On 04/19/2020 09:26 PM, Lukas Haase via USRP-users wrote:
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
There'll be quantization noise, certainly.
But RF systems with variable gain use variable attenuators, and that
attenuation value is directly added to the noise figure of the stage that
follows it.
Now the magnitude of this effect obviously depends on where that
attenuator is placed in the gain chain. A 40dB attenuator after a 20dB
gain means that the 40dB of hoise is only "washed out" by the 20dB of
the previous stage(s).
It is entirely normal and conventional to quite noise figures at
different gains settings precisely because of this architectural necessity.
Using a purely-analytic and numerical approach to determining the
magnitude of an input signal, given and intervening chain of
RF bits and pieces, whose noise figure and gain have uncertainties,
is what we call a "state of sin". You MUST calibrate with a known
signal source over your expected run-time parameter space. It's the
only way. It's the way laboratory instruments are routinely calibrated.
Using "datasheet analysis" which is basically what you're doing
above, leads to results that cannot be trusted to any degree of certainty.
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