A 2.1-to-2.8GHz all-digital frequency synthesizer with a time-windowed TDC

All-digital phase-locked loops (ADPLLs) offer the advantages of eliminating the large on-chip passive filter and not suffering from poor low-supply-voltage operation with process scaling [1,2]. However, there is a challenge in achieving low power consumption at the same time as providing the low phase noise required in modern wireless systems like WiFi and WiMAX that have higher-order modulations. Recently, efforts improve phase noise have been accomplished by increasing the time resolution of the time-to-digital converter (TDC) using a gated ring-oscillator structure by using a multipath ring oscillator [3], 2-step structures based on a vernier delay line [4] or a time-amplifier [5,6]. However, these structures require large power consumption because they require many continuously operating high-speed delay-stages.