Flex400 TX/RX to RX2 reception

Hello everyone,

I am new to this list as well as GNURadio. I recently installed all of
the software, purchased a USRP, and started learning Python. I now have
a question to a problem that has eluded me for a few days.

Let me give you a little background. Currently, I have been attempting
to build my knowledge with the use of the GMSK2 related software. The
benchmark_gmsk_tx.py and benchmark_gmsk_rx.py code in particular. I
have been successful in sending packets running benchmark_tx on one PC
and benchmark_rx on the other using the basic TX/RX daughterboards. (I
have two USRPs) Both of these cards are connected via an SMA to SMA
cable. I am also able to place both of these daughterboards on the
same USRP and do a loopback test to one PC. The loopback is successful
with both daughterboards placed on side A, or with splitting them
between both sides like this: TX daughterboard on TX A and RX
daughterboard on RX B.

Now for my question. I also have a Flex400 daughterboard. If I place
this daughterboard on my USRP, lets say on the A side, and attempt to
use the benchmark_tx and benchmark_rx software to perform a loopback
test, I am unable to receive anything at the receive end. My plan was
to do exactly as I had done with the basic TX/RX boards and perform a
loopback via the TX/RX to transmit to RX2 on the Flex400. I have tried
many different approaches to make this work. In another discussion
message, Eric stated that a line needed to be added to the RXer code in
order to set the RX2 antenna of the Flex400 daughterboard to be the
reception point. When I found this information, I was excited and
immediately added this line “self.subdev.select_rx_antenna(‘RX2’)” to
the RX code. I also added 30 dB of attunation. I figured these would
fix the problem I was seeing, but they did not help. I verified that
the antennas were indeed selected properly by probing the board and
comparing values of U209 and U202 to see if they matched up with the
selection notes on the schematics. They were set and correct: “Full
Duplex TX and RX” “0101”. (this was not the case before I added the
antenna select code, where it was “TX Only” “0110”) So, I am still
unable to do a loopback test on this board. Here are my questions…

Is it possible to perform a loop back from TX/RX to RX2 on the same
board? If so, could anyone help me figure out an approach that will
work? Should there be any changes to the benchmark gmsk tx and rx
software to make this happen (other than antenna selection)? I assumed
this would be very straight forward, but so far it has not.

I have also tried many software settings…like raising and lowering
gain, changing frequencies in the 400-500MHz range, using 60 dB of
attenuation instead of 30, changing bit rates, and setting to
discontinuous TX. None have helped!

The only thing I have seen displayed on the benchmark_gmsk_rx side that
gave me hope was “uO” being displayed. Not even sure what that means,
but it occurs at the start of benchmark_gmsk_tx.

Any help would be greatly appreciated!

Thank you for your time,