The Signal Processing Toolbox tf function requires as its argument a digital filter object created by the designfilt function. It will not give you the result you want, since you have already calculated the numerator of your transfer function as ‘b’.
There is also nothing wrong with the use of the Blackman window.
this works for me:
rzad = 21;
Wn = [0.3 0.7];
b = fir1(rzad,Wn,blackman(rzad+1));
zplane(b,1)
grid on
I cannot determine the problem with this plot and your GUI. Your filter design and the plot work.
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