[USRP-users] Quandry over B210 noise performance

David Browne decibel at mit.edu
Thu Jul 23 10:30:31 EDT 2015

I've been characterizing the noise performance of the B210 at the upper ISM band (5.8GHZ) and have been surprised to find that it is quite poor (22.3dB Noise Figure). I wanted to share my results (see below) and ask if anyone has ideas on how to improve the performance in this band. One suspicion I have is that the UHD is loading the AD9361 with a gain table that may not be optimal for this band but I have yet to dive into the UHD source code to verify this. I am also curious to know if the UHD controls AD9361 gain using the "Full Gain Mode" or "Split Gain Mode" (see AD9361_Reference_Manual_UG-570.pdf online). In general, it would be great to hear from anyone who has modified the UHD to control the AD9361 to optimize SNR or noise performance for their application's band and how much of an improvement can be achieved over the stock UHD configuration.

Test Settings
B210 with single sub-device active (i.e. single channel test)
master clock rate: 32 MHz
rx frequency     : 5.8 GHz
rx sample rate   : 400 kHz
UHD version      : UHD_003.007.001-84-gd99ce4ef

Measurements were made on the RX2 ports (both the A:A and A:B sub-devices produced similar results)
Noise power was computed after first filtering the DC component (~0.6dB) from the IQ samples to prevent LO leakage at DC from biasing the noise estimator.  
A calibrated -140dBm/Hz noise source was used to inject a controlled level of broadband noise.

Results (table formatted for fixed width font)
rx gain level                                       :  0        26      27      28      30      40
IQ sample power with rx port terminated (dBfs)      : -90.9    -90.9   -90.1   -89.9   -88.4
IQ sample power with -140dBm/Hz noise source (dbfs) :                  -78.4           -77.1   -68.5

Commment 1: The results show a gain setting of 27 is the minimum tested level that brings the input noise up to the output noise level
Comment 2 : The increase in noise power between gain levels of 30 and 40 dB is 8.6 dB (i.e. close to the expected 10dB)
Equivalent Noise Figure Calculation:
    If input noise level of -140 dBm/Hz  maps to output noise level of -78.4 dBfs
    Then output noise level of -90.1 dBfs maps to a noise level of -140 - (-78.4 - (-90.1)) = -151.7 dBm/Hz
    The equivalent noise figure with the rx port terminated at a gain setting of 27 is -151.7 - (-174) = 22.3 dB

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