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

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...

Answer 1

[Star Strider]

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.

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[Minerva Bionic]

Dear Star Strider,

Thank you for your response.

Actually, I have to implement multiple filters on a hardware.

Using high-order filters occupies most of the available areas in my device.

Another problem that I have is that its phase margin should not exceed 2*pi/3.

And the last constraint is passband ripple, which should not exceed 0.01db.

Maybe, I should conduct try and error with parameters in IIR filters to find the best solution.

Thanks alot.

[Star Strider]

As always, my pleasure!

If you are doing this in hardware, consider using the DSP System Toolbox since it is designed to do just that. (I do not have it because I do not need it, so I have no experience with it. I have exploree it to some extent in the online Run feature on Answers, however not to the point of being proficient with it.)

The one type of filter that provides the best phase-neutral performance is the Bessel filter (besself). It cannot be implemented digitally, and can only be implemented in analog hardware.

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[Minerva Bionic]

I will look for both you mentioned.

Thank you for your asistance.

[Star Strider]

As always, my pleasure!

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