[USRP-users] Odd behavior with received power on SBX 120

Hrishikesh Shelar hshelar at umich.edu
Tue Sep 16 00:03:13 EDT 2014

Hi all,

I am experiencing some odd behavior with loopback tests and received power
with the SBX-120 daughter card. After tuning both the TX/RX and RX2 side to
the same frequency I have observed that the input signal power on the RX
chain doesn't depend on the external loopback cable plus attenuators used
(its as if the two sides of the daughter card were connected internally). I
have confirmed this by removing the cable all together and applying dummy
loads to both sides.

Based on my custom logic in the RX chain I have seen the received power
swing from the 13th MSB to the 21st MSB (out of 24 bits) just by altering
the tx-gain (0-31dB) and rx-gain (0-31 dB) parameters through UHD. So 8bits
of swing is about 10*log(2^8) or ~55 dB ratio which make sense if the
combined realized gain of the tx-gain and rx-gain parameters is 0-62 dB.

With my custom logic I was able to loopback OQPSK at 10 mbps, *all with no
external loopback cable!!!* Is there anything I am missing here? I can't
make sense of these results. o.O

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