ABSTRACT
This chapter describes how an inverter-based open-loop four-/eight-phase local oscilator (LO) generator can be utilized to cover a wide spectrum and relax the speed requirement of the phase-locked loop (PLL) and voltage-controlled oscillator (VCO), when comparing with the conventional frequency-divider-based method. Many different types of multiphase LO/clock generators have been reported for wireless and wireline applications. Alternatively, a multiphase LO signals can be generated in an open-loop way by using a low noise LC VCO followed by a frequency divider. The block diagram of a four-phase LO generator is depicted. Although the phase noise can be substantially reduced by replacing all inverters with LC VCOs, the associated inductors occupy a significant part of the chip area. Moreover, the PLL and VCO will be more sensitive to parasitic capacitances, implying narrower locking and tuning ranges, respectively. Multiphase LO generators are critical blocks in modern wireless/wireline communication systems. Elementarily, a four-phase LO can be generated via a div-by-2 circuit.
