having a problem using an iir notch filter

hello everyone,

i want to filter 50 Hz sinus and therefore i implemented a notch filter such as shown in the help. i don'w know why it doesn't work. does everyone have a clue ? see also the attached file 'debuging' this is the program :

if true
  % code
end
*t=0:0.0005:0.5; % fs=2000Hz
x=sin(2*pi*50*t)+sin(2*pi*70*t);
figure;
plot(t,x);
title('Sum of sinuses - of 50Hz & of 70Hz ');
% implementation of the notch filter
wo = 50/(2000/2);  bw = wo/35;
[b,a] = iirnotch(wo,bw);
fvtool(b,a);
%signal after filterization
figure;
g=filter(b,a,x);
plot(t,g);
title('The signal after removing its 50 Hz sinus');
%figure of 70 Hz for comarison to what i need to get
figure;
y=sin(2*pi*70*t);
plot(t,y);
title('A 70 Hz sinus - for comparison');*

thanks a lot

Answer 1

[Wayne King]

Kobi, did you also supply the optional Ab input argument as I did?

nyquist = 2000/2;
 w0 = 50/nyquist;
 bw = w0/20;
 [b,a] = iirnotch(w0,bw,20);
 y = filtfilt(b,a,x);
 plot(t,y);

From your output it does not appear that you did.

If you execute the above, you should see that is pretty much a clean 70-Hz sinewave with a reduction in amplitude.

Answer 2

[Wayne King]

It is working, you just need to tweak the parameters a bit to get better results.

nyquist = 2000/2;
 w0 = 50/nyquist;
 bw = w0/20;
 [b,a] = iirnotch(w0,bw,20);
 y = filter(b,a,x);
 ydft = fft(y);
 plot(abs(ydft))

Answer 3

[kobi]

hi and thanks for the immidiate answer. I saw that you changed the Q factor. What its affect about ? in addition, when i changed the number to 20 (as you advised me) i didn't get a real 70 Hz sinus as you can see in the attached pictures.... What can i do to make it exactly the same ? what parameter should i need to change ?

thanks a lot!

Answer 4

[kobi]

O.K

I changed this part of code to yours and as you told me, i'v got a 70Hz sinwave with a reduction.

some question about the plot after filtering :

1. Does the reduction comes from the filter itself ? how can i solve it ? by multyplying it to the gain i need to get (1 - -1) ?

2. Why am I getting in the begining of the filtered plot something wierd (0-0.025 sec) ?