% 第八章: 例 8.23
%            切比雪夫-2低通濾波器設(shè)計(jì)
%                 CHEBY2函數(shù)的應(yīng)用
%
% 數(shù)字濾波器指標(biāo):
wp = 0.2*pi;                         % 數(shù)字通帶頻率(弧度)
ws = 0.3*pi;                         % 數(shù)字阻帶頻率(弧度)
Rp = 1;                              % 通帶波動(dòng)(dB)
As = 15;                             % 阻帶衰減(dB) 

% 模擬原型指標(biāo)
T = 1;                            % 置 T=1
OmegaP = (2/T)*tan(wp/2);            % 預(yù)修正原型通帶頻率
OmegaS = (2/T)*tan(ws/2);            % 預(yù)修正原型阻帶頻率
ep = sqrt(10^(Rp/10)-1);             % 通帶波動(dòng)參數(shù)
Ripple = sqrt(1/(1+ep*ep));          % 通帶波動(dòng)
Attn = 1/(10^(As/20));               % 阻帶衰減

%模擬原型階次計(jì)算
ep = sqrt(10^(Rp/10)-1);               %通帶波動(dòng)因子
A = 10^(As/20);                      %阻帶波動(dòng)因子
OmegaC = OmegaP;                  %模擬原型截止頻率
OmegaR = OmegaS/OmegaP;           %模擬原型傳輸比
g = sqrt(A*A-1)/ep;                   %模擬原型中間計(jì)算
N = ceil(log10(g+sqrt(g*g-1))/log10(OmegaR+sqrt(OmegaR*OmegaR-1)));
fprintf('\n*** 切比雪夫-2 濾波器階次 = %2.0f \n',N)
%%*** 切比雪夫-2濾波器階次 =  4 

% 數(shù)字切比雪夫-2濾波器設(shè)計(jì):
wn = wn/pi;                           %以pi為單位的數(shù)字阻帶
 [b,a]=cheby2(N,As,wn);
[b0,B,A] = dir2cas(b,a)
%%b0 = 0.1797
%%B = 1.0000    0.5574    1.0000
%%    1.0000   -1.0671    1.0000
%%A = 1.0000   -0.4183    0.1503
%%    1.0000   -1.1325    0.7183