There is a bit that can be set on the U3s. It is used to change antennas in my U3, 
https://la3za.blogspot.com/2017/02/comparing-two-antennas-with-wspr.html

If one output is attenuated, and the two outputs are then combined and fed into a power amplifier, one could regulate the output power in two steps. In theory this could be done even more accurately, as there are more than a single bit than can be set per frequency.

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Sverre

 

LA3ZA, http://la3za.blogspot.com