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Diffstat (limited to 'octave/nf_from_gr.m')
| -rw-r--r-- | octave/nf_from_gr.m | 129 |
1 files changed, 129 insertions, 0 deletions
diff --git a/octave/nf_from_gr.m b/octave/nf_from_gr.m new file mode 100644 index 0000000..bd36d02 --- /dev/null +++ b/octave/nf_from_gr.m @@ -0,0 +1,129 @@ +% nf_from_gr.m +% David Rowe Mar 2016 + +#{ + Calculate NF from GNU Radio output samples in + ...IQIQ... (32 bit float) sample files + + 1/ Take one sample with a -100dBm input carrier + 2/ Take another sample with no signal (just rx noise) + 3/ Set Fs, adjust st and en to use a chunk of spectrum without too + many birdies. + + Gotchas: + + 1/ Inspect Figure(1), the time domain plots. + 2/ Make sure plenty of ADC bits are being used with the noise-only sample, + we don't want ADC quantisation noise to dominate. Aim for about half + full scale. + 3/ Also watch out for clipping on either sample. + +#} + +1; + +function det_nf(p_filename, n_filename, title, Fs, st, en, Pin_dB, real_file=0) + + if real_file + % real samples files of 16 bit shorts + fs=fopen(p_filename,"rb"); + p = fread(fs,Inf,"short"); + fclose(fs); + fs=fopen(n_filename,"rb"); + pn = fread(fs,Inf,"short"); + fclose(fs); + else + % GNU radio complex file input + p = load_comp(p_filename); + pn = load_comp(n_filename); + end + + % skip any start up transients + + tst = floor(0.1*Fs); ten = st + Fs - 1; + P = fft(p(tst:ten)); + N = fft(pn(tst:ten)); + + PdB = 20*log10(abs(P)); + NdB = 20*log10(abs(N)); + + figure(1); clf; + subplot(211); plot(real(p(tst:tst+floor(Fs*0.1)))); + subplot(212); plot(real(pn(tst:tst+floor(Fs*0.1)))); + + figure(2); clf; + subplot(211); plot(st:en, PdB(st:en)); + subplot(212); plot(st:en, NdB(st:en)); + + #{ + ------------------------------------------------------------------------ + + From Wikipedia: The Noise Figure is the difference in decibels + (dB) between the noise output of the actual receiver to the noise + output of an “ideal” receiver + + An ideal receiver would have an output noise power of: + + Nout_dB = 10log10(B) -174 + G_dB + + The -174 dBm/Hz figure is the thermal noise density at 25C, for + every 1Hz of bandwidth your will get -174dBm of noise power. It's + the lower limit set by the laws of physics. G_dB is the Rx gain. The + 10log10(B) term takes into account the bandwidth of the Rx. A wider + bandwidth means more total noise power. + + So if you have a 1Hz bandwidth, and a gain of 100dB, you would + expect Nout_NdB = 0 -174 + 100 = -74dBm at the rx output with no + signal. If you have a 1000Hz bandwidth receiver you would have NdB_out + = 20 -174 + 100 = -44dBm of noise power at the output. + + To determine Noise Figure: + 1) Sample the Rx output first with a test signal and then with noise only. + 2) Find the Rx gain using the test signal. + 3) Find the noise output power, then using the gain we can find the noise + input power. + 4) Normalise the noise input power to 1Hz noise bandwidth and + compare to the thermal noise floor. + + ---------------------------------------------------------------------------- + #} + + % variance is the power of a sampled signal + + Pout_dB = 10*log10(var(P(st:en))); % Rx output power with test signal + G_dB = Pout_dB - Pin_dB; % Gain of Rx + Nout_dB = 10*log10(var(N(st:en))); % Rx output power with noise + Nin_dB = Nout_dB - G_dB; % Rx input power with noise + No_dB = Nin_dB - 10*log10(en-st); % Rx input power with noise in 1Hz bandwidth + NF_dB = No_dB + 174; % compare to thermal noise to get NF + printf("%10s: Pin: %4.1f Pout: %4.1f G: %4.1f NF: %3.1f dB\n", title, Pin_dB, Pout_dB, G_dB, NF_dB); +endfunction + + +% HackRF -------------------------- + +%p_filename = "~/Desktop/blogs/nf/hackrf_100dbm_4MHz.bin"; +%n_filename = "~/Desktop/blogs/nf/hackrf_nosignal_4MHz.bin"; +p_filename = "~/codec2-dev/build_linux/unittest/hackrf_100dbm_4MHz.bin"; +n_filename = "~/codec2-dev/build_linux/unittest/hackrf_nosignal_4MHz.bin"; +det_nf(p_filename, n_filename, "HackRF", 4E6, 180E3, 600E3, -100); + +#{ +% RTL-SDR -------------------------- + +p_filename = "~/Desktop/nf/neg100dBm_2MHz.bin"; +n_filename = "~/Desktop/nf/nosignal_2MHz.bin"; +det_nf(p_filename, n_filename, "RTL-SDR", 2E6, 100E3, 300E3, -100); + +% AirSpy ------------------------- + +p_filename = "~/Desktop/nf/airspy_100dbm_2.5MSPS.bin"; +n_filename = "~/Desktop/nf/airspy_nosig_2.5MSPS.bin"; +det_nf(p_filename, n_filename, "AirSpy", 2.5E6, 100E3, 300E3, -100); + +% Fun Cube Dongle Pro Plus ------------------------- + +p_filename = "~/Desktop/nf/fcdpp_100dbm_192khz.bin"; +n_filename = "~/Desktop/nf/fcdpp_nosig_192khz.bin"; +det_nf(p_filename, n_filename, "FunCube PP", 192E3, 25E3, 125E3, -100); +#} |