Transfer function of a filter

I want to plot the transfer function of a butterworth filter and the frequency should be in hertz.

I tried this code

[n wn]=buttord(2*pi*3/2000,2*pi*300/2000,3,20);

[b a]=butter(n,wn,'low');

[h w]=freqz(b,a);

stem(w,20*log10(abs(h)));

But what is 'w'? I want to represent the frequency in Hz. What should I do now so that I get -3 db attenuation at 3Hz and -20 db attenuation at 300 Hz in the graph.

Answer 1

[Wayne King]

[h w]=freqz(b,a,[],Fs);

where Fs is your sampling frequency.

[Wayne King]

Of course you might want to call the output, f.

[h,f] = freqz(b,a,[],Fs);

[i Venky]

So according to my problem Fs is 2000. But when I tried it I didn't get 20 db attenuation at 300Hz.

Answer 2

[Wayne King]

I recommend that you use fdesign.lowpass

d = fdesign.lowpass('Fp,Fst,Ap,Ast',290,300,0.5,20,2000);
Hd = design(d,'butter');
fvtool(Hd);

zoom in on 300 Hz and see what you think.

Answer 3

[i Venky]

Okay. If I do like this,is it possible to multiply this transfer function with the frequency domain of the input signal and do such kind of operations?

[Wayne King]

[h,f] = freqz(Hd);