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BNC terminatable shielded cable for low frequency measurement

Discussion in 'Electronic Design' started by Steve Parus, Jan 8, 2004.

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  1. Steve Parus

    Steve Parus Guest

    Looking for a thin flexible shielded cable with capability of BNC
    connector termination to be used at DC and low (< 100 Hz) frequencies
    for low noise signal measurement. Assuming that addition of a foil
    shield to the standard ~90% braid coverage RG-174 cable would help, is
    there a foil shield version of that cable ?

  2. You are aware that a shield only shields the electric field,
    but not a magnetic field ?
    You may gain more by using differential signals.

  3. James Meyer

    James Meyer Guest

    A braid shield, even a fairly "open" one, will be 100% effective at 100
    Hz. There will be nothing to gain by going to anything else.

  4. 1. In my experience, foil shielded cables are not very flexible. If you
    keep flexing foil shielded cables, the foil breaks.

    2. At 100Hz, you can use any impedance cable. 75 ohm, or audio types. You
    can still terminate in a 50 ohm load if that is what you have, but the
    actual cable impedance will not matter unless you have kilometres of wire.

    3. If you have a hum problem, I suggest changing the cable type won't make
    much difference, unless your cable was awful to begin with. You can amplify
    the signal before you send it down the wire. Or use microphone balanced
    cable where 2 wires of opposite phase carry the signal and a at the
    receiving end a differential amplifier or transformer responds only to the
    differential signal.

  5. Mac

    Mac Guest

    Any thin coax should do. I agree with others that you don't need the
    foil shield. Braid will do. I'm sure Digikey stocks something appropriate.

  6. I read in that Steve Parus>) about 'BNC terminatable shielded cable for low frequency
    measurement', on Thu, 8 Jan 2004:
    Be careful! You need a *low-microphony cable* for that application.
    Extreme screening seems unlikely to be necessary. If there is a lot of
    EM noise about, you'd do better to use a balanced interconnection.
  7. Bill Sloman

    Bill Sloman Guest

    Coaxial cable isn't shielded against magnetic fields, but the coaxial
    construction means that any magnetic flux moving acros the cable
    produces identical voltages in both braid and inner conductor, so the
    error voltage is strictly common mode, unless you have set up a ground
    loop that lets the induced voltage induce significant currents in the
    braid alone, producing additional resistive voltage drops in the braid

    Differential signals lend themselves to circuits with equal impedances
    to "ground" on both of the balanced conductors, and can be easier to
    get right.
  8. Jeroen

    Jeroen Guest

    Ah yes, but at low frequencies, these voltage drops directly add
    to the signal, causing interference. IOW, screen resistance
    determines the screening effectiveness of coax at low frequencies.
    The lower the screen resistance (connectors included), the better
    the shielding. This is one of the reasons I'll often use double
    shielded cable and N-connectors, even though the bandwidth of my
    signals is only 100k to 40MHz.

    Jeroen Belleman
  9. Steve Parus

    Steve Parus Guest

  10. JeffM

    JeffM Guest

    Looking for a thin flexible shielded cable
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