A Brief Overview of Novel Approaches in Designing High Performance VCOs <s> B. A novel design combined with with class-B VCO <s> A harmonic oscillator topology displaying an improved phase noise performance is introduced in this paper. Exploiting the advantages yielded by operating the core transistors in class-C, a theoretical 3.9 dB phase noise improvement compared to the standard differential-pair LC-tank oscillator is achieved for the same current consumption. Further benefits derive from the natural rejection of the tail bias current noise, and from the absence of parasitic nodes sensitive to stray capacitances. Closed-form phase-noise equations obtained from a rigorous time-variant circuit analysis are presented, as well as a time-variant study of the stability of the oscillation amplitude, resulting in simple guidelines for a reliable design. Furthermore, the analysis of phase noise is extended to encompass a general harmonic oscillator, showing that all phase noise relations previously obtained for specific LC oscillator topologies are special cases of a very general and remarkably simple result. <s> BIB001 </s> A Brief Overview of Novel Approaches in Designing High Performance VCOs <s> B. A novel design combined with with class-B VCO <s> In this letter, an adaptively biased class-C Voltage Controlled Oscillator (VCO) with a self-turn-off auxiliary class-B pair is presented for fast and robust startup in a 65-nm CMOS process. It consists of a main class-C core and an auxiliary class-B cross-coupled pair. The class-B auxiliary pair is used to solve the inherent startup problem of class-C VCOs. It works only during startup and turns itself off when the VCO reaches a steady state using an adaptive bias scheme. The dc current consumption of the VCO and bias circuit is 5 mA at 1.2 V ${\rm V}_{\rm DD}$ . Here, a good phase noise level of $-132.41~{\rm dBc}/{\rm Hz}$ at a 1 MHz offset frequency from a 2.46-GHz carrier is shown, to yield a Figure-of-Merit (FOM) of 192.45. <s> BIB002
In recent years many class-C VCOs have been proposed to have low power and improved phase noise VCO. The class-C VCO provides better phase noise performance than the other VCO structures. The mechanism for this structure has been investigated in BIB001 . The main reason for better noise performance of this class of VCO is due to the low voltage of the gate bias. As a result, a new class-C VCO has been proposed in BIB002 , which has an auxiliary class-B pair that will help to boost the startup of the VCO. The schematic of the proposed design is shown in Fig.8 . The proposed circuit in BIB002 uses an adaptive circuit which causes the auxiliary part (Class-B) turns off automatically when the output signal goes to the steady state. This adaptive circuit is shown in Fig.9 . Fig.9 . Adaptive biasing circuit. Source: Adapted from BIB002 , copyright 2016, IEEE.