Design of low phase noise fully differential LC-VCO

Abstract

In this thesis, the new differentially tuned varactor structures are proposed to improve the previous differentially tuned topologies such as an anti-parallel configuration and a complementary structure using inversion-mode MOS varactor. And NMOS cross-LC-VCO with parallel PMOS transistor that is superior to the complementary cross-coupled LC-VCO for low voltage application is proposed and verified using TSMC 0.18㎛ CMOS process. The phase noise performance of a new differentially tuned LC-VCO using the modified anti-parallel structure is improved by 10 dB at 100 kHz offset frequency and 5 dB at 1 MHz offset frequency than that of LC-VCO with the previous anti-parallel because it has a higher Q-factor due to no connection of lossy bulk substrate . Also, CMRR which is a figure-of-merit to determine the common-mode rejection is improved by 2 dB. To improve the previous complementary structure using the inversion-mode n-type and p-type varactors that experience the different noise disturbance due to the separated body bias ($V_DD$ and ground), the complementary accumulation-mode MOS AMOS) varactor structure is proposed using a triple well process such as TSMC 0.18㎛ CMOS technology. Because of the deep n-well in a triple-well process, the p-type accumulation-mode varactor is possible to use. Although the phase noise of the proposed LC-VCO using the complementary AMOS varactor is not better than the anti-parallel structure because the gate length of the implemented p-type AMOS varactor is used at 0.5㎛ to match the C-V characteristic of n-type accumulation-mode varactor which is provided by TSMC PDK, it shows the possibility to use the differentially tuned varactor structure because CMRR is improved by about 2 dB. As the feature size of CMOS technology is scaled down, the power supply is also down to sub-1V. Although the complementary cross-coupled LC-VCO has the best performance because it has higher negative-gm value at same bias current due to PMOS cross-coupled p...