Designing a lowpass filter in matlab

Hello,

I want to implement the analog LP filter having a cutoff frequency of 2.4Hz and gain of 101 as shown http://img42.imageshack.us/img42/4320/qe74.jpg in the matlab. How to implement this LPfilter having a cutoff frequency and gain in matlab. Can help me out with this.

Answer 1

[Youssef Khmou]

there is a filter design functions , you can start with the simplest function as follows :

%  given you sampling frequency, lets say 20
fs=20;
fc=2/10;
b=fir(N,fc);
freqz(b);

[Stefan]

Thanks but what about gain.

[Youssef Khmou]

i think "fir1" scales the filer to a gain of 0dB , try this and let me now :

 fs=20;
 N=200;
 fc=2/(fs/2);
 b=101*fir1(N,fc);
 freqz(b);
 [gain,f]=freqz(b);
 figure, plot(f,abs(gain))

[Stefan]

Yes,it is having a cutoff frequency of 0.2 but what is this b=101*fir1(N,fc); How to increase the gain to 101. And I have doubt that thisthread has LP filter, what will be the difference of using the both and which one should I follow. thanks.

[Youssef Khmou]

i do not have experience with the other method (filterdesign), howeve i wrote an example for testing : filtering white noise

 N=200;
 Fs=80;
 fc=2.4/(Fs/2);
 b=fir1(N,fc);
 x=randn(200,1); %  AWGN
 y=filter(b,1,x);
 NFFT=800;
 Fx=fft(x,NFFT);
 Fx=abs(Fx(1:end/2));
 Fy=fft(y,NFFT);
 Fy=abs(Fy(1:end/2));
 frequency=(0:NFFT-1)*Fs/NFFT;
 frequency=frequency(1:floor(end/2));
 figure, plot(frequency,Fx,frequency,Fy,'r'), legend(' AWGN','filtered')

[Stefan]

Hi Youssef KHMOU,

Can I know what is 'N' here and why you are using fc=2/(fs/2); is it mandatory to use this or can we directly use the required fc in the b=fir1(N,3) if fc=3Hz. How to design FIR lowpass filter having a cutoff frequency of 2.8Hz and with a sample rate of 300Hz .

[Youssef Khmou]

N is the number of coefficients in the filter. The build in function fir1 requires the cut off frequency to be 0<fc<1, that is why you compute the new cut frequency as a function of Fs : Fc2=fc/(0.5*Fs).