The Combine function of all the Siglent SDG devices
is analog - it is a very nice function that can replace an external power combiner in most applications. The
internal combiner has low port to port isolation though, thus generating some third order intermodulation distortions.
EDIT: It turns out that the channel combine function has to be a digital operation indeed.
However, this is not what I have been referring to. With the internal Combine function there is no way to precisely adjust the relative weighting of the channels. You could achieve this by using external attenuators and an external power combiner, but the comercially available components cost some money and accuracy would be limited.
So my suggestion is a voltage divider network, homemade with just a few metal film resistors, operating the SDG in High-Z mode and certainly good enough for frequencies up to a couple of MHz:
2-Way Combiner CD
V1 and V2 are the two output channels (V2 = coarse = 1/3, V1 = fine = 1/3000).
Be aware that R1, R2 just represent the output impedance of the AWG channels.
So the idea is simple: When set to High-Z reading, a V2 output set to 3V will provide 1V at the divider output. If V1 is set to 3V, it will result in a voltage rise by 1mV. So you can adjust the output voltage from 0 to 1V if you set V2 from 0 to 3V and fine adjust it by 0 to 1mV by setting V1 from 0 to 3V, see the graphs below:
2-Way Combiner V1
2-Way Combiner V2