Matlab 3 dB 12 hertz lowpass filter [closed]

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Closed 8 years ago.

I am looking for a 3dB low-pass filter with a cut-off frequency of 12Hz. I know Matlab has this function fdesign.lowpass and it should be possible to be 3dB with F3db (source / additional) but I'm not really sure yet how to implement them, that is: what features I should include and what features not. I get confused by all the other variables I think I don't need -- I just need Fc and 3dB. I also found fdatool but also don't really know how to set such a filter.

The data contains regular x and y values while it will be a speed versus time plot of a recorded movement.

Answer 1

For your application I would strongly recommend trying a plain butterworth filter, the Matlab syntax is:

[b,a]=butter(n,Wn)

Where Wn is the digital frequency, so here's how I would pose it:

% assume x is time and y is speed
Ts = mean(diff(x));
Fs = 1/Ts;
% for butter, we need Wn, which is the cutoff frequency
% where 0.0 < Wn < 1.0, where 1.0 is half the sample rate
% The cutoff is the -3 dB point of the filter
% Wn = fCutOff/(Fs/2)
% for a cutoff of 12 Hz
fCutOff = 12/(Fs/2);
% we'll start with an order of 1 which should give us about 20 db/decade attenuation
[b,a] = butter(1,fCutoff);
% plot the filter frequency response to see what it looks like
% use 512 points to plot it
freqz(b,a,512,Fs)

However, if, I understand correctly, you're sampling the data at about 66 Hz, which is about 5 times faster than your desired cutoff. Rule of thumb is often 10 times, so you may not be real happy with what you get as output. Here's my output: