Octave simulation of ADC decimation

git-svn-id: https://svn.code.sf.net/p/freetel/code@2039 01035d8c-6547-0410-b346-abe4f91aad63

diff --git a/codec2-dev/octave/adcres.m b/codec2-dev/octave/adcres.m
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+++ b/codec2-dev/octave/adcres.m
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+% adcres.m
+% David Rowe 18 Feb 2015
+%
+% ADC resamping simulation
+
+% [ ] quantisation of ADC
+% [ ] SNR at ADC input, SNR at resampler output
+% [X] decimation to 50 kHz
+% [X] 40dB ish rejection (test)
+% [X] visualise pass band flatness
+
+graphics_toolkit ("gnuplot");
+
+fs = 2E6;
+f1 = 500E3;
+f2 = f1 + 8E3;
+f3 = f1 - 7E3;
+f4 = f1 - 207E3;
+t = (0:(fs-1));
+M = 45;
+beta1 = 0.999;
+beta2 = 1 - (1-0.999)*M;
+
+s1 = [fs zeros(1,fs-1)];       % noise floor, continuous interferers 
+s2 = 100*4*cos(t*2*pi*f2/fs);  % wanted signal 40dB above interferers
+s3 = 100*2*cos(t*2*pi*f3/fs);
+s4 = 100*2*cos(t*2*pi*f4/fs);  % interferer at same level
+s = s1 + s2 + s3 + s4;
+
+s2 = filter(1,[1 0 beta1],s);  % BPF at fs/4
+s3 = s2(1:M:length(s2));       % decimate 
+s4 = filter([1 0 beta2],1,s3); % flatten filter response again
+
+figure(1)
+subplot(211)
+plot(20*log10(abs(fft(s)/fs)))
+grid
+axis([0 fs/2 -10 50])
+title('Input to ADC');
+subplot(212)
+plot(20*log10(abs(fft(s2/fs))))
+grid
+axis([0 fs/2 -10 70])
+title('After BPF');
+
+figure(2)
+subplot(211)
+plot(20*log10(abs(fft(s3)/fs)))
+grid
+axis([0 fs/2/M -10 50])
+title('After Decimation');
+subplot(212)
+plot(20*log10(abs(fft(s4)/fs)))
+grid
+title('After Flattening Filter');
+axis([0 fs/2/M -10 50])