High pass filter response at high frequencies

[DeldotB]

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"

 

[berkeman]

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...

 

[DeldotB]

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...

hmm okay, thanks Berkeman! I used a LR HPF becuase 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!

 

[meBigGuy]

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.

 

[tfr000]

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.