How to replicate results as fvtool

I generated a notch filter using the design tool, then generated the magnitude response using the fvtool. Why don't I get the same results when using freqz?

d = fdesign.notch('N,F0,BW,Ast', 2, 0.0667, 0.0023, 30); Hd = design(d); fvtool(Hd);

b = Hd.sosMatrix(1:3); a = Hd.sosMatrix(4:6); freqz(b,a)

Why do the two magnitude plots look different?

Thanks.

Answer 1

[Honglei Chen]

Are you referring to the difference in null? If so, that's because the fvtool by default uses 8192 points. If you do

freqz(b,a,8192)

You should see similar plots. The scale value does make a difference too, but in your case it's quite close to 1 so they should look approximately the same.

HTH