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% nf_from_gr.m
% David Rowe Mar 2018
#{
Calculate NF in real time from 16 bit real samples from stdin
1/ Using gqrx:
gqrx setup:
Configure I/O devices:
To switch on LNA bias for HackRF, in Configure I/O devices menu set:
Device String: hackrf,bias=1
To switch on LNA bias for airspy run for a few seconds this before starting gqrx:
$ airspy_rx -r /dev/null -f 435 -b 1
I used a sample rate of 250000 for Airspy R2, 3000000 for Airspy Mini
Input options...: start with set all gain sliders set to maximum
FFT Setting.....: freq Zoom to max
Receiver Options: On spectrum display, drag filter width until it's about 12k
Filter Shape Normal
Mode USB
Tune until tone is between 2 and 4 k
Press UDP button
Then in a Linux Term:
$ nc -ul 7355 | octave --no-gui -qf nf_from_stdio.m 48000
2/ Using command line tools. Compile airspy tools and csdr from source:
a) Airspy:
$ airspy_rx -a 6000000 -l 14 -m 15 -v 15 -r - -f 434.998 -b 1 | \
csdr convert_s16_f | csdr fir_decimate_cc 50 | csdr convert_f_s16 | \
octave --no-gui -qf ~/codec2-dev/octave/nf_from_stdio.m 120000 complex
Note: we tuned a few kHz down to put the test tone in the 2000 to 4000 Hz range.
b) HackRF:
Term 1:
$ ~/codec2-dev/octave$ nc -ul 7355 | octave --no-gui -qf nf_from_stdio.m 80000 complex
Term 2:
$ hackrf_transfer -r - -f 434995000 -s 4000000 -a 1 -p 1 -l 40 -g 32 | \
csdr convert_s8_f | csdr fir_decimate_cc 50 | csdr convert_f_s16 | \
nc localhost -u 7355
Note: HackRF needed a bit of tuning to get test tone in 2000 to 4000 Hz range. This
can be tricky with the command line method, easier with gqrx.
c) rtlsdr (assuming sig gen set to 144.5MHz, -100dBm)
Term 1:
$ ./rtl_sdr -g 50 -s 2400000 -f 144.498E6 - | csdr convert_u8_f | csdr fir_decimate_cc 50 | \
csdr convert_f_s16 | octave --no-gui -qf ~/codec2/octave/nf_from_stdio.m 48000 complex
TODO:
[ ] work out why noise power st bounces around so much, signal power seems stable
[ ] reduce CPU load, in particular of plotting
#}
graphics_toolkit ("gnuplot")
% command line arguments
arg_list = argv ();
if nargin == 0
printf("\nusage: %s FsHz [real|complex] [testToneLeveldBm]\n\n", program_name());
exit(0);
end
Fs = str2num(arg_list{1});
shorts_per_sample = 1;
if nargin == 2
if strcmp(arg_list{2}, "real")
shorts_per_sample = 1;
end
if strcmp(arg_list{2}, "complex")
shorts_per_sample = 2;
end
end
Pin_dB = -100; % level of input test tone
if nargin == 3
Pin_dB = str2num(arg_list{3});
end
printf("Fs: %d shorts_per_sample: %d Pin_dB: %f\n", Fs, shorts_per_sample, Pin_dB);
[s,c] = fread(stdin, shorts_per_sample*Fs, "short");
while c
if shorts_per_sample == 2
s = s(1:2:end)+j*s(2:2:end);
end
S = fft(s.*hanning(Fs));
SdB = 20*log10(abs(S));
figure(1); plot(real(s)); axis([0 Fs -4E4 4E4]);
figure(2); plot(SdB); axis([0 12000 40 180]);
% 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 = 5000;
noise_st = 6000; noise_en = 10000;
% find peak and sum power a few bins either side, this ensure we don't capture
% too much noise as well
[pk pk_pos] = max(abs(S));
if pk_pos > 5
Pout_dB1 = 10*log10(sum(abs(S(pk_pos-5:pk_pos+5)).^2)); % Rx output power with test signal
else
Pout_dB1 = 0;
end
Pout_dB = 10*log10(sum(abs(S(sig_st:sig_en)).^2)); % Rx output power with test signal
G_dB = Pout_dB - Pin_dB; % Gain of Rx
Nout_dB = 10*log10(sum(abs(S(noise_st:noise_en)).^2)/(noise_en-noise_st)); % 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 %d %4.1f Nout: %4.1f G: %4.1f No: %4.1f NF: %3.1f dB\n", Pout_dB, pk_pos, Pout_dB1, Nout_dB, G_dB, No_dB, NF_dB);
pause(2);
[s,c] = fread(stdin, shorts_per_sample*Fs, "short");
endwhile
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