filtering in frequency domain

I have signal say 'data'; i want to cutt off all the frequency above 25 Hz in this signal. what should i do?

Answer 1

[Honglei Chen]

Your question is incomplete. You also need to know the sampling rate of the signal. Once you have the sampling rate, together with the cutoff frequency, you can design a filter using, say butter function and then use filter function to filter the signal.

[arash]

The samplig rate is 0.01 for example;

1. clc
2. clear all
3. N=2048
4. dt=0.01
5. a=random('norm',0,1,1,N);
6. t=0:0.01:(N-1)*0.01;

for signal'a' iwant to cutoff all the frequency above 25hz..

[Honglei Chen]

so your sampling rate is 100 Hz. In that case, you can design a 4th-order (this is something you can choose but 4 is a good value to start with) Butterworth filter and filter signal x like

[b,a] = butter(4,25/(100/2)); 
filter(b,a,x)