12,269 research outputs found
A fundamental limit on the performance of geometrically-tuned planar resonators
No full textGeometric frequency tuning in planar electromagnetic resonators is common in many applications. It comes, however, at a penalty in the resonance quality, Q0. The literature traces the causes of such penalty often in terms of the shortcomings in the added elements and materials, which were used to achieve the tuning. In this paper, however, it is shown that another underlying source of quality degradation exists at the fundamental geometric level. This source, unlike other added sources of degradation during tuning, will always exist (even before tuning takes place) and will rely on the “modal areas” of the geometric modifications made to host the tuning mechanism. Hence, it forms an upper bound to the performance that can be achieved from a geometically-tuned planar resonator, carries an important insight to resonator design in general, and significantly helps in the understanding of the problem of geometric tuning in particular. We present the electromagnetic theory behind this limit and canonically demonstrate it using practical microwave resonator examples. The theory, finite-element method simulation, and experiment results are presented and good agreement is observed. It is shown that incorporating such understanding into the design process of tunable planar resonators can help optimize their performance against a given set of design requirements. Furthermore, the presented theory provides a useful electromagnetic model as a tool for estimating Q0 for geometrically modified or irregular metal patches and planar resonators in general, to assist analysis, and design at any wavelength or application. The theory also asserts that, under a given mode, a planar resonator will always have its maximum Q0 before introducing any internal subtractive geometric modifications (e.g., cuts, apertures, or slits) to its original shape.Geometric frequency tuning in planar electromagnetic resonators is common in many applications. It comes, however, at a penalty in the resonance quality, Q0. The literature traces the causes of such penalty often in terms of the shortcomings in the added elements and materials, which were used to achieve the tuning. In this paper, however, it is shown that another underlying source of quality degradation exists at the fundamental geometric level. This source, unlike other added sources of degradation during tuning, will always exist (even before tuning takes place) and will rely on the “modal areas” of the geometric modifications made to host the tuning mechanism. Hence, it forms an upper bound to the performance that can be achieved from a geometically-tuned planar resonator, carries an important insight to resonator design in general, and significantly helps in the understanding of the problem of geometric tuning in particular. We present the electromagnetic theory behind this limit and canonically demonstrate it using practical microwave resonator examples. The theory, finite-element method simulation, and experiment results are presented and good agreement is observed. It is shown that incorporating such understanding into the design process of tunable planar resonators can help optimize their performance against a given set of design requirements. Furthermore, the presented theory provides a useful electromagnetic model as a tool for estimating Q0 for geometrically modified or irregular metal patches and planar resonators in general, to assist analysis, and design at any wavelength or application. The theory also asserts that, under a given mode, a planar resonator will always have its maximum Q0 before introducing any internal subtractive geometric modifications (e.g., cuts, apertures, or slits) to its original shape
Wide tuning-range planar filters using lumped-distributed coupled resonators
No full textThis paper describes a discretely tunable filter topology based on lumped-distributed coupled transmission lines, particularly suitable for microelectromechanical systems switching devices. This topology is capable of simultaneous wide-band center frequency and bandwidth tuning, limited only by the electrical size of the transmission lines and the placement density of the switching devices. Low fractional bandwidths can be achieved without the need for large coupled-line spacings due to the antiphase relationship of the lumped capacitive and distributed electromagnetic coupling coefficients. The positions of the additional poles of attenuation due to the lumped capacitive coupling can be selected either above or below band leading to the choice of a narrow bandwidth design having good high-side performance or a design with compromised upper stopband performance, but with no bandwidth tuning limitations. The interaction between a pair of lumped-distributed coupled transmission lines is analyzed and the resulting model is used to develop a filter synthesis procedure. The synthesis procedure and filter performance are validated through theoretical and experimental comparisons using a filter with low-side attenuation poles.This paper describes a discretely tunable filter topology based on lumped-distributed coupled transmission lines, particularly suitable for microelectromechanical systems switching devices. This topology is capable of simultaneous wide-band center frequency and bandwidth tuning, limited only by the electrical size of the transmission lines and the placement density of the switching devices. Low fractional bandwidths can be achieved without the need for large coupled-line spacings due to the antiphase relationship of the lumped capacitive and distributed electromagnetic coupling coefficients. The positions of the additional poles of attenuation due to the lumped capacitive coupling can be selected either above or below band leading to the choice of a narrow bandwidth design having good high-side performance or a design with compromised upper stopband performance, but with no bandwidth tuning limitations. The interaction between a pair of lumped-distributed coupled transmission lines is analyzed and the resulting model is used to develop a filter synthesis procedure. The synthesis procedure and filter performance are validated through theoretical and experimental comparisons using a filter with low-side attenuation poles
