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Cable and equipment shielding

Discussion in 'Electronic Design' started by logjam, Mar 12, 2008.

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  1. logjam

    logjam Guest

    I've been trying to learn more about shielding equipment and cables.
    I've bought some books and read some archived posts but haven't found
    anything like what I'm trying to do.

    I have an analog sensor with a sampling rate of 1-2MHz and 12-14bit.
    The cable we use has an overall shield and each of the twisted pairs
    are also shielded. I am getting what I assume is EM interference.
    When the system is used near a magnetron at 6-15MeV we get a very
    small but annoying amount of noise. About 150mV positive only spikes.

    We have simulated the noise using a TIG welder's high voltage arc
    start feature. When not welding, just letting the torch point
    straight up we get a HUGE amount of noise, much more than we get near
    the magnetron. What ever type of noise this is, it can be picked up
    by an ionization chamber quite well. Our only choice is to use this
    source of interference as a test of our equipment before we travel to
    the customer's site. When we touch the source of the HV, turning our
    bodies into antennas I guess, the sensor quits working completely. In
    fact, a motion control system in the next room crashed and started
    free running into its hard stops!

    From what I understand the power is supplied to the magnetrons via a
    coaxial at 10,000v 1000-10,000A in extremely short bursts at a maximum
    of 200-250 pulses per second. There are then transformers which step
    that up and finally something magic happens in the magnetron and we
    get 6-15MeV.

    The high voltage pulse cables (10,000v) are extremely well shielded.
    They are running in the same cable tray with some unrelated BNC cables
    (for about 110 feet) and with a 5vp-p signal there is no
    interference. That leads me to think that the interference is coming
    from the destination. We can only modify our equipment, so I am
    looking into what it will take to shield it from everything possible.

    My biggest question is what to do with the cable shield. Which end do
    we connect it to? I've read in the past that you should connect the
    shield to the end where you care about the signal MOST. Other people
    suggested you connect the shield on the opposite end of where you care
    about the signal. Should it be connected on both ends?

    Thanks : )

  2. Mark

    Mark Guest

    connect the sensor cable shield to the sensor at the sensor end and
    connect the cable shield to your test equipment at the test equiment
    end but.... isolate the sensor and cable grounds from the other

    keep your sensor and test system electrically isolated from the other
    system except AT ONE PLACE only. This will usually be at the 3rd
    prong of the power cord.

    You want your system to have its own totally enclosed shield that
    doesn't connect to the other system except at one place only.

    This will eliminate ground loops.

    If your sensor and cables are all well shielded and you eliminate the
    ground loops and you still have a problem then we have to dig

    If your sensor MUST be electrically connected directly to the other
    equipment for some logistics reason, then the problem becomes MUCH
    more difficult.

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