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Calculating 3dB frequency.

  1. Jan 31, 2007 #1
    1. The problem statement, all variables and given/known data

    Using your results, calculate the 3dB frequency of the RC filter.

    3. The attempt at a solution

    Basically in our lab, we made a simple series RC low pass filter. We put in a 16V peak to peak signal at varying frequencies below, and measured the output voltage at the capacitor (again peak to peak) on the scope. Now if I want to calc the 3dB frequency from my results, would I plot this on a graph and look for the 3dB point? I have tried this, but I think I am going wrong somewhere. The second table would be what Im trying to plot. I took first column is just the log of the frequency. The second column is 10log(Vout/Vin). It just doesnt seem right.

    fin (Hz) Vo (V)
    50 13.8
    100 11.4
    150 9.6
    200 8.2
    250 7.2
    300 6.4

    1.698970004 -0.749974923
    2.000000000 -1.579527115
    2.176091259 -2.326214759
    3.301029996 -3.010299957
    2.397940009 -3.665315444
    2.477121255 -4.087127349

    sorry for the formatting!
  2. jcsd
  3. Feb 1, 2007 #2


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    Science Advisor
    Homework Helper

    It looks OK to me, except log 200 is not 3.301029996
  4. Feb 1, 2007 #3


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    Staff: Mentor

    I don't understand why you are taking the log of the frequency, but whatever. Keep in mind that the 3dB concept in this context is for the voltage transfer function plot (not power), so you should use 20log(), not 10log(). The 3dB point is basically where the output voltage amplitude is down by SQRT(2) compared to the input amplitude. I see one frequency on your first list that is darned close to this number....
  5. Feb 1, 2007 #4
    Firstly, you don't really have to log the frequencies. Secondly, 3dB below 16V is 16 times 10^(-3/20) which is 11.327V. Looking at the table of figures you obtained, I would say that this is roughly around the 100Hz mark.
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