Phase-Conjugated Arrays Using Low Conversion-Loss Resistive Phase-Conjugating Mixers and Stub-Loaded Patch Antennas

A 1-D four-element and 2-D eight-element phase-conjugated retrodirective arrays employing low conversion-loss resistive field-effect transistor mixers and stub-loaded patch elements are proposed. The mixer, systematically designed using a multiharmonic loading technique, offers a low 5.2-dB conversion loss at a 4.5-dBm local oscillator power level. The stub-loaded patch element is a square patch with a capacitive open stub loaded at each of its four corners, leading to patch-size reduction, lower mutual coupling between adjacent elements, and broader beamwidth. With the same element spacing of half a free-space wavelength at 5.8 GHz, the mutual coupling is suppressed by 3.1 and 3.3 dB in the $E$- and $H$-planes, respectively, when compared to the conventional square patches. Within an angle of ${pm}{hbox{60}}^{circ}$ normal to the arrays, distinct linearly polarized retrodirectivities in terms of pattern deformations are experimentally confirmed for both the $H$-plane of the 1-D array and both $E$ - and $H$-planes for the 2-D array. Additionally, circularly polarized retrodirectivity are also experimentally confirmed for 2-D arrays.      

Highly Efficient Grouping Strategy for the Analysis of Two-Port Arbitrarily Shaped $H$-Plane Waveguide Devices

A new grouping strategy for the analysis of two-port arbitrarily shaped $H$-plane devices in rectangular waveguides is presented. The new grouping strategy accelerates a previous method based on the method of moments and the Green's function of an infinite line source placed inside two parallel plates. The computational cost of analyzing several $H$ -plane filters of different geometries is reduced by about 50%, while maximum accuracy is maintained, as a result of using the new grouping strategy.      

Very Compact Stacked LC Resonator-Based Bandpass Filters With a Novel Approach to Tune the Transmission Zeros

This letter presents very compact stacked LC resonator structures and their coupling and excitation techniques for designing the bandpass filters operating in the 3.5 GHz WiMAX band. Using a coupled stacked LC-resonator structure not only greatly reduces the overall size but also can create multiple transmission zeros to enhance roll-off rate or desired stopband rejection for the designed bandpass filters. Furthermore, this letter develops a simple technique of etching a rectangular C-shaped slot in the bottom ground plane to flexibly adjust the transmission-zero frequencies without changing the size and shape of the filter.      

Coplanar Waveguide Filters Based on Multibehavior Etched-Ground Stubs

In this paper, coplanar waveguide filters with etched-ground stubs that have multiple behaviors are proposed. The response of a unit cell is a bandreject with one or more transmission zeros according to the number and the lengths of the stubs. A library of transmission line models is being built for the basic cells and then used for the design of more elaborated structures. The potential of these new topologies is highlighted by providing different filtering structures with interesting features: a second-order narrow bandpass filter, and two configurations for a lowpass filter with a wide stopband. All theoretical predictions have been verified by electromagnetic simulation and measurements.      

Substrate Integrated Waveguide Filter With Improved Stopband Performance for Satellite Ground Terminal

In this paper, patent pending substrate integrated waveguide (SIW) bandpass filters with moderate fractional bandwidth and improved stopband performance are proposed and demonstrated for a $Ka$ -band satellite ground terminal. Nonphysical cross-coupling provided by higher order modes in the oversized SIW cavities is used to generate the finite transmission zeros far away from the passband for improved stopband performance. Different input/output topologies of the filter are discussed for wide stopband applications. Design considerations including the design approach, filter configuration, and tolerance analysis are addressed. Two fourth-order filters with a passband of 19.2–21.2 GHz are fabricated on a single-layer Rogers RT/Duroid 6002 substrate using linear arrays of metallized via-holes by a standard printed circuit board process. Measured results of the two filters agree very well with simulated results, showing the in-band insertion loss is 0.9 dB or better, and the stopband attenuation in the frequency band of 29.5–30 GHz is better than 50 dB. Measurements over a temperature range of $-{hbox{20}} ^{circ}$C to $+{hbox{40}} ^{circ}$C show the passband remains almost unchanged.      

