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Electromagnetic Interference

  1. Jul 16, 2007 #1
    Dear all,
    If I have a strong magnetic field present (an oscillating field at say 60Hz), how can I eliminate its interference effect on a neighboring circuitry that has a transmitter operating at say 1 MHz? Would simple filtering at the receiver be enough? I just want to check the feasibility of a functional transmitter without getting into using magnetic shielding.

    Thanks,
    Mike
     
  2. jcsd
  3. Jul 17, 2007 #2

    xez

    User Avatar

    Well because 60Hz is so different than 1MHz you could
    often use simple signal frequency based discrimination
    to reduce the interference of the 60Hz on the 1MHz.

    the frequency ratio is 1.66*10^4, or 14 octaves,
    so even a 6dB/octave roll-off filter would give
    6*14 = 84dB of rejection of 60Hz relative to a cut-off
    frequency of 1MHz.

    The interesting question is to ask HOW AND WHY the
    60Hz couples into the transmitter circuitry.

    One way would involve the 60Hz ripple appearing as
    a differential signal on the power supply leads of
    circuitry in the transmitter. In that case you'd look at
    how well the differential 60Hz power supply signal variance
    is amplified or attenuated by your transmitter circuitry.
    This kind of power supply ripple noise based interference
    is commonly due to inadequate filtration of 60Hz variance
    in a linear 60 Hz mains operated power supply.

    Another way 60Hz interference can form is magnetic
    field coupling from the magnetic field of a wire or
    coil / transformer carrying 60Hz signals through mutual
    inductance with wires, coils, transformers that carry
    other signals. Using geometric means of distancing and
    orienting the circuits to minimize their mutual inductance
    can be helpful. Using thick high permeability magnetic
    materials to enclose the 60Hz noise inductors and/or
    the susceptable inductors that can pick up the noise
    can help.

    Electric field pickup can happen if there were 60Hz
    voltage sources that capacitively coupled to parts of
    the sensitive circuit, but this is usually less dominant of
    a mechanism than others.

    Finally, common mode coupling can occur where a
    common mode 60Hz noise voltage can appear on
    your equipment or various cables running into the
    equipment. Generally devices are insensitive to common
    mode voltages, but the common mode noise present can
    be turned into differential (unbalanced) noise by any
    unbalance of part of the circuit so that a differential
    noise voltage or current can be present between the
    common mode potential and earth or whatever.

    Usually it's best to design the system so that it has both
    low EMI generation capability as well as low EMI
    noise susceptability for whatever EMI is present in
    the environmental space, in the power supply, coupled
    in through cables/wires/signals, et. al.
     
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