butter filter

hi

suppose there are three signal like this

syms t x

a=2*sin(2*pi*t*300)

b=2*sin(2*pi*t*600)

c=2*sin(2*pi*t*900)

i I'll pass the frequency of 900 with butter filter

and my script is

syms t x

a=2*sin(2*pi*t*300)

b=2*sin(2*pi*t*600)

c=2*sin(2*pi*t*900)

z=a+b+c

[z,a]=butter(3,90/100)

fvtool(z,a)

where is the problem ?

please help

[Walter Roberson]

What problem? Is the code producing an error message, or is the result not matching your expectation?

Is there a reason that you carefully compute z=a+b+c and then throw away the value for z by assigning to z in the statement [z,a]=butter(3,90/100) ?

[amir]

there are no problem in my script

but the result not expectation

can you help me ?

I want pass the 900Hz and rejected another signal or frequence

Is there another solution?

[Walter Roberson]

Since you are overwriting "z" in your assignment of the output of butter(), you might as well not calculate the initial "z". The initial "z" is formed from the variables "a", "b", and "c", and those variables are not used after that point (the "a" that appears later is the "a" that is output from butter()), so you might as well not calculate them either. This reduces your script to

[z,a]=butter(3,90/100)

fvtool(z,a)

Are you sure that is correct and sufficient ??

If you want to pass 900 Hz and reject other frequencies, then you need a very narrow band-pass filter. The syntax you have used for butter() is not correct for creating a band-pass filter.

[amir]

suppose that if we get back on first question, with a different frequency signal we want to separate one

What is your solution?

The band-pass filter can be used?

Can you write script fot me ?

thanks for your help!!!

Answer 1

[Wayne King]

If you want to pass 900 Hz and reject the other frequencies, you need a highpass filter. You have designed a lowpass filter. Why are you using symbolic variables? And we need to know your sampling frequency in order to design a useful filter.

In this example, I'll assume that the sampling rate is 10 kHz.

   Fs = 1e4;
   [b,a] = butter(15,(2*650)/1e4,'high');
   t = 0:1/Fs:1;
   x = 2*sin(2*pi*t'*[300 600 900]);
   x = sum(x,2);
   % view your filter's magnitude response
   fvtool(b,a,'Fs',Fs);
   % filter the data
   output = filter(b,a,x);

Although with such a stringent filtering problem, I think you're better off using fdesign.highpass.

d = fdesign.highpass('Fst,Fp,Ast,Ap',650,700,80,0.5,Fs); 
Hd = design(d,'butter');
output = filter(Hd,x);

I think you see the above filter removes all but the 900 Hz component.

[amir]

perfect ,thanks a lot