A key element of transceivers for wireless communications is the voltage-controlled oscillator (VCO) used as local oscillator. In CMOS technologies inductance-capacitance (LC) tank oscillators are preferred over ring oscillators due to better relative phase noise performance and the necessity of low power consumption (battery operated systems). CMOS technologies allow low-cost integration of digital and analog functions on the same die, but pose problems for the passive elements. To facilitate improved VCO designs an important task is to optimize the performance of inductors and varactors. Their quality factors need to be high, the capacitance tuning range of varactors must be increased as well as non-linearities reduced. This work proposes and analyzes several passive devices (varactors and inductors) for a fully integrated RF circuit in standard MOS technologies. Performance data is obtained by test structures, but also by application in VCOs (especially frequency tuning range and phase noise have been studied). All devices and circuits have been fabricated in standard, digital CMOS technologies (except one device in BiCMOS). Accumulation mode (A-Mode) varactors with STIs (shallow trench isolation, i.e. oxide) are introduced. STIs between the n+ source/drain regions (in n- well) and the gate reduce parasitic overlap and fringing capacitances between the gate and the n+ regions. This increases the capacitance tuning range by approx. 40%. In CMOS technology the quality factor is reduced compared to the conventional device, but to a large extent this reduction can be balanced by simple means, e.g. small, direct n+ connections between the n+ regions and the active area beneath the gate (at few points). Implemented in (otherwise identical) VCOs the A-mode varactors with STI allow a frequency tuning range of up to ±13% compared to ±7% with a conventional device. In each case the phase noise is low enough to fulfill UMTS Tx specifications. In a BiCMOS technology deep n+ sinker implants replace the shallow source/drain regions of the CMOS process. The sinkers feature large lateral outdiffusion underneath the STIs. Thus, the BiCMOS devices achieve the same high capacitance tuning range as the CMOS varactors with STI, but twice as high quality factors. In conventional MOS varactors the gate poly silicon and the source/drain regions have the same type of doping, due to a self-aligned process step. With STIs a non-zero distance between the gate and the source/drain regions gives the freedom to choose the doping of each gate-finger as n or p (always n+ source/drain). As the flatband voltage of a varactor with n-gate doping and p-gate doping differs by approx. 1 V, using both doping types within one varactor leads to a significantly smoother CV characteristic. Implemented in (otherwise identical) VCOs the np doped varactor results in a somewhat reduced frequency tuning range compared to the homogeneously n doped device. However, the np doping allows highly linear frequency tuning behaviour, up to 10dB reduction in phase noise and 25% lower maximum pushing (frequency sensitivity to supply voltage variations). A physical and scalable VHDL-AMS model of the proposed varactor for circuit simulations is developed. Simulation results of the varactors capacitance and resistance as well as results of a VCOs frequency tuning and phase noise are compared with measurement results. In each case good agreement is achieved. To evaluate the varactors potential for application in VCOs several parameters are introduced, that can easily be extracted from device measurements. The varactors influence on the 1/f2 and 1/f3 regions of VCO phase noise, the frequency tuning and pushing are captured accurately. The switched varactor concept is studied experimentally and observations are explained. Increased circuit complexity makes a large frequency tuning range, low pushing and a reduction in phase noise possible.    «

A key element of transceivers for wireless communications is the voltage-controlled oscillator (VCO) used as local oscillator. In CMOS technologies inductance-capacitance (LC) tank oscillators are preferred over ring oscillators due to better relative phase noise performance and the necessity of low power consumption (battery operated systems). CMOS technologies allow low-cost integration of digital and analog functions on the same die, but pose problems for the passive elements. To facilitate i...    »