High pass filter response at high frequencies

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Discussion Overview

The discussion revolves around the behavior of a passive high pass filter constructed with a hand-wound inductor and a resistor, particularly focusing on its response at high frequencies (5-10 MHz) where an unexpected roll-off occurs. Participants explore the underlying causes of this phenomenon, including the effects of parasitic capacitance and the characteristics of real inductors.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that the filter worked effectively within a frequency range of 100 Hz to 1,000,000 Hz but exhibits roll-off at higher frequencies, prompting the inquiry into the cause.
  • Another participant suggests that parasitic capacitance may be influencing the filter's performance at high frequencies and questions the choice of an LR high pass filter over an RC or LC configuration.
  • A participant emphasizes that all real inductors possess parallel capacitance, which can lead to high-frequency roll-off when paired with a series resistor, and recommends measuring the inductor's self-resonance.
  • It is mentioned that the combination of the inductor and parasitic capacitance may create a tuned circuit effect, which could explain the observed behavior.

Areas of Agreement / Disagreement

Participants express various hypotheses regarding the roll-off behavior, with some agreeing on the role of parasitic capacitance while others provide different perspectives on the filter's design and performance. No consensus is reached on the definitive cause of the observed behavior.

Contextual Notes

Participants discuss the limitations of the filter's design and the assumptions regarding component behavior, particularly the ideal versus real characteristics of inductors. The discussion includes references to circuit modeling tools like SPICE, indicating a potential for further exploration of the circuit's behavior.

DeldotB
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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"
 
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DeldotB said:
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"

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:
 
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berkeman said:
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:
hmm okay, thanks Berkeman! I used a LR HPF because part of the experiment 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!
 
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.
 
meBigGuy said:
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.
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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