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What screening to protect mic from cellphone interference?

Discussion in 'Electronic Design' started by Mozzy, Feb 21, 2007.

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  1. Mozzy

    Mozzy Guest

    What screening is necessary if I want to protect a domestic-grade
    electret microphone from the RF coming from a cellphone which is in use?

    I sometimes hear radio or TV news broadcasts which still get cellphone
    interference so I would guess it is not all that easy to provide a
    reasonale level of screening.

    On the other hand, you can get a cheap electret microphone designed to
    be worn in the ear (as a phone-recording microphone) which is remarkably
    resistent to cellphone RF.

    How do I make a domestic electret microphone capsule with its co-ax
    microphone cable (cdonsisting of shield and one core) resistent to
    cellphone RF?



    Note: xposted to 4 relevant groups (GKSA limit) = alt.cellular
  2. Guest

    Generally the way intereference gets in is by speaker or power or
    signal wires acting as antennas. If a wire doesn't need to carry a
    high speed signal, you can bypass it with an RF choke in series and/or
    capacitors to ground. And put the circuit in a shielded enclosure.
    Chokes, and perhaps coaxial microphone cable, should help quite a bit.

    With GSM phones, the worst interference seems to be when the network
    is getting ready to ring them. In fact, I can often tell when my
    phone is about to ring because of what happens to my computer
    speakers, or car radio. Chances are the phone won't be heard in its
    own headphone because it probably has it's audio jack bypassed with
    chokes and its amplifier not only well shielded, but also muted at
    this point. And nobody is yet on the other end to hear if the
    microphone is being interfered with. Presumably a phone that does
    voice recognition could even know exactly when it is transmitting its
    stronger pulsed signals, and thus if necesssary not listen right then,
    to avoid confusing itself.
  3. Err...I hope that it doesn't do something similar to our brains?(Just now
    reading Stephen King's cell).I have measured the electrostatic field of my
    Nokia 1100, and is 11 V/m at, maybe, 1/2", when calling (when idle,
    something like 0.44 V/m)
  4. Dave Higton

    Dave Higton Guest

    In message <[email protected]>
    It is exceptionally unlikely to be the microphone that's causing
    the problem; it will be the electronics that it's connected to
    that is demodulating the signal, and therefore that needs to be
    filtered and/or screened.

    Unless you understand electronics fairly well, it's difficult or
    impossible to explain how to do this. If you /do/ understand
    electronics, then the best hint is to beware of the inductances
    of even short lengths of wire. Surface mounted components and
    tiny track lengths are your friends.

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