function [b,a] = afd_butt(Wp,Ws,Rp,As);
% Analog Lowpass Filter Design: Butterworth
% -----------------------------------------
% [b,a] = afd_butt(Wp,Ws,Rp,As);
%  b = Numerator coefficients of Ha(s)
%  a = Denominator coefficients of Ha(s)
% Wp = Passband edge frequency in rad/sec; Wp > 0
% Ws = Stopband edge frequency in rad/sec; Ws > Wp > 0
% Rp = Passband ripple in +dB; (Rp > 0)
% As = Stopband attenuation in +dB; (As > 0)
%
if Wp <= 0
        error('Passband edge must be larger than 0')
end
if Ws <= Wp
        error('Stopband edge must be larger than Passband edge')
end
if (Rp <= 0) | (As < 0)
        error('PB ripple and/or SB attenuation ust be larger than 0')
end

N = ceil((log10((10^(Rp/10)-1)/(10^(As/10)-1)))/(2*log10(Wp/Ws)));
fprintf('\n*** Butterworth Filter Order = %2.0f \n',N)
OmegaC = Wp/((10^(Rp/10)-1)^(1/(2*N)));
[b,a]=u_buttap(N,OmegaC);