. VI filtering IIR and response: response of Butterworth filter size depends on sampling rate - why?

Hi people,

I'm not an expert in the design of the filter, only a person in applying them, so please can someone help me with an explanation?

I need to filter signals very infrequent using a buttherwoth filter 2. or 3. order of the bandpass 0.1 to 10 Hz.

Very relevant amplitudes are BELOW 1 Hz, often less than 0.5 Hz, but there is as well the amplitudes beyond 5 Hz to observe.

It's fixed and prescribed for the application.

However, the sampling rate of the measuring system is not prescribed. It may be between say between 30 and 2000 Hz. Depends on the question of whether the same set of data is used for analysis of the higher up to 1000 Hz frequencies on the same measure or this is not done by the user and he chooses a lower sampling rate to reduce the size of files, especially when measuring for longer periods of several weeks.

To compare the response amplitude of 2nd and 3rd order filter, I used the example of IIR filtering .vi and response:

I was very surprised when I found that the response of greatness is considerably influenced by the SAMPLING RATE I say the signal generator in this example vi.

Can you please tell me why - and especially why the filter of order 3 will be worse for the parts of low frequency below 1 Hz signal. Told me of people experienced with filters that the 3rd oder will less distort the amplitudes which does nothing for my the frequencies below 1 Hz.

In the attached png you see 4 screenshots for 2 or 3 command and sampling rate of 300 or 1000 Hz to show you the answers of variable magnitude without opening labview.

THANK YOU very much for your ANSWERS!

Chris

Hello Cameron and thanks for my lenses of compensation.

I can now proudly present the solution of my problem.

It seems to be purely a problem of the visualistion information filters through the cluster of the scale.

After looking in the front panel of the IIR, I suddenly noticed that the "df" of the pole size is changing with the Fs of the input signal.

For a Fs to 30 Hz, the "df" is 0.03 Hz so you see the curve of the filter with more points, see png.

For a Fs 300 Hz "df" is 0.3 Hz, so the curve is larger with only 3 points between 0 and 1 Hz.

For a 1 kHz Fs the df is 0,976 Hz, so there is no point in the graph between 0 and 1 Hz.

It's strange that for constant Fs, df of this cluster NOT reduced with the increase in the number of samples, as it does in an FFT.

However, I hope now the filter used now for the curves obtained with the proposed Lynn way and the response of greatness from the filter information fit together.

Thank you for your support.

Merry Christmas and a happy new year to all.

Chris

Tags: NI Software

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