Butterworth lowpass filter design code with a specific cutoff frequency, pass-band and stop-band attenuation

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sH
sH 2017-9-17
评论： Star Strider 2017-9-18
Hello I want to design an eighth-order Butterworth lowpass filter with the cutoff frequency of 35 Hz , stop-band attenuation of 1 dB and the pass-band attenuation of 8 dB , but I can not find a function in matlab that covers all these features.
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Star Strider
Star Strider 2017-9-18
That is not the filter you described, or perhaps the authors described it incorrectly. It has a passband of 30 Hz and stopband (half-power, or -6 dB) frequency of 35 Hz
It will be relatively straightforward to design it as a discrete filter. See my Answer.
If you want to design it as a continuous filter in hardware, that is beyond the scope of this forum. There are several textbooks that discuss hardware filter design and implementation.

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Star Strider
Star Strider 2017-9-17
Your filter design defies logic. I do not understand your passband attenuation being greater than your stopband attenuation. Usually the passband is at 0 dB and the stopband is -50 dB or less.
That aside, see if the designfilt (link) function will design your filter.

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