Design IIR filter with filterDesigner function

I want to design a 3rd degree ( order 3 ) IIR lowpass butterworth filter using " filterDesigner function " with defualt Fs and Fc. I set quantization parameters, choose filter arithmetic to "fixed-point" and set coeffcient and input word length to 8 bits. then I realize model and choose build model using basic elements and click to realize model. when simulink started and I add step input and oscilloscope to see step response, output in simulink isn't same with that ouput that is shown in filterDesigner window.( pls ask me for further explanation in case the text was not clear. thx )

[Jan]

Note: This is not twitter - no # before the tags. Thanks.

Answer 1

[Abhishek]

Hi S.M,

I understand you have designed a lowpass Butterworth filter and are seeing different step responses in “filterDesigner” and Simulink. The issue is reproducible with the specified quantization parameters in MATLAB R2024b and can be resolved by setting the “Input fraction length parameter” under “Input/Output” tab to “7”.

Since, the input word length is changed to 8 bits, the input fraction length should also be changed from 15 (which is used for a 16-bit input) to 7. This ensures the input values are interpreted correctly, making the step response output in Simulink match the output shown in “filterDesigner”.

For more information on the Input/Output parameters of filter designer, please refer to the below MATLAB documentation:

https://www.mathworks.com/help/releases/r2024b/dsp/ug/use-fdatool-with-dsp-system-toolbox-software.html

Hope this helps!