Passband filter

[zelka]

Hi Weizang,

I am trying to do a passband filter using a fomula. Right now I do a lowpass then highpass filter because I cannot find (even using help) the right synthax for passband. Indeed, the IRRFilter in the Filtering menu does not works. I have:

• Filter(V, IIRFilter(FILTER_BUTTERWORTH, FILTER_LOWPASS, 3, 150000, , , , , 1000000), FALSE) to filter the V signal avoiding frequencies above 150kHz.
• Filter(VFiltered, IIRFilter(FILTER_BUTTERWORTH, FILTER_HIGHPASS, 3, 360, , , , , 1000000), FALSE) to filter my first signal avoiding frequencies under 360 Hz.

First, my highpass filter does not do a filtering under 360Hz (I still have harmonic at aprox 300Hz). Same for the lowpass filter, frequencies above 150kHz still there.

Could you help me with both formula and figure out the frequencies filtering issue please? Or help me with a passband filter?

 

 

[Bernhard Kantz]

If you are not sure which argument values to apply, you may want to use the Analysis Wizard. In the category Filters you can select the analysis for IIR Filter. In the second step you can play around with the filter type, characteristics and parameters and immediately check the results. After Finish you can Open the created analysis object (from the context menu) and look at the FPScript code generated. Alternatively, you can convert the object into a formula and adjust the code directly.

An additional remark: From the parameters supplied in your example, you want to filter a signal sampled at 1 MHz. The low pass filter at 150 kHz will not be problematic since the normalized frequency will be 0.15. But the highpass at 300 Hz will cause trouble, normalized to the sampling frequency the cut-off will be at 0.0003, a value needing extremely high orders tending to unstable behaviour.

[Autor]

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