granted, there are some differences in setup. For instance, due to space constraints, we arranged the cables in a serpentine, whereas the standard wants the cables arranged in a straight line. This could have a big effect. Additionally, the test setup is positioned directly against the metal chamber wall, whereas the standard wants at least 1m. These type of issues require more investigation.

While it is foolish to think we can get 100% comparable results to a lab with a (probably) $500,000 chamber, I'm curious to see if we can knock down some of the resonances without also spending $500,000.  8)

----

So, here's my question, which will probably neatly highlight my lack of understanding:

>>>Would there be any value in using a small number of lossy ferrite tiles in the direct vicinity of the test setup? Take a look at my attached photo as an idea. The actual coverage of the chamber would probably be 5% or less.

My idea is to try to limit reflections "at the source". Intuitively, it seems that placing tiles near the emitting source would be more effective than on a far wall, but I wonder how much coverage you really need to make a dent. I guess I want to understand if the incremental improvement with additional tiles is 'linear', or if the incremental improvement is 'diminishing'. Does that make sense? Anyone have an idea?

The usual approach is to cover the entire room in ferrite tiles, but given the price of the tiles, that's not practical. Plus, it would probably just be plain stupid to spend so much money on a setup which is not compliant with any particular standardized setup.

Thanks,
Tim

Dulus:
Are these resonances here in the room with us now?  >:D

Well, with nothing to absorb the RF in the air, your shielded room is a reverb chamber for sure.
Yet for the size of the room, resonance frequencies you've observed seem unlikely to be caused by the chamber resonance.
First simple test that comes to mind is to open&close the chamber door while the test is running, and observe those resonances. Any amplitude or frequency change?

My guess would be the resonances are occuring between the metal table and your circuit...

TimNJ:

--- Quote from: Dulus on May 19, 2022, 02:54:34 pm ---Are these resonances here in the room with us now?  >:D

Well, with nothing to absorb the RF in the air, your shielded room is a reverb chamber for sure.
Yet for the size of the room, resonance frequencies you've observed seem unlikely to be caused by the chamber resonance.
First simple test that comes to mind is to open&close the chamber door while the test is running, and observe those resonances. Any amplitude or frequency change?

My guess would be the resonances are occuring between the metal table and your circuit...

--- End quote ---

Thank you for sharing your knowledge and intuition.

I think your comments about the resonance between metal table and circuit is definitely on the right path. In fact, for our first test of the setup, we placed the DUT directly on the metal plane (no 2" / 50mm spacer). The 10MHz, 20MHz, and 60MHz peaks were certainly much higher amplitude and we also had a severe hump at about 1-2MHz or so. After adding the spacer, the 1-2MHz hump was squashed to the noise floor, and the 10MHz, 20MHz, and 60MHz humps came down about 10dB (~about 30% reduction).

Still, compared to the reference measurement from the accredited test lab, there's still some serious peaking at the above mentioned frequencies. So, if it was purely resonance between plane and DUT, I would figure we might see those resonances on the accredited scans too??

As I mentioned, there is still some larger structural differences like serpentine cable configuration and distance from wall. I think the electrical bonding of the table to the wall is also quite poor in our setup. So, there are certainly many other potential issues.

Thanks for the idea about opening the chamber door. Will try it! What would be the range of expected resonance frequencies for the mentioned chamber size?

I am still curious to know if anyone has any intuition of using a small number of tiles, and the ideal arrangement/position...although I'm sure it will depend a lot on DUT and chamber geometry.

Thanks!

TimNJ:
My microwave theory and knowledge of cavity resonators/waveguides is a very shaky, but ballpark estimate based on 1λ would probably indicate at least 50MHz fundamental resonance, and more like 75 or 100MHz and above.

https://learnemc.com/EXT/calculators/Cavity_Resonance_Calculator/rect-res.html

On the other hand, the cable length is in the ballpark of 3m, so based on 1/4λ, that might indicate something like 25MHz due to the cables over the ground plane. Again, I'm a total dunce with respect to most of this.

radiolistener:

--- Quote from: TimNJ on May 19, 2022, 12:18:31 am --->>>Would there be any value in using a small number of lossy ferrite tiles in the direct vicinity of the test setup? Take a look at my attached photo as an idea. The actual coverage of the chamber would probably be 5% or less.

My idea is to try to limit reflections "at the source"

--- End quote ---

Note that there is near field coupling at distance less than lambda / 2, which is very large for a long wavelength like 30 MHz and below. So if you place some ferrite or metal thing at near your equipment you can make interference worse in some cases (where main roles plays near field coupling). Because that ferrite or metal thing will work like antenna for interference and your DUT will be coupled through near field with that antenna.  :)

In order to limit reflections you're needs to use special coating with high absorption properties and special geometry of walls to change angle of reflection and absorb it. Something like that:

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