Yes you are right. The gain should be set once and should change rarely.
The I and Q will pass the AD9763 10bit-DACs, then Low Pass Filters (pass band 0-10MHz), then (possible) step attenuators and then AD8346 Quadrature Modulator (output 900-1500 MHz).
According to AD8346 modulator overview for differential I/Q inputs: the drive signal should be a 2 V p-p (differential) signal with a bias level of 1.2 V, that is, each input swings from 0.7 V to 1.7 V.
The maximum modulator output power is 0dBm. I need to control this in the range between -20dBm to 0 dBm.
As I know the AD9763 DAC has "GAINCTRL MODE" and "REFERENCE CONTROL AMPLIFIER" but I am not sure that I can use it to control the gain. May be I am wrong and the problem is much simpler then I think.
Please let me know what do you think.
Regards,
Olxx