Filter Designer setting Apass and Astop for given gain values.

I am designing multiple IIR and FIR Lowpass filters with Filter Designer. I need to meet maximum and minimum gain values in the passbnad (40 dB and 37 dB), as well as maximum gain in stopband (-55 dB). To do this, I have set the Apass to 3, and the Astop to 92. However, the pass band of the plotted filter is 0 to -3 dB, and the stop is -92. Is this the correct way to have the filter desinged, or is there a way to adjust the gain level to meet the requirements? This is a school project, so please point out learning resources as I did not see this covered on the "Using Filter Designer" page or in any associated info.

Answer 1

[Srija Kethiri]

Hi Dustin,

From my understanding, you want to design a IIR and FIR lowpass filter with maximum and minimum gain values in the pass band is 40DB and 37DB and the maximum gain in stop band is -55DB.

To achieve this, you need to set Apass to 3 and Astop to 55, as Apass is the amount of ripple allowed in the pass band in decibels (the default units) and Astop is the attenuation in the stop band in decibels (the default units). 

You can refer to the below link for the better understanding of lowpass filter design:

Lowpass filter specification - MATLAB fdesign.lowpass (mathworks.com)

Hope this helps!