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A non contact way of messuring Johnson Noise?

Discussion in 'Electronic Design' started by Jamie, Oct 29, 2012.

  1. Jamie

    Jamie Guest

    We have a resistor wire (open type) that I took some measurements
    with and applied the usual Vj math Sqrt(4KTRB) etc... to verify it.

    We used an out board instrument amp to bring it up to an acceptable
    level for analysis however, The idea is to not measure this being
    connected to the circuit..

    Any one have an idea how I could couple to or very close to the
    resistor wire to get a measurement? this resistor is on a long ceramic
    form and I was thinking of putting a parallel plate near it to possibly
    get the noise reading from that as a capacitor coupling, but that would
    involve a fet input and lots of gain, I think I would be doing myself
    more harm than good with that ?

    You see the problem is, we'd like to measure this while in operation
    but don't want a direct connection to the circuit, there are things that
    go wrong in this area at times. (large voltages)

    As for the final task, I need to look around for a good instrument
    Op-amp that has very low offset drift and low noise for this.


    I was also contraplating a small coil around the R but not touching it
    but I don't know if I would be able to detect J-noise in this fashion?

    Jamie
     
  2. Phil Allison

    Phil Allison Guest

    "Jamie" <>

    ** The resident radio ham fuckwit is on a real howler this time ........


    ** The replies to this one will be highly entertaining indeed.



    ..... Phil
     
  3. Sounds quite challenging. In my field guys do something related,
    noise thermometry at sub-kelvin temperatures, using a SQUID as the
    amplifier and inductive non-touching pickup from the Cu block
    whose temperature is to be measured. My boss here has done
    a much higher temperature version of effectively the same thing,
    namely an instrument for non-touching measurement of the
    temperature of molten steel in blast furnaces. The steel is covered
    by slag, so that pyrometric measurement is typically not possible.
    Something about the blast furnace instrument was published
    in Rev. Mod. Instr. vol. 64, p.1593 (1993).

    The key idea is to construct a reactive pickup, whose losses
    (i.e. real part of the impedance) are predominantly due to
    eddy currents in the conductive medium whose temperature is
    to be measured. Fluctuation-dissipation theorem then allows
    one to deduce the temperature of the medium.

    Your case is not exactly the same, however. You are not
    trying to measure the temperature of some material in thermal
    equilibrium, but rather to measure some sort of excess noise,
    at some predefined terminals to the medium, right? (1/f when
    there is dc bias? White noise from some external device
    which merely passes through your resistor?)
    If there are other signals present in addition to the noise, the
    measurement is more difficult still. You'd need enough dynamic
    range to detect simultaneously the noise and the largest signal
    present in your system.

    Regards,
    Mikko
     
  4. Yes, an effect we try to minimize by eliminating any material that is
    not either dielectric or superconducting from near the SQUIDs.
    Interesting paper, thanks. They claim about 1% agreement with a
    thermocouple.

    Molten steel is easier- they use disposable platinum alloy
    thermocouples- this seemed to be aimed at intermediate temperature
    range up to 1000°C in applications such as rolling mills where the
    material is solid but may have nasty slag on the outside.
     
  5. Hi Jamie, I'm not sure of your circuit configuration, but can you
    just attach a cap to the resistor and measure the noise coupled
    through the cap? What's the resistance and temperature of the wire?
    (There are bjt opamps with ~1nV/rtHz of noise and FET's with
    ~4-8nV..) If it's got long cables attached then sometimes the cable
    capacitance can limit the bandwidth of the system. A good way to
    measure a small signal is to look for noise differences. If you can
    switch between having the input shorted and then looking at the
    resistor you can see noise levels below that of the opamp. (But
    you've got to keep the bandwidth the same!)

    George H.
     
  6. You do noise thermometry? Do you use PTB gear or have you
    rolled your own?

    I have only done contacted stuff, like measuring Kapitza resistance
    of our resistive thin films from a van der Pauw pattern. Heated it
    by current through one contact pair, and coupled the SQUID
    to the orthogonal contact pair.

    I mean Rev. Sci. Instr. (mixing it with Rev. Mod. Phys. ...)
    Right. However I remember they did field tests at blast
    furnaces, too. It seems the steel mill workers have a habit of
    throwing in all scrap metal they see lying around - one can cast
    almost anything into such a furnace and it just vanishes. My boss
    told they had left one instrument prototype lying around in the
    vicinity of the furnace after one of their field trips. The next time
    they came around the instrument had disappeared.
     
  7. Robert Macy

    Robert Macy Guest

    Inductive probably is the way to go. Although interpretation may be a
    bit 'fun'.
    I make Eddy Current NDE Instrumentation that maps/displays a 3D
    rendition of the resistivity of aluminum sheeting - based upon the
    changes caused by work hardening of the material during crack
    formation. These are fuselage layers and not a cylinder, like your
    wire.

    Essentially better than 0.05% relative and 1% absolute resisitivity.

    I think the principle can be applied to non-contact resistive
    measurement for your wire. Do you need values 'along the wire' or a
    simple aggregate number?
     
  8. Jamie

    Jamie Guest

    nulling out signals would not be issue, that we already do else where
    with no issues.

    The inductive idea is intriguing though and I may experiment with that
    a little.

    Thanks.

    Jamie
     
  9. Jamie

    Jamie Guest

    And there is the problem, I really can't make a connection to this
    circuit although I already have, to test the noise and I got a
    reasonable reading. But the bigger problem is when the High voltage
    is turned on, Sometimes we get a little problem near by when the voltage
    comes up faster than the beam current does. A static field builds, it
    likes to drain itself into this area. The effect does not hurt the
    R but i am sure it would kill anything sensitive attached to it. The R
    sits in side a pressured vessel of SF6 that seems to blanket other
    methods of detection.

    This is just a pet project idea I am trying and if works, it'll replace
    the method we currently use, which is a calculation of 3 other factors
    to come up with an assumption of what is happening inside.

    Jamie
     
  10. Jamie

    Jamie Guest

    I think I have come up with a way to commutate the signal down the
    throat of the 3Mvolt rectifier stack to get statistics on the R.

    Problem is, you can't directly acquire values of pretty much anything
    at that location. Even the heater (gun) on the beam tube uses the ripple
    voltage into an RF transformer to create the 3 volts or so needed,
    instead of just feeding some wires down the vessel to it. This means
    of course you need the stats driven with delwin rods from servo motors,
    one as a minimum set point and the other as the beam current gain.

    I think I can use the same source of voltage for the heater to operate
    a local circuit and convey the information via a fiber option thread,
    one that has a blocking jacket on it, because we also have optical
    monitors in the vessel. This thread would pass down the center of the
    rectifier stack into a receiver via feed through's that makes
    connections at a much lower penitential.

    By connecting only to the electrical that sits at the end for power
    and optically coupled, I think I can be safe with a direct connection.

    If I spiral the fiber on its wall down the stack I should be fine,
    famous last words! :)

    Jamie
     
  11. josephkk

    josephkk Guest

    Noise wise corona is far worse.

    ?-)
     
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