We are testing our custom RF board based on AD9361. While testing for spectral flatness, the recived signal seems to have a variation in the flatness of around 3 to 4 dB. The transmitted spectrum looks ok with a maximum variation of +/-0.5dB in the flatness. TX and RX paths have external PA and LNA. Testing is done with OFDM waveform of 10MHz bandwidth and 5MHz bandwidth. In the 10MHz bandwidth testing, 1MHz each from both sides (outermost 1MHz), are getting affected. Similarly in 5MHz BW testing, 500KHz each on both sides.
I have tried increasing the Analog and Digital filter bandwidths by setting the register values, but not improving. Can someone help me in understanding the source of the issue.