ellip3db.m

IEEE文章仿真FIR滤波器,附带源代码程序,便于理解与深入学习.

function [b,a] = ellip3db(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)
%
%   NOTE 1: This function requires the Filter Design Toolbox.
%
%   NOTE 2: To design highpass, bandpass, and bandstop filters use the
%   resulting filter from this function along with IIRLP2HP, IIRLP2BP, and
%   IIRLP2BS respectively. These functions are included in the Filter
%   Design Toolbox.
%
%   Example 6th order filter with cutoff at 0.3:
%   [b,a] = ellip3db(6,.3,1,80);
%   fvtool(b,a)

%   Author(s): R. Losada, V. Pellissier
%   Copyright 2005 The MathWorks, Inc.

% Design an elliptic filter for any fp
fp = 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);