Hi, https://kb.ettus.com/UBX#Noise_Figure lists 2-3dB noise figure, UBX_Data_Sheet.pdf 2-4dB for UBX-160 (at 915 MHz).
I connect a 50 Ohm load to RX and create a simple gnuradio application with USRP Source that calculates the RMS value of the sampled data (and plots the noise in time domain). samp_rate = 5 MHz. RX Gain = 0 dB. The value is 0.000113 RMS. The time domain waveforms look a bit like quantization noise. I repeat the experiment, this time with RX Gain value set to 37.5dB. The value is 0.000841 RMS. According to https://files.ettus.com/performance_data/ubx/UBX-without-UHD-corrections.pdf, NF=2-3dB for 37.5dB gain and ~23dB for 0dB gain. The high gain value coincides with the values from the datasheet above. Question 1: Is the reason that for 0dB it looks like quantization noise, that the noise is smaller than LSB so I am effecively seeing the ADC noise? Question 2: Is the ADC quantixation noise the main reason for the difference in NF? (Note: I am familiar with Friis' equation) Question 3: The noise difference for both cases is 20*log10(0.000841/0.000113)=17.43dB. According to the measurement PDF it should be ca. 23-3=20dB. The difference is ~2-4dB. Is this just measurement uncertainty/part-to-part mismatch or is something wrong with my approach? Question 3: I repeat the last experiment by setting the "Ch0 Bandwidth [Hz]" to 0 (default), 5e6 and 1e6. I would expect that my rms value decreases by a factor of sqrt(2) for each halfing of bandwidth. However, the value always stays around 0.000841 RMS, regardless of the bandwidth value. Why? Question 4: According to theory, my captured signal is -174 + NF + 10*log10(BW) = -174+3+10*log10(5e6)=-104dBm. Is it correct that my 0.000841 corresponds to -104dBm input power? (If not, why not?). Or differently, -104dBm input power corresponds to 20*log10(0.000841 / 1) = -61.5dBFS ? Thank you, Lukas _______________________________________________ USRP-users mailing list USRP-users@lists.ettus.com http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
