How to convert the passband ripple from dB to linear in the Filter Designer?

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Tong Xu 2019-1-26

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https://ww2.mathworks.cn/matlabcentral/answers/441560-how-to-convert-the-passband-ripple-from-db-to-linear-in-the-filter-designer

评论： paravai s 2020-4-24

Hi everyone,

I'm using the Filter Designer in Matlab 2018b. I designed a low pass FIR filter using the equiripple design method. In the filter specifications, I set the passband ripple as Apass = 1dB and the stopband attenuation as Astop = 40dB. Then I generate the matlab code by selecting Filter Design Function.

Here is a section of the generated Matlab code:

% All frequency values are in MHz.
Fs = 1600;  % Sampling Frequency
Fpass = 90;              % Passband Frequency
Fstop = 110;             % Stopband Frequency
Dpass = 0.057501127785;  % Passband Ripple
Dstop = 0.01;            % Stopband Attenuation
dens  = 20;              % Density Factor

I knew that the values of Apass and Astop have been converted from dB to linear, so I get Dpass = 0.057501127785 and Dstop = 0.01. Here I figured out the linear value of stopband attenuation can be caculated using Dstop = 10^(-Astop/20), which is the inverse of Astop = -20*log10(Dstop).

I also assume Apass = -20*log10(1-Dpass) and the inverse is Dpass = 1 - 10^(-Apass/20). However, I'm unable to get Dpass = 0.057501127785 by substituting Apass = 1dB.

Can anyone tell me how Dpass is calculated? I.e. how to convert the passband ripple from dB to linear?

Thank you very much.

Regards,

Tong