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I have to design high-pass IIR Biquad filter with cutoff frequency of 80Hz, sampling frequency 24kHz, and filter slope of 24dB/oct @ 80Hz. I have used formula for HPF biquad coefficients from here. However, the slope is less steep for frequencies lower than 80Hz, please see the picture. How I can design a filter to give me 24dB/oct for the frequencies lower than 80Hz, to match the reference curve on the picture?

enter image description here

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  • $\begingroup$ You might find that one biquad is not enough for the slope you want. Try cascading two of them in series and you will often get a steeper slope. $\endgroup$ – panthyon May 27 '15 at 20:36
  • $\begingroup$ Try a 4th order Butterworth. 2nd order (biquad) is not enough. We've talked about order and dB/octave when discussing your other question, haven't we? $\endgroup$ – Matt L. May 27 '15 at 20:38
  • $\begingroup$ Anthony Parks, thank you, that worked. Matt, yes we did discuss, I learned a lot. I used frequency sampling method to obtain custom slope for FIR filters. Anthony, you can write your comment as the answer for my question. $\endgroup$ – Nebojsa May 28 '15 at 12:11

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