Browsing by Author "Zhao, Jun"
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- Development of Integrated "Chip-Scale" Active Antennas for Wireless ApplicationsZhao, Jun (Virginia Tech, 2002-07-26)With the rapid expansion of wireless communication services, ultra-miniature, low cost RF microsystems operating at higher carrier frequencies (e.g. 5-6 GHz) are in demand for various applications. Such applications include networked wireless sensor nodes and wireless local area data networks (WLANs). Integrated microstrip antennas coupled directly to the RF electronics, offer potential advantages of low cost, reduced parasitics, simplified assembly and design flexibility compared to systems based on discrete antennas. However, the size of such antennas is governed by physical laws, and cannot be arbitrarily reduced. The critical patch antenna dimension at resonance needs to be ~ λg/2 (where λg is the guided wavelength given by λg=λ₀/√(𝜖r) . Several methods are available to reduce the physical size of the antenna to enable on-chip integration. A high dielectric constant substrate reduces the guided wavelength. Grounding one edge of the microstrip patch enables the resonant antenna length to be further reduced to ~ λg/4. However, these techniques result in degraded antenna efficiency and bandwidth. Nonetheless, such antennas still have potential for use in low power/short range applications. In this work, "electrically small" (small with respect to λ₀) square-shaped microstrip patch antennas, grounded on one edge by shorting posts, have been investigated. The antenna input impedance depends on the feed position; by adjusting the feed point, the antenna can be tuned to match a 50 Ω or other system impedance. The antennas were designed on a GaAs substrate, with a high dielectric constant of 12.9. The size of the patch antenna is further reduced by utilizing shorted through substrate vias along one edge. The size of the antenna is about 4.2mm × 4.2mm, which is ~1/13 of λ₀ at ~5.6GHz. The antennas are practical for integration on chip. Due to the size reduction, the simulated peak gain of the antenna is only −10.2 dB (~3.2% radiation efficiency). However, this may be acceptable for short-range wireless communications and distributed sensor network applications. Based on the above approach, integrated GaAs "chip-scale" antennas with matching power amplifiers have been designed and fabricated. Class A tuned MESFET power amplifiers (PAs) were designed with outputs directly matched to the antenna feed point. The antenna is fabricated on the backside of the chip through backside patterning; the PA feeds the antenna through a backside via. The structure is then mounted such that the antenna faces up, and is compatible with flip-chip technology. The measurement of a 50 Ω passive (no PA) antenna indicates a gain of -12.7dB on boresight at 5.64 GHz, consistent with the antenna size reduction. The measurement of one active antenna (50 Ω system) shows a gain of -4.3dB on boresight at 5.80 GHz. The other version of active antenna (22.5 Ω system) shows a gain of −2.9 dBi on boresight at 5.725 GHz. The active circuitry (PA) contributes an average of ~9 dB gain in the active antenna, reasonable close to the designed PA gain of 12.7dB. The feasibility of direct integration of a PA with an on-chip antenna in a commercial GaAs process at RF frequencies was successfully demonstrated.
- Digital Eversion of a Hollow Structure: An Application in Virtual ColonographyZhao, Jun; Cao, Liji; Zhuang, Tiange; Wang, Ge (Hindawi, 2008-07-22)A new methodology is presented for digital eversion of a hollow structure. The digital eversion is advantageous for better visualization of a larger portion of the inner surface with preservation of geometric relationship and without time-consuming navigation. Together with other techniques, digital eversion may help improve screening, diagnosis, surgical planning, and medical education. Two eversion algorithms are proposed and evaluated in numerical simulation to demonstrate the feasibility of the approach.
- Silicon-Based RFIC Multi-band Transmitter Front Ends for Ultra-Wideband Communications and Sensor ApplicationsZhao, Jun (Virginia Tech, 2007-03-27)Fully integrated Ultra-Wideband (UWB) RFIC transmitters are designed in Si-based technologies for applications such as wireless communications or sensor networks. UWB technology offers many unique features such as broad bandwidth, low power, accurate position location capabilities, etc. This research focuses on the RFIC front-end hardware design issues for proposed UWB transmitters. Two different methods of multiband frequency generation ----- using switched capacitor VCO tanks and frequency mixing with single sideband mixers ----- are explored in great detail. To generate the required UWB signals, pulse generators are designed and integrated into the transmitter chips. The first prototype UWB transmitter is designed in Freescale Semiconductor 0.18μm SiGe BiCMOS technology for operation over three 500 MHz bands at center frequencies of 4.6/6.4/8.0 GHz, and generates pulses supporting differential BPSK modulation. The transmitter output frequency is controlled by a two-bit code which sets the state of a switched capacitor tank array for coarse tuning of the VCO. While selecting between the different bands, the transmitter is capable of settling and re-transmitting in less than 0.7μs using an integrated, wide band phase-locked loop (PLL). Various issues such as mismatch/inaccuracy of the pulses and high power consumption of the prescaler were identified during the first design and were addressed in subsequent design revisions. The pulse generator is a critical part of the proposed UWB transmitter. The initial pulse generator design used CMOS delay lines and logic gates to synthesize the required pulse bandwidth; however this approach suffered from inaccurate pulse timing control due to delay time sensitivity to device modelling and process variations. Subsequently, a novel pulse generator design capable of achieving accurate timing control was implemented using digital logic and a fixed oscillator frequency to provide timing information, integrated into a modified transmitter circuit, and subsequently fabricated in Jazz Semiconductor's 0.18μm CA18 RFCMOS process. Experimental results confirm the generation of accurate one-nanosecond pulses. Finally, a new multiband UWB transmitter based on a new single sideband (SSB) resistive mixer with superior linearity and zero static power consumption was also designed and fabricated using Jazz CA13 0.13μm RF CMOS process. This design is based on a fixed frequency phase-locked VCO and generates different bands through frequency mixing. In the prototype design, two additional carrier frequencies are generated from the VCO center frequency (5 GHz) by mixing it with its output divided-by-4 (1.25 GHz). By switching the relative I/Q phases of the LO/IF inputs to this single side band mixer, either the upper side band (6.25 GHz) or lower side band (3.75 GHz) frequency is selected at the mixer output, while the other sideband is rejected. Simulation results show that the transmitter is capable of generating the desired carrier frequencies while suppressing the image component by more than 40 dB. Overall, this work has explored various aspects of UWB transmitter design and implementations in fully integrated silicon chips. The major contributions of this work include: proposed hardware architectures for pulse-based multiband UWB transmitters; implemented a fully integrated multiband UWB transmitter with embedded phase-locked switched-tank VCO capable of wide frequency tuning; demonstrated an all digital pulse generator capable of generating accurate one-nanosecond pulse trains in the presence of various mismatches; and investigated resistive SSB mixer topologies and their implementation in a multiband UWB generation architecture.