cheby1_3db_hp.m

编写一个函数

function [b,a] = cheby1_3db_hp(N,wc,wp)
% N is the order
% wc is the 3dB cutoff frequency of the highpass filter we need
% wp is the passband-edge frequency of the highpass fiter we need
% wclp is the 3dB cutoff frequency of the original lowpass filter 
% wplp is the passband-edge frequency of the original lowpass filter 

% choose any walue of wclp,here we set 0.2
wclp=0.2

% alfa is one parameter of the Constantinides transfomation
alfa=-(cos((wclp+wc)*pi/2))/(cos((wclp-wc)*pi/2));

wplp=angle(-(exp(-j*wp*pi)+alfa)/(1+alfa*exp(-j*wp*pi)))/pi;

OmegaC = tan(pi*wclp/2); 
OmegaP= tan(pi*wplp/2); 

% find out the value of Ap
est=1/cosh(N*acosh(OmegaC/OmegaP));
Ap=10*log10(est^2+1)  

% use classic methods to design our highpass filter
[b,a] = cheby1(N,Ap,wp,'high')
fvtool(b,a);