The growing demand for higher data rate in wireless transceivers continues to consume available bandwidth and move towards multi-band/multi-channel mm-wave systems to satisfy latest specifications and be compatible with legacy standards. One of the critical components, the LC-voltage controlled oscillator (LC-VCO), is required to have low phase noise while maintaining wide tuning range to achieve low bit error rates (BERs). Due to the large parasitic capacitance and high losses in silicon based technologies, extending the tuning range and reducing phase noise is becoming very challenging. This dissertation studies and proposes novel architectures and circuit techniques to overcome the limitations of high frequency LC-VCOs.
Different LC-VCO architectures are reviewed and the performance is summarized and compared. The components in LC-VCOs are theoretically analyzed and a prototype VCO is designed. An inductance redistribution technique to equally space sub-band coarse tuning characteristics is proposed and implemented. To predict the tuning range, a detailed analysis of the frequency dependent quality factor (Q) of the LC-tank is performed to characterize the tank loss. The relationship between the cross-coupled pair gm and the operating frequency is further derived.
To extend the tuning range, active negative capacitance (NC) circuits are investigated and integrated to bottom-biased and top-biased cross-coupled pair CMOS LC-VCOs in order to cancel the fixed parasitic capacitance in the LC-tank. Various NC circuits with low power consumption are designed and analyzed. A figure of merit (FOM) is proposed to compare the performance of different NC circuits. A power and area efficient NC scheme is then selected for mm-wave applications.
Applying the NC circuit to a bottom-biased mm-wave LC-VCO, the tuning range is increased with minimal impact on power consumption, silicon area or phase noise. The NC structure is further modified to be tunable based on switched varactors, enabling additional expansion of the bottom-biased VCO tuning range. By manipulating the quality factor (Q) of the NC tuning varactor pair, a prototype VCO achieves a maximum tuning range of 27% from 34.5 GHz to 45.4 GHz, while dissipating 13 mA from a 0.9 V supply.
A method of combining the tunable NC circuit and a top-biased VCO is also proposed and verified. A switched MOS transistor NC circuit is analyzed and demonstrated to have a wide tuning range. With this technology, the VCO achieves a maximum tuning range of 26% with worst-case phase noise of -100.1 dBc/Hz at 1 MHz offset.
For VCOs in SiGe BiCMOS technology, combining NMOS and BJT cross-coupled pairs to reduce the sizes of transistors and current consumption is proposed and analyzed. The tuning range is improved by adopting small MOS transistors while the reduced BJT current lowers phase noise. To verify the results, a 37.8 GHz VCO is designed and fabricated with a 30% tuning range and an average phase noise of -103.6 dBc/Hz at 1 MHz offset. The corresponding figure of merit with tuning (FOMT) is -193.5 dBc/Hz, which is the highest reported to date in Ka-band.