Why does increasing the channel count increase the spurious performance?

On behalf of Mike Jones:
Spurious performance can be improved with multiple channels by thinking about how one can calibrate multiple channels at a particular frequency in both phase and amplitude so that the desired signal sums constructively. However, if the noise and spurious components are uncorrelated, then certain spurious components can be instead forced to add destructively or not completely constructively, thereby improving the spurious-free dynamic range of the system. Certain methods, in which the LOs of many channels are offset, and numerically-controlled-oscillators are used to align the desired RF output, can be implemented to ensure that image frequencies and third-order harmonics do not fall on top of each other within the spectrum as well.