Design a low pass filter for time domain data

Hi,all I wish to design a low pass filter for the time domain data.I have transformed the data into frequency domain, which showed below. But I have no idea on how to design a low pass filter, because I can not determine the sampling frequency and cutoff frequency. The data is a 87238x1 vector. The total time for recording this data is 300 seconds.

Answer 1

[Star Strider]

You already have much of the information you need, particularly the length of the vector and the total time.

Assuming a constant sampling interval, you can calculate these:

Lv = 87238;                                                 % Data Vector Length (samples)
Tv = 300;                                                   % Data Vector Length (sec)
Ts = Tv/Lv;                                                 % Sampling Interval (sec/sample)
t = linspace(0, 1, Lv)*Ts;                                  % Time Vector (sec)
Fs = 1/Ts;                                                  % Sampling Frequency (Hz)
Fn = Fs/2;                                                  % Nyquist Frequency (Hz)

You have to choose the cutoff frequency.

There are several ways to design filters in MATLAB, including dfilt and designfilt, however the IIR filter design procedure I usually use may be easier. It is here: How to design a lowpass filter for ocean wave data in Matlab?