Analysis And Design Of Dual-band VCO With Automatic Band Switching

Thanks to the excellent characteristics of wireless communication technology,it has been widely applied in various fields and plays an important role in modern society,and then brings about fast growing in communication requirements of multiband.Therefore,Multi-frequency RF system has become one of the research hotspots.The study of multi-frequency voltage controlled oscillator has raised concerns,which is a major element of transceivers.Based on a tunable dual-resonance LC tank and a complementary LC oscillator,a dual-band voltage controlled oscillator is proposed in the SMIC 0.18?m CMOS process,which switches between 2.4-GHz band and 5-GHz band with the variation of input RF signal frequency.Firstly,the oscillator performance parameters and basic principles are elaborated.Besides,detailed analysis are provided in these aspects:Q of an oscillator,noise shaping in oscillators,conversion of additive noise to phase noise,noise of bias current source,linear time invariant modeling of phase noise and linear time varying modeling of phase noise.And on this basis,phase noise optimization methods are given including incorporating PMOS transistors rather than NMOS transistors,removing tail current source,the use of the large-valued filter capacitors,second harmonic resonance filter and class-C oscillators.Secondly,the design idea of dual-band VCO with automatic band switching is demonstrated.It is divided into two parts:a band control circuit and a dual-band VCO.The band control circuit consists of a 2.4-GHz low noise amplifier,a 16-stage rectifier and a two-stage inverter.According to the post simulation results,the designed LNA has S21 of 14.61dB,S11 of-36.67dB,noise figure of 2.41dB at 2.4GHz,which plays a role in detecting and amplifying input RF signals.The band control voltage is 1.8V on 2.4-GHz band.While on 5-GHz band,it is OV.And then,the dual-band VCO design combines a tunable dual-resonance LC tank with a complementary LC oscillator and utilizes the key features of class-C oscillators to improve phase noise.In the end,the layout design of the whole circuit and the micrograph of the fabricated VCO are shown.The post simulation results show that when the input RF signal frequency is 2.4-GHz band,it has a frequency range of 2.399GHz to 2.665GHz,10.51%.While the input RF signal frequency is 5-GHz band,this circuit has a frequency range of 5.785G to 6.643GHz,13.81%.Phase noise of-124.33dBc/Hz and-120.41dBc/Hz at 1MHz offset is achieved at 2.4GHz and 5.8GHz respectively,neglecting the influence of band control circuit.