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IQ constellation distortion in AD9361

Category: Hardware
Product Number: ADRV9361-Z7035

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.

Thanks Regards,

Jitendra Kapse

Parents Reply
  • When the signal is kept on LO, then there will be degradation in performance because of the LO leakage calibrations running on the actual signal and trying to correct that. You can disable the RFDC offset and BBDC tracking cals and then check the performance levels. Else, you may  need to offset the signal from the LO, such that a subcarrier doesnot fall on the LO frequency, or you need to shift the signal such that the full signal BW falls outside the LO, in order to avoid this degradation. 

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