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Bode plot with given transfer function

  1. Apr 15, 2014 #1

    Maylis

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    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    Hello,

    I am having serious problems being able to draw bode plots. At first the professor said to put everything into standard form with an (ω/ω_c)...and then there is also a table in the textbook with standard forms that don't all include an ω_c, and he retracted his statement. So now I am just so confused how to draw these things.

    I am working on the first transfer function in the image, and with two poles at 50 and 100, I'm not sure how to represent that.
     

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    Last edited: Apr 15, 2014
  2. jcsd
  3. Apr 15, 2014 #2

    Maylis

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    Here is what I have for my magnitude plot


    I don't see anything in this table that looks like this transfer function anyway, leaving me even more confused

    ImageUploadedByPhysics Forums1397538664.496334.jpg
     

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  4. Apr 15, 2014 #3

    rude man

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    Your Bode gain plot needs help. But you have the right idea.

    It's OK (flat) until you get to w=2, then a -20dB/decade fall from w=2 to w=10 is still OK, but then for some reason you give it a +40dB/decade rise when it should be just +20 dB. So make that correction and try again to finish the plot.
     
  5. Apr 15, 2014 #4

    donpacino

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    I would recommend you get some semi-log paper. It will make things much easier
     
  6. Apr 15, 2014 #5

    donpacino

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    To add to what Rude man said the slope is a running total
     
  7. Apr 15, 2014 #6

    Maylis

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    Should the paper be log scale on horizontal axis and linear on vertical axis? I found a website that I can print semi log paper, thanks for the suggestion
     
  8. Apr 15, 2014 #7

    donpacino

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  9. Apr 15, 2014 #8

    psparky

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    The gain and frequency are both logrithmic if refering to gain. If referring to dB, then only the horizontal is logrithmic.

    Keep in mind that your transfer function will give you the exact gain and phase angle for any value of omega. So if you ever get confused, start plugging in your limits for 0 and infinity to see where your gain sits. Then plug in other values of interest, the break frequencys for sure. If there is an "s" in your transfer function, simply substiture "jω" for "s" and solve for that frequency. You will always get a vector with magnitude (gain) and an angle (phase).

    If you plug in 0, 2, 10, 50 and 100 and infinity for omega, you can never miss.

    And if you are "guessing" with the zeros and poles....at least you can back check your guess.
     
    Last edited: Apr 15, 2014
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