SiGe based multiple-phase VCO operating for mm-wave frequencies

Abstract

The ever-increasing demand for higher speed in wireless consumer applications has increased the interest in the unlicensed spectrum of 7 GHz around 60 GHz. The high atmospheric oxygen absorption at 60 GHz and small size of the antennas at this frequency requires the use of integrated phased-array systems to overcome the deficiencies of lossy channels at these frequencies. The phased arrays combine signals from multiple paths to obtain higher receiver sensitivity and directivity. The system thus requires phase-shifted voltage-controlled oscillator (VCO) signals to implement phase shifting in the local-oscillator (LO) path. In this research, the vector sum method to generate various phases of the signal at 60 GHz was investigated for its suitability in phased-array systems. The main focus was on improving the phase noise performance of the VCO. The VCO was implemented using a fully differential common-collector Colpitts oscillator in the cascode configuration, which was found to be the VCO configuration with acceptable phase noise performance and stability in the millimetre-wave range. The research focus was on modelling the phase noise of the VCO, and was performed by identifying the impulse sensitivity function for various noise sources, followed by analysing its effect on the linear time varying (LTV) model of the oscillators. The analysis led to a closed-form expression for the phase noise of the oscillator in terms of process and design parameters. The design was then optimised in terms of identified parameters to attain minimum phase noise. The phase noise expression using LTV theory and SpectreRF simulations reported the same optimum value for the design parameter, of around 0.3 for the capacitor ratio. The simulation results utilising the vector sum phase shifting method to generate multiple phase oscillator signals suggest its suitability in implementing phased-array systems in the millimetre-wave range. The vector sum was realised by generating quadrature signals from the oscillator using hybrid couplers. Variable gain amplifiers (VGAs) based on Gilbert mixer topology were used to combine the in-phase and quadrature phase signals to generate the phase-shifted oscillator signal. The gains of the VGAs were linearised by using a pre-distortion circuit, which was an inverse tanh cell. A fully differential 60 GHz VCO was fabricated using a SiGe process with a fT of 200 GHz. The fabricated integrated circuit (IC) measured at the wafer level had a centre frequency of 52.8 GHz and a tuning range of 7 GHz. It demonstrated a phase noise performance of -98.9 dBc/Hz at 1 MHz offset and a power dissipation of 140 mW, thus providing a VCO figure of merit of 172 dBc/Hz. It delivered a differential output power of 8 dBm and the IC occupied an area of 0.54 mm2, including the bondpads. It was thus concluded that a 10 % design margin for the tuning range is required while using SiGe BiCMOS technology. The simulation results demonstrate that the VCO, along with an active interpolator, provides a range of phase-shifted signals from 0° to 360° in steps of 22.5° for various gain settings of the VGAs. The power dissipation of the active interpolator is around 60 mW and the system could thus be employed in LO path shifting architecture of the phased arrays with increased power consumption.