Lost constellation while transferring signals using USRP N210

[Hompeaz]

Hi everyone,

I'm using USRP and gnuradio for 16QAM communication. The flowgraph was linked according to this tutorial example : https://github.com/gnuradio/gr-tutorial/blob/master/examples/tutorial7/mpsk_stage3.grc ( I change the Constellation Object type from QPSK to QAM )

It works fine when I use "Channel Model" as in the tutorial. The constellation graph shows perfectly. But when I adopt the flowgraph to USRP transimmiting, the constellation somehow lost. The signal points in the graph have very small range and keep spinning around.

The transmitter flowgraph shows as follow:

The receiver flowgraph shows as follow:

The receiver constellation figure shows as follow:

And here's some basic parameters of the USRP N210 :

Do you guys have any idea what's going wrong?

[Sastry]

Do you have a good reference clock ?

[Hompeaz]

In my receiver flowgraph, I used the "Polyphase Clock Sync" block to synchronize the clock and set the USRP sync to "PC Clock". However, I realized that this might not be sufficient.

After consulting the gnuradio wiki page, I learned that I should not change the "clock rate" option in the USRP Sink or USRP Source blocks and leave it as default. Do you think this is the option I need to modify?