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Does there has "Half-Order Filter" (3dB/oct) ?

Alvis
on Nov 7, 2019

Hello,

I am trying to make a 3dB/oct High-Pass Filter, maybe it can call "half-order"? 

Because as I know the basic order for the first-order filter is 6dB/oct, but how can I make a 3dB/oct filter?

here is the 200Hz 6dB/oct High pass filter:

but how to make 200Hz 3dB/oct High pass filter, which slope is more gentle than the 6dB/oct.

Thanks!

BR.

Alvis

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  • +1
    •  Super User 
    on Nov 10, 2019 9:00 PM

         Hello Alvis,

         You can trick a low-shelf filter into approximating your desired response.  By setting its fo = 26 Hz, slope factor = 0.21, and boost = -25 dB, you get the result shown below -- a highpass whose -3 dB point is near 200 Hz, with a 3 dB / octave (10 dB / decade) slope down to below 3 Hz:

         Note that the stock shelf filter only allows for a -10 -- +10 dB boost, so I calculated the needed coefficients with the attached spreadsheet, then typed them manually into the filter as shown above.  Small coefficient changes result in large response swings, suggesting that double-precision arithmetic should be used.

         Best regards,

         Bob

    5706.1st-2nd-Order_IIR_Coefficient_Calculator.zip

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  • +1
    •  Super User 
    on Nov 10, 2019 9:00 PM

         Hello Alvis,

         You can trick a low-shelf filter into approximating your desired response.  By setting its fo = 26 Hz, slope factor = 0.21, and boost = -25 dB, you get the result shown below -- a highpass whose -3 dB point is near 200 Hz, with a 3 dB / octave (10 dB / decade) slope down to below 3 Hz:

         Note that the stock shelf filter only allows for a -10 -- +10 dB boost, so I calculated the needed coefficients with the attached spreadsheet, then typed them manually into the filter as shown above.  Small coefficient changes result in large response swings, suggesting that double-precision arithmetic should be used.

         Best regards,

         Bob

    5706.1st-2nd-Order_IIR_Coefficient_Calculator.zip

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