Hello @ecekbb,
To prevent drift, the actual sample rate has to divide into the sample rate 1.25Gs/s. For example, f_micwave=500kHz * 201 pts = 100500000 pts/s and 1.25Gs/s / 100500000 = 12.438 so this setting would cause a bit of drift. You can adjust the settings of your AWG to accommodate this, or you can use the normal wave form generator with settings similar to this
I hope this was helpful and if you have any more questions please let me know!
(Forum post for reference: AWG frequency error/offset - #4 by nnosk55)