Problem in Designing Linear Phase (Zero Phase) FIR Filter!

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Minerva Bionic 2021-9-15

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https://ww2.mathworks.cn/matlabcentral/answers/1453904-problem-in-designing-linear-phase-zero-phase-fir-filter

评论： Star Strider 2021-9-16

采纳的回答： Star Strider

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Hi everyone,

I'am going to design a linear phse low pass FIR filter.

It seems that I'm defining something wrong.

lpf = designfilt('lowpassfir','FilterOrder',6,'CutoffFrequency',5000,'PassbandRipple',1,'DesignMethod','cls','ZeroPhase',true,'SampleRate',1e6);

IIR filter works fine but with considerable phase shift.

lpf = designfilt('lowpassiir','FilterOrder',3,'PassbandFrequency',5000,'PassbandRipple',1,'StopbandAttenuation',55,'DesignMethod','ellip','SampleRate',1e6);

I dont have any idea...

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Star Strider 2021-9-15

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1453904-problem-in-designing-linear-phase-zero-phase-fir-filter#answer_788199

在 MATLAB Online 中打开

I do not see that you are doing anything wrong.

However I made some changes that might improve things —

lpf1 = designfilt('lowpassfir','FilterOrder',46,'CutoffFrequency',5000,'PassbandRipple',1,'DesignMethod','cls','ZeroPhase',true,'SampleRate',1e6);

↑ ← CHANGED

figure

freqz(lpf1.Coefficients,1,2^16,1E6)

lpf2 = designfilt('lowpassiir','FilterOrder',3,'PassbandFrequency',5000,'PassbandRipple',1,'StopbandAttenuation',55,'DesignMethod','ellip','SampleRate',1e6);

figure

freqz(lpf2.Coefficients, 2^16, 1E6)

These are two different filters with different characteristics.

The phase delay/distortion will likely disappear with fitlfilt rather than filter to filter the signal. Tha is true for FIR and IIR filters in MATLAB.

In general, FIR filters require a much higher order (and so are llss computationally efficient) than IIR filters to get the same response. Here, I increased the IIR filter order from 6 to 46.