Thomas Wong

Construction and testing of an 11.4 GHz dielectric structure based traveling wave accelerator

We report on the design, numerical modeling, and experimental testing of a cylindrical dielectric loaded traveling wave structure for charged particle beam acceleration. This type of structure has similar accelerating properties to disk-loaded metal slow wave structures but with some distinct advantages in terms of simplicity of fabrication and suppression of parasitic wakefield effects. Efficient coupling of external rf power to the cylindrical dielectric waveguide is a technical challenge, particularly with structures of very high dielectric constant e. We have designed and constructed an X-band structure loaded with a permittivity e=20 dielectric to be powered by an external rf power source. We have attained high efficiency broadband rf coupling by using a combination of a tapered dielectric end section and a carefully adjusted coupling slot. Bench testing using a network analyzer has demonstrated a power coupling efficiency in excess of 95% with bandwidth of 30 MHz, thus providing a necessary basis fo...

Modeling and Analysis of SiC MOSFET Switching Oscillations

SiC MOSFETs exhibit extremely fast switching characteristics, which are unfortunately accompanied by undesirable switching oscillations. In this paper, equivalent circuit models incorporating all parasitic elements are developed for the turn-ON and turn-OFF of a SiC MOSFET. Simple mathematical formulas are derived to provide the theoretical analysis of the switching oscillation phenomenon, and to guide the snubber or damping circuit design. Both circuit simulation and experimental measurement are carried out to validate these simple equivalent circuit models.

A Q-band low phase noise monolithic AlGaN/GaN HEMT VCO

A Q-band 40-GHz GaN monolithic microwave integrated circuit voltage controlled oscillator (VCO) based on AlGaN/GaN high electron mobility transistor technology has been demonstrated. The GaN VCO delivered an output power of +25dBm with phase noise of -92dBc/Hz at 100-KHz offset, and -120dBc/Hz at 1-MHz offset. To the best of our knowledge, this represents the state-of-the-art for GaN VCOs in terms of frequency, output power, and phase noise performance. This work demonstrates the potential for the use of GaN technology for high frequency, high power, and low phase noise frequency sources for military and commercial applications

A reservation-based multiple access protocol with collision avoidance for wireless multihop ad hoc networks

A flexible and effective adaptive acquisition collision avoidance (AACA) multiple access protocol is proposed. It integrates the concept of multichannel and random reservation with piggyback to effectively solve hidden terminal and exposed terminal problems caused by the multihop architecture. In the protocol, every node adaptively reserves an idle traffic channel by request-to-send and clear-to-send (RTS/CTS) dialogue on the common channel. After successful reservation, the packet transmission of related other nodes do not interrupt other nodes. The protocol can use any number of channels. It performs better than the single channel RTS/CTS protocol under the assumption of the same total bandwidth if the number of the channels is not too large experiments.

Space-charge wave considerations in MIS waveguide analysis

A transport-based small-signal analysis of the fundamental mode of propagation in a metal-insulator-semiconductor (MIS) waveguide is presented. The formulation incorporates the full set of Maxwells equations and the equations of motion of the carriers based on a drift-diffusion model, providing a quantitative description of the space-charge wave induced of the surface of the semiconductor. Effects of an external DC bias on the propagation characteristics are also accounted for. Numerical solutions to the system of equations for a waveguide with typical material parameters and dimensions are obtained using an iterative algorithm. Results indicate that the transverse component of the electric field in the semiconductor is strongly influenced by the screening effect of the charge carriers, whereas the longitudinal component is governed mainly by energy dissipation arising from the conduction current. >

Multi-Nanosecond High Power Pulse Generation at 7.8 GHz With a Dielectric-Loaded Power Extractor

Power extraction from charged particle beams is a prospective way to develop future high power radio frequency (RF) sources. We have designed and tested a 7.8 GHz power extractor based on a dielectric-loaded waveguide. Building upon earlier work on single electron bunch tests, 10 ns and 22 ns megawatt-level RF pulses have been generated with trains consisting of 16 electron bunches each, by using a laser splitting-recombination scheme. In addition, 44 MW of peak power has been generated with a train consisting 4 electron bunches. Behaviors of higher-order-modes are also explored.

Coupled-wave small-signal transient analysis of GaAs distributed amplifier

The analysis takes into consideration the effect of c/sub dg/ in the active device, leading to a coupled-mode formulation. Dispersions within the transmission lines and the presence of two normal modes make it impractical to obtain broadband matching. Numerical results for specific terminations and various degrees of passive and active coupling clearly indicate the influence of c/sub dg/ and the necessity for coupled-mode analysis. The numerical scheme, which is based on Bromwich integration, can be incorporated into CAD routines for time-domain response optimization. >

Root nature of the transverse electric characteristic equation for a dissipative sphere

Natural frequencies of the transverse electric (TE) modes of a dissipative sphere with Debye-type permittivity are evaluated numerically for different relaxation times. Results provided strong evidence for the influence of dielectric dispersion and dissipation on the resonant behavior. It is show that the TE characteristic equations of the lossless spheres have only simple roots. Based on numerical studies on the dissipative sphere, it is deduced that if a higher-order root should occur, it is not an intrinsic feature of the problem, but merely a consequence of the choice of the numerical values for the physical parameters. From the continuity of the scattering coefficients on the branch cut associated with the square root of the complex permittivity, it is concluded that the branch cut does not contribute to the transient scattered field. It is shown that the branch points do not give rise to extra poles. >

An electrically small transverse bilateral helical antenna for portable personal wireless applications

Simulations comparing the TBH and normal mode helical antennas were conducted and shown to be in close agreement with measured data. Based on this comparison, the TBH antenna provides greater efficiency and an omni directional pattern that is maintained when placed next to the human hand. The TBH further exhibits reduced sensitivity to detuning. Agreement between measured and simulated data also indicates that the three layer hand model is a practical approximation for simulation of the human hand.

Performance analysis of UPMA protocol for wireless multihop mobile ad hoc networks

Based on the concept of contention reservation and polling transmission, user-dependent perfect-scheduling multiple access (UPMA) protocol for supporting node mobility and multihop architecture in wireless multihop mobile ad hoc network (WMMANET) is described. Based on clustering architecture acquired by self-organizing algorithm, it provides wireless access in a WMMANET with any kinds of topologies. On the other hand, the problem of intercluster traffic relay is sufficiently designed for nodes only having one set of half-duplex transceiver. In the protocol, channel utilization is improved greatly with the help of appropriate arrangement scheme between access period and polling period and any active nodes are guaranteed to access channel rapidly by means of an effective collision avoidance and resolution protocol. Moreover, we provide performance analysis on its throughput, delay and message dropping probability. Finally, the analytical results are shown to validate the effectiveness of the proposed protocol.