filtm.m

数字通信第四版原书的例程

function [at,bt,ct,dt,freq,mag] = filtm(filt,trans,cutoff,bw,npts,P1,P2,P3);
%FILTM	Analog filter design for masked blocks in SIMULINK system MASKFILT.
%
%	This M-file is used to return the state-space matrices and frequency response
%	for given filter design routines and frequency ranges.
%
%	[A,B,C,D,FREQ,MAG] = FILTM('FILT','TRANS',CUTOFF,BW,NPTS,P1,P2,P3); returns
%	A, B, C, D state-space matrices and the magnitude response FREQ, MAG for
%	a given filter design function 'FILT' (e.g. 'BUTTAP') and filter
%	transformation 'TRANS' (e.g. 'LP2BP') .
%	CUTOFF and BW are the cutoff frequency and bandwidth of the filter (rads/sec).
%	NPTS defines the number of points for the frequency response.
%	P1, P2, P3 are the parameters for the filter design.
%
%	See also: MASKFILT, BUTTAP, LP2BP

%	Copyright (c) 1990-94 by The MathWorks, Inc.
%	Andrew Grace 5-30-91.

% Get LP analog prototype
a=-1;b=1;c=1;d=1;freq=1:10; mag=1:10;
if nargin == 8
	[z,p,k] = feval(filt,P1,P2,P3);
elseif nargin == 7
	[z,p,k] = feval(filt,P1,P2);
else
	[z,p,k] = feval(filt,P1);
end

% Conversion e.g. lp2bp
if length(z)
	k = prod(-p)/prod(-z); 
else 
	k = prod(-p);
end
[a,b,c,d]=zp2ss(z,p,k); 
if length(bw)
	%[at,bt,ct,dt] = feval(trans,a,b,c,d,cutoff,bw)); (Buggy)
	[at,bt,ct,dt] = eval([trans,'(a,b,c,d,cutoff,bw)']);
else
	%[at,bt,ct,dt] = feval([trans,a,b,c,d,cutoff,bw)); (Buggy)
	[at,bt,ct,dt] = eval([trans,'(a,b,c,d,cutoff)']);
end
% Frequency response
[num,den]=ss2tf(at,bt,ct,dt,1); 
w=logspace(log10(cutoff/10),log10(10*cutoff),npts); 
h=freqs(num,den,w); 
freq = 20*log10(w);
mag = 20*log10(abs(h));