New Miniaturized Dual-Mode Dual-Band Ring Resonator Bandpass Filter With Microwave -Sections

A new miniaturized dual-mode dual-band ring resonator bandpass filter is implemented by a cascade of several microwave $C$-sections. Each $C$-section is used to substitute a transmission line section of designated electrical length. Through proper design of input/output coupling configuration, two transmission zeros can be created on both sides of each passband. Two circuits with four and six microwave $C$-sections are fabricated for confirmation. They occupy less than 30% of the area of a traditional ring resonator filter. Measured responses agree very well with the simulation.      

An Inline Coaxial Quasi-Elliptic Filter With Controllable Mixed Electric and Magnetic Coupling

Based on the concept of using controllable mixed electric and magnetic coupling to design planar filters in our earlier work, we propose a novel inline coaxial filter, in which the electric and magnetic coupling can be separately controlled. Due to this controllable mixed coupling, the filter can operate with a quasi-elliptic characteristic. By modifying resonator structure and inserting additional coupling components, the filter size is substantially reduced and the coupling becomes more easily controllable, thus being able to produce a highly selective filter with a compact size. In addition, asymmetric response can be easily achieved. For the $N$-order filter, a maximum of $N - 1$ finite transmission zeros can be realized.      

Inline TM/sub 110/-mode filters with high-design flexibility by utilizing bypass couplings of nonresonating TE/sub 10/01/ modes

This paper presents a new class of pseudoelliptic function filters that are based on compact inline TM/sub 110/-mode cavity configurations. No structural folding is required. The bypass couplings are implemented through the nonresonating TE/sub 10/01/ modes so that arbitrarily positioned transmission zeros can be implemented. Design guidelines to generate a given transmission zero on the desired side of the passband and how to control it are presented. To demonstrate its flexibility, the approach is illustrated at examples of four-pole inline filters providing Chebyshev, elliptic-function-type, and asymmetric characteristics. Performance comparisons with different numerical codes validate the designs. A fourth-order pseudoelliptic filter with four transmission zeros is then designed, constructed, and measured. Excellent agreement between simulated and experimental results verifies the approach.     

A Narrow Band Dual Mode HTS Patch Filter With High Power Capability

A novel structure is presented to optimize the design of narrow band dual mode patch filters. Using this method, a narrow band dual mode high power HTS filter with center frequency 2.015 GHz, fractional bandwidth less than 0.35%, return loss greater than 20 dB, near band-edge rejection close to ${-}45$ dB, and insertion loss less than 0.2 dB has been designed and fabricated. The filter can work at a power of 11.7 W without obvious deterioration of its transmission response.      

Two-Dimensional Subwavelength Focusing Using a Slotted Meta-Screen

A transmission screen with subwavelength slots is described which can focus electromagnetic radiation to a 2-D subwavelength spot. Unlike negative-refractive-index focusing implementations, this “meta-screen” does not suffer from image degradation when losses are introduced and is easily scalable from microwave to Terahertz frequencies and beyond. The slotted geometry is designed using a theory of shifted beams to determine the necessary weighting factors for each slot element, which are then converted to appropriate slot dimensions. The screen was designed using a pair of rotated orthogonal focusing axes to produce a spot on a focal plane located $0.25 lambda$ away. Full-wave simulations are corroborated by experimental measurements at 10 GHz reporting a full-width half-maximum beam width of $0.27 lambda$ along the $x$-axis and $0.38 lambda$ along the $z$-axis.      

