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PI Filter and Cable Design

  1. May 14, 2009 #1
    Is it possible to design a cable with properties (Capacitance and Inductance) to reduce the number of components needed for the PI filter? I know I can design a cable to have a specific capacitance and specific inductance, but I am not sure if or how i would get these to work with the pi filter. A pi filter has a shunt capacitor, a series inductor and another shunt capacitor If I use the shield of a coax cable as my gound and design the capacitance correct for one of my capacitors in the filter could I get by eliminating one of the capacitors and just placing a series inductor and shunt capacitor in the circuit? Is it possible to use both a designed in capacitance and inductance and only add the second shunt capacitor?
     
  2. jcsd
  3. May 14, 2009 #2
    A coax with its capacitance and inductance will exhibit a constant impedance. All cables are designed to have a 50 ohm impedance, or 75 ohm for the cable industry. The dielectric will also determine the cutoff frequency. So to use it as a low pass filter for any arbitrary frequency is just too much work.

    What you have to do is match impedance of the PI filter to the coax.


    At higher frequencies we take advantage of microstrip transmission lines that can be made with different impedances that can be arranged as a PI filter and matched to another transmission line.
     
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