I am streaming the data from RF SOM Board (ADRV9361-Z7035 with FMC carrier card) on Windows system IIO scope. There I am experiencing IQ constellation distortion. The setup to receive the data is generating 80 MHz signal with B-PSK modulation (symbol rate: 40 Msps). The source of the data is external. Input level is -40 dBm to SOM. board. Rx LO set is 80 MHz . Input signal offsets were given from 0 Hz to 100KHz, 200KHz and 300KHz. With no offset (i.e. board LO and external are at the same frequency 80 MHz) there were more distortion were seen. As we increase the offset the distortion is getting diminishing. What can have the issue? As per my understanding the constellation diagram shall have circular rotation in the IQ plan, but it didn't look so.
The data also tried to acquire via MATLAB but same thing observed. Constellation diagram attached for your reference.
For 0 Hz from internal LO Frequency-
For 100 KHz from internal LO Frequency-
For 200 KHz from internal LO Frequency-
For 300 KHz offset from internal LO Frequency -
Please help me out to resolve this issue.