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High pass filter response at high frequencies

  1. Sep 13, 2015 #1
    Good day all,

    I have a question:

    I built a passive high pass filter in an experiment using a hand wound inductor (toriod) and a resistor. The filter worked great as a high pass at frequencys ranging from 100 Hz - 1,000,000 Hz (rolling off around 10,000 Hz). After creating a bode plot, I noticed that at very high frequencies (relative) around 5-10 MHz, the filter began to roll off slightly. What causes this unwanted behavior?

    * feel free to be "mathy"
  2. jcsd
  3. Sep 13, 2015 #2


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    Staff: Mentor

    Probably parasitic capacitance came into play. Why did you use an LR HPF instead of an RC or LC HPF? Can you post a drawing of your circuit, including any component values you've measured? Do you have access to a SPICE program? We can help you with characterizing the components and modeling them with SPICE... :smile:
  4. Sep 13, 2015 #3
    hmm okay, thanks Berkeman! I used a LR HPF becuase part of the experiemnt was to create an inductor of a specific impedance that when combined with the resistor rolled off values at a specific frequency. I do have a spice program but I didnt run the circuit on it (I didnt feel it necessary - I just calculated by hand what the component values needed to be). I was just curious why this happened.Thanks!
  5. Sep 14, 2015 #4


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    Gold Member

    All real inductors have parallel capacitance which will roll off the high frequencies when fed by a series R.

    Measure the inductor self resonance. Actually, I expect you saw the high pass peak before it started to rolloff.
  6. Sep 14, 2015 #5
    In other words, no inductor is perfect. You actually get a sort of tuned circuit with the inductor/parasitic capacitance combination. If you sweep the inductor by itself, you should see it.
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