On Parallelepiped-Shaped Passbands for Multidimensional Nonseparable ${bf M}$th-Band Low-Pass Filters

The “shape” of the desired frequency passband is an important consideration in the design of nonseparable multidimensional ($M$ -D) filters in $M$-D multirate systems. For $M$-D ${bf M}$th-band filters, the passband shape should be chosen such that the ${bf M}$th-band constraint is satisfied. The most commonly used shape of the passband for $M$-D ${bf M}$ th-band low-pass filters is the so-called symmetric parallelepiped (SPD) ${rm SPD}(pi {bf M}^{- {rm T}})$ . In this paper, we consider the more general parallelepiped passband ${rm SPD}(pi {bf L} ^{rm T})$, and derive conditions on $ {bf L} $ such that the ${bf M}$ th-band constraint is satisfied. This result gives some flexibility in designing $M$-D ${bf M}$th-band filters with parallelepiped shapes other than the commonly used case of $ {bf L} = {bf M}^{- 1}$. We present design examples of 2-D ${bf M}$th-band filters to illustrate this flexibility in the choice of $ {bf L} $.      

Matrix Methods for the Dynamic Range Optimization of Continuous-Time $G_{m}$- $C$ Filters

This paper presents a synthesis procedure for the optimization of the dynamic range of continuous-time fully differential $G_{m}$-$C$ filters. Such procedure builds up on a general extended state-space system representation which provides simple matrix algebra mechanisms to evaluate the noise and distortion performances of filters, as well as, the effect of amplitude and impedance scaling operations. Using these methods, an analytical technique for the dynamic range optimization of weakly nonlinear $G_{m}$- $C$ filters under power dissipation constraints is presented. The procedure is first explained for general filter structures and then illustrated with a simple biquadratic section.      

Dual-Mode Dual-Band Filter With Band Notch Structures

A novel approach for designing dual-mode dual-band bandpass filters is proposed and experimentally studied. By embedding a pair of slits in the square patch, the dual-band response is realized and one transmission zero is introduced between the two passbands. Two degenerate modes are achieved by loading with a small square perturbation, which can be placed at the diagonal line. There are three transmission zeros. The slits control the first and third transmission zeros. On the other hand, the right crossed slots control the first and the second transmission zeros.      

Novel Substrate Integrated Waveguide Filters With Mixed Cross Coupling (MCC)

A new type of substrate integrated waveguide filter with mixed cross coupling (MCC) is proposed, with its mechanism of generating extra transmission zeros using MCC investigated. Less mixed coupling is needed to obtain the same number of transmission zeros in comparison with inline filters, with good agreements obtained between the measured and the simulated S-parameters of the developed filters.      

Limitations of Voltage-Oriented PI Current Control of Grid-Connected PWM Rectifiers With $LCL$ Filters

Voltage-oriented PI control of three-phase grid-connected pulsewidth-modulation rectifiers with $LCL$ filters is addressed. $LCL$ filters require resonance damping. Active resonance damping is state of the art to face the problem, but it is still under investigation because of the manifold solutions. It is often realized using many sensors and/or complex control algorithms. In contrast, pure PI control requires only one set of current sensors, and its implementation and design are rather simple and well known from the $L$ filter control. PI control has already been shown to be a suitable solution also for $LCL$ filters, but there are limitations. These are investigated in this paper. System stability is analyzed with respect to different ratios of $LCL$ filter resonance and control frequencies. The latter are important parameters for system design and control. Both line and converter current control are analyzed. For a certain range of frequency ratios, the voltage-oriented PI control gives stable performance without additional feedback, but for ratios outside this range, stable operation is impossible. Experimental tests validate the theoretical results. In addition, an experimentally determined $LCL$ filter transfer function is shown in this paper, which shows a lower resonance peak as expected from commonly used filter models.      

General Extracted Pole Synthesis Technique with Applications to Low-Loss TE/sub011/ Mode Filters

A novel synthesis technique is developed for two-port networks which possess finite real frequency transmission zeros. The low-pass prototype is synthesized in the form of a network with complex conjugate symmetry where the real frequency transmission zeros are extracted from both ends and realized by simple resonators separated by phase shifters. The remaining transmission zeros are realized by the central part of the filter in the form of a cross-coupled double array. This prototype is particularly suitable for designing waveguides bandpass filters and each real frequency transmission zero is independently tunable. Furthermore, in the case of the most complex transfer function with all possible types of transmission zeros, the realization requires only one type of coupling which is necessary in the important case of TE/sub011/ cylindrical mode cavity resonators. The general synthesis technique is given and the process illustrated by a nontrival example. Additionally, from the results of a computer program based upon the synthesis techniques, the important differences between the possible prototype forms for the same transfer function resulting from extracting the transmission zeros in different orders are cited.     

