Calculate the Break Frequency

Discussion in 'Homework Help' started by millotorres, May 19, 2012.

  1. millotorres

    Thread Starter New Member

    May 18, 2012
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    I need help calculating the break frequency of an RC and an RL circuit when given only the resistor of 1000 ohms value and the magnitude and phase plots.

    I've been trying to solve this without luck. Please guide me here of the steps I should take.

    [​IMG]

    RL - Red Plot
    RC - Blue Plot

    [​IMG]

    [​IMG]

    Thanks
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
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    When the gain Vout/Vin reach 1/√2 ≈ 0.707 ≈ -3.01dB
    The Xc = R and also XL = R
     
  3. millotorres

    Thread Starter New Member

    May 18, 2012
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    Ok I still don't get it, can you show me how I get fc. I cannot tie the knot on this one. I have tao = RC, fc=1/2piRC. I have two unknowns. Show me the light here someone!
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    When Xc = R.

    i.e. when R = 1/ωC
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    The light is that you have to supply two of the values to calculate the third. ;) For example, if you want a certain fc, then you need to also specify a value for either R or C, and solve for the missing value.
     
  6. millotorres

    Thread Starter New Member

    May 18, 2012
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    Ok, This is the puzzle, I must get fc by looking at the graphs and the frequency i.e. 4.6 is told not to be = fc. I only was given what I posted..
    A magnitude and a phase graph of the circuit and the resistor value of 1000 ohms. So can you solve this one?? fc = ...

    Thank you in advanced!!
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    The formula for Xc is:

     X_c = \frac{1}{2\pi fC}

    We solve for f when R = Xc

     f = \frac{1}{2\pi RC}
     
    Last edited: May 20, 2012
  8. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Use the plot to find frequency when gain drop to -3.01dB or 45 degree of a phase shift.
     
  9. millotorres

    Thread Starter New Member

    May 18, 2012
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    Please no more theory, fc = ? someone???
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    Geez... did you not see my posts #4 and #7? Also post #2.

    We keep giving you the answer.
     
  11. millotorres

    Thread Starter New Member

    May 18, 2012
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    0
    I'm getting ~50000 rad/sec but still it is marked as incorrect
     
  12. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,361
    What is the answer you are expecting?
     
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