Digitally Assisted RF Architectures: Two Illustrative Designs
What exactly is “digitally assisted radio frequency (RF)”? The descriptor “digitally assisted” is the leading candidate in the race to capture, in a single word or phrase, the latest trend in RF, analog, and mixed-signal circuit design. Other candidates include “mostly digital,” “all-digital,” and most awkwardly, “analog/digital hybrid”. Boiled down to its essence, the trend is that the optimal partioning of functionality between the analog and digital domains in RF circuits is changing. It is changing because digital circuits in CMOS have become blindingly fast, vanishingly small, and laughably cheap. The amount of digital signal processing power now available in a square millimeter of silicon is nothing short of astounding. At the same time, modern MOSFETs as analog-processing elements have become more difficult to work with. They have low output resistance, tolerate only low supply voltages, and have lots of gate leakage. And then there’s the big one: They are so difficult to model, and are prone to so much variation, that even digital designers are complaining. The good news is that in RF transceivers, this state of affairs has created a tremendous opportunity for designers. For the first time in decades, RF and mixed-signal designers have both the need to be creative at the architectural level and the means ...