Fiber Ring Laser Operated by Dynamic Local Phase Shifting of a Chirped Grating

An ultranarrow linewidth erbium-doped fiber ring laser is presented. It is based on the filtering properties of a phase-shifted chirped fiber Bragg grating, which is inserted inside the cavity of the laser. A dynamic control of the phase shifting, which is induced by a magnetostrictive transducer, permits both tunable continuous-wave and actively $Q$-switched operation. The use of a chirped grating overcomes the limitations imposed by the narrow spectra of uniform gratings observed in previously reported ring lasers based on intracavity transmission filters.      

Nonpolar AlGaN/GaN Metal–Insulator–Semiconductor Heterojunction Field-Effect Transistors With a Normally Off Operation

Nonpolar AlGaN/GaN metal–insulator–semiconductor-heterojunction field-effect transistors (MIS-HFETs) with a complete normally off operation have been demonstrated for the first time. To realize the normally off operation of the MIS-HFETs, a 2-nm-thick SiN (as an insulator) and GaN-based nonpolar epitaxial layers that are free from polarization charges are employed. We have found that a thicker nonpolar AlN buffer layer achieves a GaN layer with a narrower full width at half maximum of the X-ray rocking curve and higher electron mobility. The fabricated MIS gate structure on the AlN buffer layer successfully decreases the gate leakage current and enables a more positive gate bias of up to $+$4 V, which is advantageous to achieve a higher drain current. Moreover, the n-GaN capping layer between gate and ohmic electrodes helps to reduce parasitic resistance and suppress current collapse. The fabricated $a$-plane MIS-HFET exhibits a threshold voltage of $+$ 1.3 V with a high drain current of 112 mA/mm. The presented MIS-HFETs will be desirable in next-generation radio-frequency and power switching application fields.      

Novel half-mode substrate integrated waveguide bandpass filters using semi-hexagonal resonators

In this paper, a novel bandpass filter is presented, designed, and implemented by using a triple mode semi-hexagonal Half-Mode Substrate Integrated Waveguide (HMSIW) cavity, which has the advantages of compact size, low insertion loss, and high selectivity in upper and lower passband. To realize a triple mode filter, the first resonant mode is shifted to near the next two modes using a via hole perturbation. Two microstrip open stubs connected to open edge of HMSIW resonator are introduced to generate two transmission zeros in the lower passband. The position of transmission zeros could be controlled by adjusting the coupling gap between the microstrip open stub resonators. By etching an E-shape slot on the top plate of HMSIW resonator, two other transmission zeros are produced in the upper passband. A wide-band planar six-pole bandpass filter, which has the advantages of wide bandwidth and small size, is also proposed and fabricated by cascading two triple mode resonators. Measured results indicate that 29% and 47% fractional bandwidth, as well as approximately 1 dB and 1.1 dB insertion loss, are achieved for the proposed filters. Measured results of all those filters agree well with the simulated results.     

Design of generalized Chebyshev filters with asymmetrically located transmission zeros

A simple design technique is introduced for generalized Chebyshev filters with asymmetrically located transmission zeros. The efficient and exact procedure is given for the calculation of transmission zeros for a class of generalized Chebyshev low-pass prototype filters with a maximum of four asymmetrically located transmission zeros of any multiplicity. This technique is based on obtaining the frequencies of magnitude characteristic extreme values in closed form, and after this, transmission zeros can be calculated by solving a system of nonlinear equations. Novel formulas for orders of zeros of maximally selective filters with equiripple characteristics in the passband and stopband are deduced. The obtained results are verified and illustrated by given examples.