USRP Sink Block

[Q W]

Hi there,
I am new to USRP, Gnuradio and UHD. I am currently starting to use an Ettus 
X310 with a UBX-160, a LFTX and a LFRX daughterboards, and trying to implement 
a simple communications using the USRP.
I am working on a gnuradio environment and planning to send a sine wave from RF 
A TX/RX to RF A RX2. The TX/RX and RX 2 ports are connected to the UBX 
daughterboard. At the moment, the two ports are connected with a SMA cable and 
a 30dB attenuator.
The script for Gnuradio is shown as below.


When running the script, I can see the correct output of the signal Source, 
being sine wave from the GUI time and frequency sinks. However, the received 
signal from USRP Source looks incorrect, which is very noisy with extremely low 
amplitude. I suspect the sine wave seen at the output of the USRP Source is the 
LO leakage other than the transmitted wave. (See the waveforms below. The 
second diagram is the output of the USRP source which is extremely low in 
amplitude and very noisy.) This has been confirmed when the TX/RX port was 
connected to an oscilloscope which showed noise only. When the script is 
running, I can see the LED indicators were on, (red light for TX/RX and green 
light for RX2). Thus I think there should be transmissions in the wire.



One more thing I observed is that when the SMA wire is connected to other 
receiving ports such as RF B RX 2, the USRP still think it is RX2 of RF A is 
connected, and thus I can see the green LED below RX2 of RF A , other than RX2 
of RF B, is on.
I suspect I didn't configure the USRP Sink and USRP Source blocks correctly in 
Gnuradio, but couldn't find a solution to resolve the problem.
Kind regards,Tom