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Faraday Cage Noise Problem

  1. Jun 27, 2011 #1
    Hi everyone, I have recently built a Faraday Cage. However it seems to be working in reverse and amplifying the noise!

    My test setup:

    I place a 3.3 Mohm Bulk metal precision resistor across the terminals of the meter and measure the pk-to-pk noise using the meter.

    I now place the meter inside of the cage (the cage is floating) but now the noise increases!

    I now ground the cage and repeat the measurement and it makes no difference.

    N.B The noise that I am seeing is 50Hz mains pick up. I have sealed the door of the faraday cage with copper tape and I am quite confident that the sealing is effecting.

    Does anyone have any ideas?


    I thank you all in advance :cry:
     
  2. jcsd
  3. Jun 27, 2011 #2
    This is not really an answer, but is the meter electronic? Perhaps it's own circuitry is responsible for the noise. A related question is how relatively old (lower frequency) mobile phones operate successfully inside lifts.

    P.S. By lifts I mean 'elevators'.
     
    Last edited: Jun 27, 2011
  4. Jun 27, 2011 #3
    No this is defiantly caused by 50Hz pickup. As I open and close the cage of the door I can see the noise increase on the DVM.

    The cage is capable of almost killing mobile 3G mobile phone signals. I suspect the elevators are not sealed sufficiently to shield effectively.
     
    Last edited: Jun 27, 2011
  5. Jun 27, 2011 #4

    f95toli

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    Faraday cages do not work for inductive pickup, meaning they are useless for 50 Hz.
    Whenever you see a lot of 50 Hz in a setup it is almost inevitable due to a ground loop. Hence, the cage should not really have any effect on the noise at all, if it does it is probably due to you accidentally creating a loop or perhaps making an existing loop worse (by e.g. separating the cables even further)
     
  6. Jun 27, 2011 #5
    I have connected the meter directly to the cage and the cage is grounded, so I dont think there is ground loops. I am seeing an increase in 50Hz noise pickup when I place the meter inside the cage, when I open the door to the cage, all the cables remain stationary, but the noise now decreases.
     
  7. Jun 27, 2011 #6

    sophiecentaur

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    You need to look at the 'noise' in more detail. Have you tried a receiver in there, or an oscilloscope on max gain? A DVM is far too crude an instrument. A spectrum analyser may be over the top but could you not borrow a small hand-held scanner? Without more information you could be chasing your tail.
    What did you build your Faraday Cage for?
     
  8. Jun 27, 2011 #7
    This is a 8 1/2 Digit Meter, the best on the market, it is no ordinary DVM. It is connected to a PC via GPIB and we are able to monitor the signal waveform on the PC. The cage is built out of 6mm chicken wire.
     
  9. Jun 27, 2011 #8

    sophiecentaur

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    And does the meter also measure frequency / spectrum? It may be 'extraordinary' as a DVM but if you need another instrument, it won't do the job. How is your "noise" defined? What bandwidth etc.?
     
  10. Jun 27, 2011 #9
    *******************
    UPDATE
    *******************

    I would like to add a update. I have conducted some experiments. I noticed the following, atm I have a resistor connected to the HI and LO terminal of the DVM, I am using this as a fixed noise source. When I connect a short from the LO terminal to Ground the 50 Hz noise decreases by half.

    I have also noticed that the 50 hz pickup is the same outside the cage as it is inside the cage. However whenever I open the (trap) door of the cage the 50Hz pickup decreases dramatically. Its like the door is providing extra shielding when it is open?!?1? I am very confused.
     
  11. Jun 27, 2011 #10

    sophiecentaur

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    IS this Noise, Hum or what? How does your system distinguish?
     
  12. Jun 27, 2011 #11
    The PC that the meter is connected to has a function that will allow me to export the data to excel. I can then perform fast fourier transform on it to obtain the frequency spectrum. I have tried placing a spectrum analyzer inside the cage and I could not detect any noticeable changes in 50Hz pickup. In fact despite my spectrum analyzer having a 30Hz -3 GHz BW, I could not even pickup 50Hz when it was outside the cage.

    My spectrum analyzer has no antenna so that is probably the reason why it isn't picking up anything.
     
    Last edited: Jun 27, 2011
  13. Jun 27, 2011 #12
    Its defiantly 50Hz pickup, the DVM samples many times per second and the oscilloscope software is showing a 50Hz wave form.
     
  14. Jun 27, 2011 #13

    sophiecentaur

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    So we are looking at Hum and not noise. As I suspected,, your equipment just can't show noise (at least not above a few Hz bandwidth), but this may not be relevant.

    Earth currents can flow all around your cage in a totally unpredictable way so there is just no way of knowing what levels of hum you could be picking up. A faraday cage - certainly not a homemade one of chicken wire is really not the best way to control Hum. A copper box with soldered seams might work quite well but the door would need a very good contact all way round, with spring fingers etc.. Then you would need to be getting signals in and out - which is another path for noise and Hum.
    As usual, I would suggest going back to your original requirement and address that first. Then decide whether you need this cage and what would be the required spec. This puts me in mind of enthusiastic guys who think they are acoustically treating a room by lining it with egg boxes.

    Re your comment about the spectrum analyser. Why would you have an antenna on it if you were interested in the signals from your DVM? Better to put a balanced antenna inside your cage and connect that directly to the spectrum analyser.
     
  15. Jun 27, 2011 #14
    Thanks for your help. I think it was some type of current loop. On my DVM I have a HI and LO terminal and a terminal. There is also a earth terminal. I had the cage connected to the earth terminal and not the guard terminal. I don't know why but when I switched it to the guard terminal, the cage started behaving as you would expect. Now when I open the door the noise goes up and when I close it the nose goes down.
     
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