remove noise from measured data

Hi All,

I am reading this page from Matlab, and I have the following question from it. Can you please explain? It says: "The filter is designed to allow all frequency components with a period less than 48 hours and block all components higher than that." So my question is: based on what it says, how did it come up with the following values?

% Filter Order
N = 3;
% Passband frequency
Fpass = 1/(24*60*60);
% Stopband frequency
Fstop = 1/(6*60*60);
% Ripple Factor and Attenuation in stop band
Rp = 0.5;
Astop = 50;

Thanks

[masih]

Remove High-Frequency Noise in Measured Data

Note: DSP System Toolbox software is required to run this example.

[masih]

To be more specific, where does (24*60*60) and (6*60*60) come from?

[John BG]

1/(24*60*60) is the slowest fastest signal that the filter is going to let in, or should let in, without significant attenuation. 1/(6*60*60) is a 6 hours cycle signal that the filter will block. 6 hours cycle is the frequency (f=1/T) where the 1st notch of the filter is going to be placed to attenuate faster signals.

The question reads 'let through frequencies lower than 1/2days' the 48 hours, but the pass / stop frequencies chosen are not that stringent.

John BG [e-mail]