Question: FAQ: Can AD8346 be operated at frequencies below 800 MHz?
Answer: For starters, there is only one good reason to try and use the AD8346 at frequencies below its specified 800 MHz minimum. And that reason is the fact that the device has relatively low supply current (45 mA) and operates from supply voltages as low as 2.7 V. One alternative idea would be to use AD8345 which is essentially a down-banded version of AD8346. It is pin-compatible with AD8346 and is specified to operate from 140 MHz to 1 GHz. It does consume more current than AD8346 (65 mA vs 45 mA) but also operates with supply voltages down to 2.7V.
If power consumption is not an issue, there are a number of newer devices, ADL5370, ADL5375, ADL5385 and ADL5386 which cover various portions of the sub-800 MHz band and some of which operate all the way down to 50 MHz.
But let's come back to the original question. The only real issue with operating the AD8346 below its specified minimum frequency of 800 MHz is that the sideband suppression will start to degrade. As you can see from the plot, Sideband Suppression appears to be tending upwards as you go below 900 MHz. But it's starting out from a very good place (-46 dBc at 900 MHz).
So you can operate the device below 800 MHz, especially if you have the capability to the skew the I and Q inputs from your DAC to compensate for the degrading sideband suppression. If you can live with sideband suppression in the mid-30s, I think that it will be fine out of the box (i.e. uncompensated). It is hard to say what the lowest frequency of operation really is as we have not characterized the device much below 800 MHz