function [b,a] = ellip_3db_lp(N,Fc,Apass,Astop)% ELLIP3DB Design a lowpass elliptic filter for a given 3 dB point.%   Inputs:%       N     - Filter order%       Fc    - 3 dB cutoff frequency (0 to 1)%       Apass - Passband attenuation (dB)%       Astop - Stopband attenuation (dB)% Design an elliptic filter for any fpfp = Fc; % To keep things simple, we make fp equal to Fc% Design prototype filter[btemp,atemp] = ellip(N,Apass,Astop,fp);% Find 3 db point[H,w]=freqz(btemp,atemp,1024);indx = max(find(abs(H).^2 >.5));% Refine search [H2,w2]=freqz(btemp,atemp,linspace(w(indx),w(indx+1),1000)');indx2 = max(find(abs(H2).^2 >.5));f3db = w2(indx2)/pi% Apply frequency transformation to obtain final filter[b,a] = iirlp2lp(btemp,atemp,f3db,Fc)fvtool(b,a);