real time NF calculation script, giving sensible results with gqrx

git-svn-id: https://svn.code.sf.net/p/freetel/code@3404 01035d8c-6547-0410-b346-abe4f91aad63

diff --git a/codec2-dev/octave/nf_from_stdio.m b/codec2-dev/octave/nf_from_stdio.m
new file mode 100644 (file)
index 0000000..fb58dd9
--- /dev/null
+++ b/codec2-dev/octave/nf_from_stdio.m
@@ -0,0 +1,38 @@
+% nf_from_gr.m
+% David Rowe Mar 2018
+
+#{
+  Calculate NF in real time from 16 bit real samples from stdin
+#}
+
+N = 48000;
+Pin_dB = -100;
+
+graphics_toolkit ("gnuplot")
+
+[s,c] = fread(stdin, N, "short");
+
+while c
+  S = fft(s.*hanning(N));
+  SdB = 20*log10(abs(S));
+  figure(1); plot(real(s)); axis([0 N -3E4 3E4]);
+  figure(2); plot(SdB); axis([0 12000 40 160]);
+
+  % assume sine wave is between 2000 and 4000 Hz, and dominates energy in that
+  % region.  Noise is between 5000 - 10000 Hz
+  
+  sig_st = 2000; sig_en = 4000;
+  noise_st = 5000; noise_en = 10000;
+  
+  Pout_dB = 10*log10(var(S(sig_st:sig_en)));      % Rx output power with test signal
+  G_dB    = Pout_dB - Pin_dB;                     % Gain of Rx                           
+  Nout_dB = 10*log10(var(S(noise_st:noise_en)));  % Rx output power with noise
+  Nin_dB  = Nout_dB - G_dB;                       % Rx input power with noise
+  No_dB   = Nin_dB - 10*log10(noise_en-noise_st); % Rx input power with noise in 1Hz bandwidth
+  NF_dB   = No_dB + 174;                          % compare to thermal noise to get NF
+  printf("Pout: %4.1f  Nout: %4.1f G: %4.1f  No: %4.1f NF: %3.1f dB\n", Pout_dB, Nout_dB, G_dB, No_dB, NF_dB);
+
+  pause(2);
+  [s,c] = fread(stdin, N, "short");
+endwhile
+