Dropoff in white noise produced by AWG
This is a sinx/x roll off. You can compensate 


mathematically in the waveform files
Some AWG have List mode". You can perform amplitude compensation by writing into the AWG List entries, amplitude fields, first by determining if you are sending a signal in the band edge.
Some commercial vendor comes with automated software to perform frequency flatness calibration (which they write the compensation table as per item 2).




Example: Keysight M8195A sampling rate at 65 GSa/s and its compensation for sinx/x roll off.