fir.m

对接收到的磁场信号进行低通滤波

N=yesinput（'input the length of the windows N=:\n',15,[10,64]）;
w=yesinput('Input The Wc:\n',pi/4,[0.5,pi]);
b=1;
close all;
i=0;
while(b);
       n=[0:(N-1)];
       hd=ideal(w,N);
       k=yesinput('请选择窗口类型：1（boxcar）2(hamming)3(hanning)4(blackman):\n','1','1|2|3|4');
       k=str2num(k);
       if k==1
           B=boxcar(N);
           string=['Boxcar','N=',num2str(N)];7
       else if k==2
               B=hamming(N);
               string=['Hamming','N=',num2str(N)];
       else if k==3
               B=hanning(N);
               string=['Hanning','N=',num2str(N)];
           else if k==4
                   B=blackman(N);
                   string=['Blackman','N=',num2str(N)];
               end
            end
          end
       end  
         h=hd.*(B)';
         [H,m]=freqz(h,[1],1024,'whole');
         mag=abs(H);
         db=20*log10((mag+eps)/max(mag));
         pha=angle(H);
         i=i+1;
         figure(i)
            subplot(2,2,1);
            n=0:N-1;
            stem(n,h,'.');
            axis([0 N-1 -0.1 0.3]);
            hold on;
            n=0:N-1;
            x=zeros(N)；
            plot(n,x,'-');
            xlable('n');
            ylable('h(n)');
            title('实际低通滤波器的h(n)');
            text((0.3*N），0.27，string);
            hold off;
            subplot(2,2,2);
            plot(m/pi,db);
            axis([0 1 -100 0]);
            xlable('w/pi');
            ylable('dB');
            title('衰减特性（dB）');
            grid;
            subplot(2,2,3);
            plot(m,pha);
            hold on;
            n=0:7;
            x=zeros(8);
            plot(n,x,'-');
            title('相频特性')；
            xlable('频率（rad）');
            ylable('相位（rad）');
            axis([0 3.15 -4 4]);
            subplot(2,2,4);
            plot(m,mag);
            title('幅频特性')；
            xlable('频率W（rad）');
            ylable('幅值')；
            axis([0 3.15 0 1.5]);
            text(0.9,1.2,string);
            
            b=yesinput('Do You want To Continue:1(Continue),0(Exit):\n',
            1,[0 1]);
            if (b==1)
                N=yesinput('Input The length of the windows:\n',15,[10 50]);
                w=yesinput('Input The Wc:\n',pi/4,[0.5,pi]);
            end
end
% 子程序：产生理想低通滤波器单位脉冲响应hd(n)
function hd=ideal(w,N);
alpha=(N-1)/2;
n=[0:(N-1)];
m=n-alpha+eps;
hd=sin(w*m)./(pi*m);