MATLAB: How to design a lowpass filter for ocean wave data in Matlab

bandpassfilterjulian dataMATLABoceanographywaves

Hi there.

What frequency specification should I use in order to create a low-pass filter ? ( Specifically filter order and Frequency specifications) I have a huge data set contains unfiltered water level data and it is in meters relative to mean lower water (MLW) at hourly intervals. The time stamp is in Julian data.

thank you in advance.

Best Answer

You need to know the passband of the filter you want to design (the frequencies you want to either keep or filter out), and the sampling frequency (Fs) at the very least. Since you have Fs, the Signal Processing Toolbox makes the rest easy. From your description, a Butterworth design is likely the best. You are always free to experiment with other designs, but the scheme I’m outlining for the Butterworth design is a prototype for all of them. You have already decided on a lowpass design, the default for all of them, so you don’t have to specify a filter type. (I usually prefer a bandpass with a very low cutoff, in the event that I want to filter out baseline drift. That’s simply my personal bias, because my signals usually have baseline drift.)

To determine the order, start with the buttord function;
Use the output of buttord to design a transfer function (b,a) realization of your filter with the butter function, (I usually use 1 dB for Rp and 10 dB for Rs, but these are not relevant for Butterworth designs);
Use the tf2sos function to create a second-order-section representation for stability;
Use the trapz function (be sure to give it your sampling frequency, ‘Fs’, to make the output understandable) in order to check the filter performance to be sure it is stable and does what you intend it to do;
Use the filtfilt function for the actual filtering, since unlike filter, filtfilt does not introduce any phase distortion.

The documentation is reasonably clear on all these, so their usage should be straightforward, but that assumes some experience in discrete signal processing. If you have any problems, follow up here and I’ll do my best to help you solve them.

Related Solutions

MATLAB: How to design FIR lowpass filter with cutoff frequency, fc1 = 20 Hz and FIR bandpass filter with cutoff frequency fc2 = 10 Hz and fc3 = 35 Hz.

Déjà vu

how to design IIR highpass filter with cutoff frequency of 20Hz and FIR bandpass filter with cutoff frequency of 10Hz and 15Hz?

Make the appropriate changes to get the cascaded filter you want.

MATLAB: How to filter? (Kalman, Narrow Bandpass , Low Pass)

Filter design, implementation, and plots:

D = load('Xaris load.mat');
t = D.t;
Xs_ddot = D.Xs_ddot;
Xu_ddot = D.Xu_ddot;
Ts = mean(diff(t));                                         % Sampling Interval
Fs = 1/Ts;                                                  % Sampling Frequency
Fn = Fs/2;                                                  % Nyquist Frequency
Wp = [3  25]/Fn;                                            % Normalised Paassband
Ws = Wp .* [0.2  1.1];                                      % Normalised Stopband
Rp =  1;                                                    % Passband Ripple
Rs = 30;                                                    % Stopband Ripple
[n, Wn] = buttord(Wp,Ws,Rp,Rs);                             % Bandpass Filter Design
[b,a]   = butter(n,Wn,'bandpass');                          % Choose Butterworth
[bp_sos,bp_g] = tf2sos(b,a);                                % Use SOS For Stability
Rs = 10;
Rp = 20;
Ws = [9.95  10.05]/Fn;
n = 2;
[b,a] = cheby2(n,Rs,Ws,'stop');                             % Bandstop (Notch) Filter Design
[bs_sos,bs_g] = tf2sos(b,a);
figure(1)
freqz(bp_sos,1024,Fs)                                       % Bandpass Filter Bode Plot
figure(2)
freqz(bs_sos,1024,Fs)                                       % Bandstop Filter Bode Plot
Xs_bp  = filtfilt(bp_sos, bp_g, Xs_ddot);                   % Filter Signals
Xs_out = filtfilt(bs_sos, bs_g, Xs_bp);
Xu_bp  = filtfilt(bp_sos, bp_g, Xu_ddot);
Xu_out = filtfilt(bs_sos, bs_g, Xu_bp);
figure(3)
subplot(2,1,1)
plot(t, Xs_ddot)
title('Unfiltered  ‘Xs\_ddot’')
grid
subplot(2,1,2)
plot(t, Xs_out)
title('Filtered ‘Xs\_ddot’')
grid
figure(4)
subplot(2,1,1)
plot(t, Xu_ddot)
title('Unfiltered ‘Xu\_ddot’')
grid
subplot(2,1,2)
plot(t, Xu_out)
title('Filtered ‘Xu\_ddot’')
grid

Best Answer

Related Solutions

MATLAB: How to design FIR lowpass filter with cutoff frequency, fc1 = 20 Hz and FIR bandpass filter with cutoff frequency fc2 = 10 Hz and fc3 = 35 Hz.

MATLAB: How to filter? (Kalman, Narrow Bandpass , Low Pass)

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