Why do I receive two different responses when I try to discretize the same analog filter using the ELLIP

John Williams answered . 2025-07-04 23:06:45

Fs=2e9; 

FN=Fs/2; % Sampling and Nyquist frequencies

fp=0.4*FN; 

fs=0.42*FN; %Passband and Stopband frequencies

wp=fp*2*pi; 

ws=fs*2*pi; %Passband and Stopband edge frequencies

Ap=1.2; 

As=100; % Passband and Stopband attenuation

% Calculate filter coefficients and frequency responses

[N, wc]=ellipord(wp, ws, Ap, As, 's'); % analog filter

%--------------------------------------------------------------------------

% Design using ELLIP for the analog prototype

%--------------------------------------------------------------------------

[A, B, C, D] = ellip(N, Ap, As, wc, 's');

[Ad, Bd, Cd, Dd] = bilinear(A, B, C, D, Fs);

[sos, g] = ss2sos(Ad, Bd, Cd, Dd);

hd1 = dfilt.df2sos(sos, g);

%--------------------------------------------------------------------------

% Design using ELLIPAP for the analog prototype

%--------------------------------------------------------------------------

[z, p, k] = ellipap(N, Ap, As);

[A, B, C, D] = zp2ss(z, p, k);

[At, Bt, Ct, Dt] = lp2lp(A, B, C, D, wc);

[Bs, As] = ss2tf(At, Bt, Ct, Dt);

[Ad, Bd, Cd, Dd] = bilinear(At, Bt, Ct, Dt, Fs);

[sos, g] = ss2sos(Ad, Bd, Cd, Dd);

hd2 = dfilt.df2sos(sos, g);

fvtool(hd1, hd2)

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