http://www.researchonline.mq.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:23292 Based on a new variational method, an even- and odd-mode analysis of transverse coupling slot between waveguides is presented. The proposed method is capable of dealing with slots of finite wall thickness. It uses multiple incident waves with symmetry to simplify the field distribution in the vicinity of the slot, enabling the adoption of one-expansion-term trial functions with sufficient accuracy, even in the instance of wide slots. Analytical solutions are provided, and the calculated results demonstrate excellent agreement with those of numerical simulation. The computation time with the new formulation is, however, significantly shorter. 2012-12-10T01:51:10.045Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:7024 In implanted biomedical devices, due to the presence of surrounding dissipative biological tissue, the antenna suffers poor impedance matching. This causes degradation in the performance of a wideband or ultra-wideband (UWB) implanted device. Moreover, the electrical properties of tissue change from organ to organ, and possibly from time to time. In this paper, it is shown that loading of antennas with suitable insulators can deliver broadband matching across a range of dissipative medium properties. An impedance-matched UWB antenna designed to operate inside a lossy medium, which has varying electromagnetic properties within the range expected in biological tissues, is presented. The operating bandwidth of the proposed design is 3.5-4.5 GHz, which is an interference-free subset of the unlicensed UWB band in the US. It is demonstrated that once the dielectric loading is applied, the conventional procedure for antenna design in free space can be followed. The proposed implantable small capsule-shaped slot antenna has been characterized using numerical simulations. Details of a proof-of-concept experiment are presented. 2011-02-07T01:45:45.811Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:10843 Baseband impedance has been identified as having a positive or negative effect on the intermodulation distortion of microwave circuits. The effect can be assessed or utilized with the aid of previously proposed figures-of-merit. Under certain situations, intermodulation cancellation can be achieved simply by adding resistance to the bias network. The impact of baseband impedance on the performance of derivative superposition amplifiers is analyzed. A bias region was studied that exhibits a good second- and third-order intermodulation with minimal intermodulation dependence on baseband impedance. This allows the effective use of the derivative superposition technique in baseband amplifiers, as well as giving wide-band linearization performance in RF amplifiers. 2010-12-03T09:30:24.909Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:8830 Diode mismatch in an antiparallel-diode (APD) mixer results in an unwanted virtual local-oscillator (LO) leakage. At a radio system level, the virtual LO leakage is primarily a challenge of meeting spurious emission requirements. At a device level, it is a challenge of circuit yield, determined by the statistical variation in diode mismatch of a semiconductor fabrication process. This paper introduces methods of characterizing diode mismatch in APD mixers. The parameters of these characterization methods can be used to make an informed selection of the fabrication process, diode size, and LO pump power for reduced virtual LO leakage and improved yield. 2010-07-02T07:21:21.577Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:2091 This paper presents an efficient technique to evaluate the Green’s functions of single-layer and multilayer structures. Using the generalized pencil of function method, a Green’s function in the spectral domain is accurately approximated by a short series of exponentials, which represent images in spatial domain. New compact closed-form spatial-domain Green’s functions are found from these images using several semi-infinite integrals of Bessel functions. With the numerical integration of the Sommerfeld integrals avoided, this method has the advantages of speed and simplicity over numerical techniques, and it leads to closed-form expressions for the method-of-moments matrix coefficients. Numerical examples are given and compared with those from numerical integration. 2010-01-27T23:08:19.996Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:2100 The variation of high electron-mobility transistor (HEMT) large-signal behavior with a change in operating condition is examined with a view to understanding the dynamics involved and developing a modeling strategy. The observed variation exhibits the dynamics of thermal, impact ionization, and trapping effects. A novel measurement of drain characteristic transients gives time-evolution information that clearly shows these as separate quantifiable phenomena with significant dependence on initial operating conditions. A drain-current model that describes high-frequency characteristics with pinchoff, gain, and drain feedback parameters is adapted to describe the variation of the characteristics with changing operating conditions. The results reported give insight and grounding for simulation of HEMT circuits. 2010-01-27T23:08:13.374Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:2165 The intermodulation performance of an FET in the common-source configuration is dependent on the impedance presented to its gate and drain terminals, not only at fundamental, but also at harmonic and baseband frequencies. At baseband frequencies, these terminating impedances are usually determined by the bias networks, which may have varying impedance over the frequencies involved. This can give rise to asymmetry in two-tone intermodulation levels, and changing intermodulation levels with tone spacing, as previous studies have shown. In this paper, an FET is analyzed to gain an understanding, useful to the circuit designer, of the contributing mechanisms, and to enable the prediction of bias points and the design of networks that can minimize or maximize these effects. Compact formulas are given to facilitate this. An amplifier was tested, showing good agreement between the theoretical and measured results. 2010-01-27T23:07:30.229Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:2166 A novel measurement of the dynamics of high electron-mobility transistor (HEMT) and MESFET behavior permits classification of rate-dependence mechanisms and identification of operating regions that they affect. This reveals a simple structure to the otherwise complicated behavior that has concerned circuit designers. Heating, impact ionization, and trapping contribute to transient behavior through rate-dependence mechanisms. These are illustrated by a simple description. Each has an effect on specific regions of bias and operating frequency. With this insight, it is possible to determine true isodyamic characteristics of HEMTs and MESFETs and to predict operating conditions that will or will not be affected by rate dependence. It is interesting to note that, for some devices, rate dependence can be seen to exist at microwave frequencies and may, therefore, contribute to intermodulation distortion. 2010-01-27T23:07:30.052Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:2350 The temperature response of field-effect transistors to instantaneous power dissipation has been shown to be significant at high frequencies, even though the self-heating process has a very low time constant. This affects intermodulation at high frequencies, which is examined with the aid of a signal-flow description of the self-heating process. The impact on broad-band intermodulation is confirmed with measurements over a range of biases. Intermodulation measurements are then used to obtain parameters that describe the heating response in the frequency domain. This description is then implemented in a time-domain model suitable for transient analysis and compared with measured heating and cooling step responses. 2010-01-27T23:05:32.400Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:2632 Advances in microwave instrumentation now make it feasible to accurately measure not only the magnitude spectrum, but also the phase spectrum of wide-bandwidth signals. In a practical measurement, the spectrum is measured over a finite window of time. The phase spectrum is related to the position of this window, causing the spectrum to differ between measurements of an identical waveform. It is difficult to compare multiple measurements with different window positions or to incorporate them into a model. Several methods have been proposed for determining the phase spectrum such that multiple measurements can be effectively compared and utilized in models. The methods are reviewed in terms of the information required to determine the phase and compared in terms of their robustness in the presence of measurement noise. 2010-01-27T23:02:17.649Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:2910 The uncertainty, due to random noise, of the measurements made with a mixer-based nonlinear vector network analyzer are analyzed. An approximate covariance matrix corresponding to the measurements is derived that can be used for fitting models and maximizing the dynamic range in the measurement setup. The validity of the approximation is verified with measurements. 2010-01-27T22:59:13.118Z ]]>