These systems use a software radio architecture. To minimize phase noise, we use direct digital synthesis techniques to generate fundamental signals in the 7 MHz - 42 MHz range. When converted to analog by the DAC, the unfiltered output contains aliases which are weaker, but otherwise perfect, replicas of the fundamental signals at frequencies appropriate for use as local oscillators.
Good linearity is achieved in the design of the frequency block up- and downconverters.
The resulting system displays no observable distortion when n-QAM signals are passed through it. I described above, except it is designed specifically for field use.