I connected Tx2 and RX2 of my fmcomms3 board, and load a file to the Tx side. I created a QPSK.txt file with 128 points in that file, and the four points are
Then I got some data from the receiver side of AD9361. Although the data I got show squares, there existed an angel shift of the squares when I changed RX_LO and TX_LO frequencies. I attached a picture to show how the data changed when I changes the LO frequencies.
I am wondering how this happens, as I didn't change the file loaded to Tx. Is the number of samples affect the results?