[USRP-users] Quandry over B210 noise performance
Marcus D. Leech
mleech at ripnet.com
Thu Jul 23 16:17:24 EDT 2015
On 07/23/2015 10:30 AM, David Browne via USRP-users wrote:
> 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.
Making noise-figure measurements anywhere other than at maximum RX gain
will give you bad results.
Typically, (I don't know for sure about the AD9361), front-end gain
adjustment is implemented with a variable attenuator between the output
of the first LNA
stage and a subsequent gain stage. What this means is that the
attenuator equivalent noise figure contributes significantly to the
overall noise figure of
the system, and the degree to which is does that is based on:
(A) The amount of attenuation
(B) The gain of the first-stage LNA
Let's say that at 27dB gain setting, there's 35dB of attenuation
inserted between the first LNA and subsequent gain stages. If the first
gain stage has 15dB
of gain (this is a guess), then that 35dB attenuation is still
contributing roughly 20dB of equivalent noise figure to the system. As
you increase the gain,
the attenuation is reduced, and your system noise figure goes down.
Doing useful noise-figure measurements is also usually done with a
calibrated noise source with a known ENR, which you can then use in a
calculation to derive noise-figure.
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