Re : why the signal amplitude changes sosososo much????after i used a filter

[labview测控小能手]

i am so puzzled !

i used butterworth coefficient  vi get the fs=8000hz,low frequency 500hz filter coefficient and used in labview vi ,put 50hz sine wave in ,i dont know why the siginal changed sososo much amplitiude ?i just put a pure wave in the filter no noise no other frequency wave !please help me!i think the labview makes me so puzzled!

[ben64]

You didn't wired anything to the high cutoff frequency input so it is using the default value of 0.45 Hz.

Ben64

[labview测控小能手]

lowpass filter dont need higher cutoff frequency

[ben64]

You're right, my bad.

Ben64

[dan_u]

I'm not sure why, but try using the "IIR Cascade Filter.vi" instead of the "IIR Filter.vi".

Or use the Butterworth Filter.vi which combines the coefficient and actual filter VIs.

[jamiva]

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@dan_u wrote:

I'm not sure why, but try using the "IIR Cascade Filter.vi" instead of the "IIR Filter.vi".

Or use the Butterworth Filter.vi which combines the coefficient and actual filter VIs.

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dan_u is correct. Use the IIR Cascade Filter.vi

[Kallis · Accepted solution]

The IIR filter.vi requires coefficients in direct form. 

So, in your original code, we should convert the coefficients to direct form VI before passing it to the IIR filter.vi.

[labview测控小能手]

yes,u are right man!