gps-sdr / gps-sdr

Open source GPS receiver based on USRP and GN3S

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gps-sdr / accessories / test_agc.m
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clear all; close all;
 
fp = fopen('/tmp/GPSPIPE','r');
pts = 4096;
paxis = 1:pts/2;
% edges = -2048:128:2048;
 
B = zeros(pts,1);
A = B;
 
low = 4;
high = 16;
 
bits = 5;
edges = [-2^(bits+1):1:2^(bits+1)];
 
 
scale = 1;
 
while(1)
    
    B = fread(fp,pts,'int16');
 
     mask = B < 0;
     A(mask)  = fix(2*B(mask)/scale-1.0);
     A(~mask) = fix(2*B(~mask)/scale+0.0);    
     
%    A = B;
 
    num = sum(abs(A) > 2^bits);
    
    
    if(num < low)
        scale = scale - 1;
    end
    
    
    if(num > high)
        scale = scale + 1;
    end
    
    if(scale == 0)
        scale = 1;
    end
    
    
 
    
    figure(1)
    subplot(211)
    plot(paxis,A(1:2:end),'b',paxis,A(2:2:end),'r');    
%    plot(10*log10(abs(fft(A))))
%     axis([0 pts/2 -2^(bits+1) 2^(bits+1)])
    title(sprintf('%f\t%f\t%f\n',num,scale,3*std(A)))
    grid on;
    subplot(212)
    plot(10*log10(abs(fft(B))))
    plot(edges,histc(A,edges));
    axis([min(edges) max(edges) 0 10*pts/length(edges)])
    grid on;
    drawnow;
    
end