Switched-Tank VCO Designs and Single Crystal Silicon Contour-Mode Disk Resonators for use in Multiband Radio Frequency Sources
To support the large growth in wireless devices, such as personal data assistants (PDAs), wireless local area network (WLAN) enabled laptop computers, and intelligent transportation systems (ITS), the FCC allocated three high-frequency bands for unlicensed operation. Of particular interest is the 5-6 GHz Unlicensed National Information Infrastructure (UNII) band intended to support high-speed WLAN applications. The UNII band is further split into three smaller 100 MHz sub-bands: 5.15 - 5.25 GHz; 5.25-5.35 GHz; and 5.725-5.825 GHz.
VCOs that can be switched between each of the three UNII sub-bands offer flexibility and optimum phase-locked loop (PLL) design versus non-switchable VCOs. This work presents switched-tank voltage controlled oscillators (VCOs) designed in MotorolaÃ Âs 0.18 Ã µm HIP6WRF BiCMOS process that could be used in multiband receivers covering the three UNII sub-bands. The first VCO was optimized for low power consumption. The VCO draws a total of 6.75 mA from a 1.8 V supply including buffer amplifiers. The VCO is designed with a switched-capacitor LC tank circuit that can switch to two center frequencies, 5.25 GHz and 5.775 GHz, with 200 MHz of varactor-supplied tuning range. The simulated output voltage swing is 2.0 V peak-to-peak and is kept constant between sub-bands by an active PMOS load integrated into the biasing circuitry. The second VCO was optimized for a high output voltage swing by replacing the current biasing circuit with a degenerating inductor. This design targeted three center frequencies, 5.2 GHz, 5.3 GHz, and 5.775 GHz, with 100 MHz of tuning range. This design has an output peak-to-peak voltage swing of 5.2 V but consumes an average of 16.5 mA from a 1.8 V supply. The two fabricated circuits exhibit tuning ranges similar to the simulated results; however, the center frequencies of each decrease due to interconnect parasitics there were unaccounted for in the designs. The measured center frequencies are 4.4 GHz and 5.37 GHz for the first design, and 4.4 GHz and 4.7 GHz for the second design (with one state inoperative due to a faulty switch).
The phase noise of the fabricated VCO designs was limited primarily by the low quality factor (Q-factor) of the on-chip LC tank circuits. Oscillators referenced with high-Q off-chip components such as quartz crystal references and surface acoustic wave (SAW) resonators in a PLL can exhibit much improved performance; however, these off-chip components add packaging/assembly cost and higher bill of materials, impedance matching issues, and parasitics that can significantly affect performance for RF applications. Thus, there is tremendous incentive for integrating high-Q components on-chip with the eventual goal of consolidating all of the RF/analog/digital components onto a single wireless-enabled chip, commonly called RF system-on-a-chip (SoC).
Microelectromechanical (MEM) resonators have received significant attention based on their ability to provide high on-chip Q-factors at RF frequencies using fabrication techniques that are compatible with modern IC processes. MEM resonators transduce electrical signals into extremely low-loss mechanical vibration and vice versa. Consequently, this thesis also describes the modeling, simulation, and fabrication of contour-mode disk-shaped MEM resonators. This resonator geometry is capable of providing high-Q oscillation at frequencies exceeding 1 GHz at sizes easily within the limits of modern photolithography techniques. Finite element analysis is used to predict the frequency response of disk resonators under various operating conditions and to determine variables that are most critical to the resonator design. A silicon-on-insulator (SOI) fabrication process for constructing the disk is also discussed. Finally, the possible future integration of MEM resonators with multiband VCOs in a common IC process is proposed.