MATLAB: Is it possible to change the FIR filter -6 dB break point to -3 dB when using the FIR function or the FDATOOL in the Signal Processing Toolbox

-3db36breakdbfilterfirpointSignal Processing Toolbox

I want to design an FIR filter with a -3 dB break point. All the IIR filters have a breakpoint of -3 dB break point.

Best Answer

The cutoff frequency in FIR designs usually corresponds to -6 dB. There is no way to change the break point to -3 dB point. The one exception is the maximally flat filter which can design both IIR and FIR filters. From the FDATOOL, choose "Maximally flat" for the design method. From the command line, use the MAXFLAT function and be sure to include the 'sym' option for FIR design.

Related Solutions

MATLAB: Fir and iir filtering

You have to know your filter specifications. If you do, you can do both with fdesign.lowpass and fdesign.highpass

fdesign.lowpass and fdesign.highpass have different specification strings which support different filter designs.

The default string 'Fp,Fst,Ap,Ast' supports both FIR and IIR designs.

Here is an example:

Design a lowpass filter with a passband frequency of 1 kHz, a stopband frequency of 1.5 kHz, passband ripple of 0.5 dB, and stopband attenuation of 40 dB. The sampling frequency is 20 kHz

   d = fdesign.lowpass('Fp,Fst,Ap,Ast',1e3,1.5e3,0.5,40,20e3);
   designmethods(d)  %shows which designs are valid for this spec
   Hdeq = design(d,'equiripple'); % FIR filter
   Hbutt = design(d,'butter'); %IIR design

MATLAB: Designing BPFs using butter function

As the frequency response of the Butterworth filter is maximally flat (no ripples), you do not have an option to specify the passband ripple and stopband attenuation values in butter function. In designfilt, you can specify the passband ripple and stopband attenuation values, however, they are not accepted for butterworth filter. You can use designfilt to design a butterworth bandpass filter as shown below.

d = designfilt('bandpassiir', 'FilterOrder', 20, 'HalfPowerFrequency1', 0.4, 'HalfPowerFrequency2', 0.6, 'DesignMethod', 'butter');
fvtool(d)

For more information, refer the following link

https://www.mathworks.com/help/signal/ref/designfilt.html

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MATLAB: Fir and iir filtering

MATLAB: Designing BPFs using butter function

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