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Attenuator resistor formulas ?

  1. May 8, 2012 #1
    Hello, I am trying to work out the resistor values for an 'O' attenuator.
    I am using the attached formula to work out resistor 3's value.
    My values to insert into this formula are:

    Input impedance (Z1) = 50 Ohm
    Load impedance (Z2) = 600 Ohm
    a =1.995

    However when I substitute these figures I am getting a negative number result.
    Obviously I can't have a negative Ohm resitor, so how do you interpret that ?

    Thanks for any help
     

    Attached Files:

    Last edited: May 8, 2012
  2. jcsd
  3. May 8, 2012 #2

    gneill

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    Staff: Mentor

    Your circuit diagram looks like that of a ##\Pi## attenuator, not an O-pad.

    There's a minimum attenuation that is determined by the source and load impedances. Have you checked to see whether or not your "a" value meets this criterion?
     
  4. May 8, 2012 #3
    sorry, I attached the wrong image.
    This is the O-pad attachment.

    What does it mean when I get a negative value from one of the resistor formula calculations (as I am in this case).
    Obviously I cant have negative Ohm resistors ?
     

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  5. May 8, 2012 #4

    gneill

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    Staff: Mentor

    Usually it means that you can't match the impedances while achieving the desired attenuation. There's a minimum amount of attenuation incurred when matching impedances that are not close; if your desired attenuation is less than this then some other type of matching circuit would be necessary.
     
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