[email protected] wrote:
> Hello group,
>
> I recently carried out a conducted emission in our unlined screen room
> with no
> EUT connected. I.e just the LISN inside and measuring receiver outside
> the
> room. I was expecting the plot to be a horizontal line and very low
> perhaps in
> the region of 10 or 20dB. Instead I noticed that the plot starts at
> about 40dB
> at 150kHzand decreases to about 20dB at around 1MHz and then after
> that is a
> horizontal line all the way to 30MHz. Bearing in mind that the room is
> very
> well grounded and I use a reputable measuring device. My questions
> are:
>
> 1-Is this what one would expect to see?
> 2-Why the noise floor so high in an screen room?
> 3-What can we do to reduce the noise floor?
> 4-What is the best method to ensure that the room is O.K?
>
> Any help and advice is appreciated.
>
> With regards
> Peter
>
> E-mail: [email protected]
Hi Peter
I made a similar experience while measuring in a relatively noisy
environment.
-LISN without power line, bonded to a large, earthed, metallic
surface
-Receiver (ESHS30, Rohde & Schwarz) placed on top of the LISN. The
only connection
between LISN and receiver through a 2m coax cable.
-Receiver connected to the a power line socket.
The measurement errors occurred even without a LISN, just by bonding the
other
end of the cable to the metallic surface (of course we placed a 50 Ohm
terminator at
that end)
The measurement disappeared by:
-placing the receiver on the metallic surface
-placing a power line filter in the power line of receiver
(filter bonded to the metallic surface)
-receiver bonded to the metallic surface
Conclusion:
HF fields n the environment or voltages between the earth pin in the
plug and
the metal sheet induced currents in the loop metal sheet, coax cable,
receiver
earth pin of receiver.
Assumption:
The measuring results were the voltages induced in the central conductor
by the
currents in the shield. These voltages disappeared in the moment
receiver and
LISN were on the same HF potential.
Suggestion:
I assume that your measuring coax cable was bonded to the wall of the
screened room. In this case you might have the same problem we had.
Try following:
1. Bond the receiver to the screened room and place a power line filter
(bonded to the outside screened room) in the power line of the receiver.
2. Avoid large loops (Place the coax cable close to the wall of the
screened room.
3. Ferrites on the coax cable might also help.
Good luck
George
--
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* Dr. Georg M. Dancau * HAUNI MASCHINENBAU AG *
* [email protected] * Manager EMC Lab *
* TEL: +49 40 7250 2102 * Kampchaussee 8..32 *
* FAX: +49 40 7250 3801 * 21027 Hamburg, Germany *
**************************************************************
* home: Tel: +49 4122 99451 * Hauptstr. 60a *
* Fax: +49 4122 99454 * 25492 Heist, Germany *
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