Flexible linearity profile amplifier for software defined applications
No full textA highly linear Low Noise Amplifier (LNA) using a flexible feedforward linearisation scheme is described. The wideband LNA is designed for use in the frequency spectrum between 1 and 2GHz. It has a tuneable linearity profile which allows it to be adjusted for wideband moderate linearity or narrowband high linearity. The linearity can be tuned so a wideband Third Order Intercept Point (IP3) improvement of 4dB can be achieved from 1.3 to 1.7GHz or a narrowband (1MHz) 13dB improvement can be realised anywhere between 1.1 and 1.7GH
Flexible linearity profile low noise feedforward amplifiers for improving channel capacity
No full textThis paper describes a highly linear low noise amplifier (LNA) exhibiting a flexible linearity profile intended as a way to meet future flexible mobile communications standards. This has the potential of inclusion in ad-hoc software defined radio (SDR) networks where a frequency band and modulation scheme can be chosen based on the characteristics of the frequency spectrum and user requirements. The linearity profile of the LNA is tailored to cancel distortion generated as a function of the received power profile. This improves the receiver's tolerance to unwanted signals present in the frequency band of interest and reduces the requirement of coding and equalization to compensate for receiver limitations whilst increasing the available capacity. The flexible linearity profile is achieved by the use of a feedforward amplifier structure which has the capability of large distortion suppression determined by the gain and phase balances within its two loops. To ensure maximum distortion cancellation in the band, adaptive cancellation is used to maximize the cancellation of harmful distortio
TRUST approach to software defined radio: RF considerations
No full textThe design of the RF component of a Software Defined Radio (SDR) is a challenging task. This paper sets out to examine the important issues concerning the design of both the transmitter and the receiver. Techniques of achieving wideband linearity in the transmitter are reviewed. The importance of being able to design a low phase noise oscillator to operate over a wide range of frequencies is discussed. Receiver architecture is also reviewed. The major problems associated with the design of the receiver component of an SDR(linearity and image signals) are reviewed. The relationship of blocker specifications to receiver linearity is described. The paper concludes with a discussion of some problems unique to SDRs, and in particular those that need to be transparently reconfigure
Distortion mechanisms in varactor diode-tuned microwave filters
No full textThis paper examines the broadband distortion behavior in flexible filters employing varactor-diode tuning elements. Series- and parallel-resonant varactor-loaded transmission-lines, both commonly used in bandpass and bandstop microwave filters, are analyzed. Nonlinear Volterra-series analysis is employed to determine the second- and third-order distortion ratios dependent on the frequencies of the incident signals. It is shown that in a bandpass filter (employing parallel tuned resonators), maximum distortion occurs in the passband, while in a bandstop filter (employing series tuned resonators), minimum distortion occurs at the minimum-loss passband. The analysis is verified by practical measurement of filters employing the two modes of resonator
Characteristics of PA cost-effectiveness studies by the first author, setting, number of PAs & design.
No full textCharacteristics of PA cost-effectiveness studies by the first author, setting, number of PAs & design.</p
South African responses to Open Access publishing: a survey of the research community
No full textOpen access publishing offers wide benefits to the scholarly community and may also afford relief to financially embattled academic libraries. The progress of the open access model rests upon the acceptance and validation of open access journals and open archives or institutional repositories by the academic mainstream, particularly by publishing researchers. To what extent are the key actors in the South African research system aware of the advantages of open access? This article reports on the findings of a recent survey undertaken to assess the current awareness, concerns and depth of support for open access amongst local researchers, research managers and policy makers in South Africa. The study focuses on issues of quality, article or author charges and the established academic reward system. It concludes that within the prevailing framework, there is little prospect that academics would choose to publish within open access
journals. Recommendations for advocacy by the library community are proposed
A novel mixer linearisation technique using frequency retranslation
No full textThe inherent non-linearity of mixers in communication systems creates numerous undesired effects, which are particularly acute in broadband receiver design for software defined radio (SDR) applications. In this paper, previously investigated mixer linearisation techniques are summarised, and a new technique using frequency retranslation is presented. To the author’s knowledge, this linearisation technique is new. Two-tone-test results show up to 33dB reduction in the distortion products and a π/4-DQPSK modulated carrier yield 22dB suppression of adjacent channel interference (ACI
Author Co-Citation Analysis (ACA): a powerful tool for representing implicit knowledge of scholar knowledge workers
No full textIn the last decade, knowledge has emerged as one of the most important and valuable organizational assets. Gradually this importance caused to emergence of new discipline entitled ―knowledge management‖. However one of the major challenges of knowledge management is conversion implicit or tacit knowledge to explicit knowledge. Thus Making knowledge visible so that it can be better accessed, discussed, valued or generally managed is a long-standing objective in knowledge management. Accordingly in this paper author co- citation analysis (ACA) will be proposed as an efficient technique of knowledge visualization in academia (Scholar knowledge workers)
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