I added a page describing the used measurement principle.
Hope this explains why it is possible to have only minimal HW and still very good performance.
Basically the performance is made possible by the extreme linearity of the PC audio signal path and a perfect DSP IQ mixer.
All IMD in the HW mixers is eliminated and the only requirement is to stay below if 1dB compression point
All harmonics from the SI5351 are also eliminated with the measurement approach.
Calibration will remove the imperfections of the Bridge
