Coaxial 2D Bragg structures for microwave electronics

Peculiarities of mode spectrum in two-dimensional Bragg structures

Making use of field expansion and the equivalent boundary, an analytical model is set up to describe the multi-mode intercoupling in two-dimensional coaxial Bragg structures with either one or both of the conductors corrugated with helical ripples. The coupled-mode equations and the explicit formulae of the coupling coefficients for all possible mode combinations are derived, and the general mode coupling rules are discussed. Based on the analytical model, the dependence of the coupling coefficients on structural parameters of the two-dimensional coaxial Bragg structures with overmoded operation is investigated by numerical calculation. The results show that the additional conductor provides more flexibility for adjustment of coupling coefficients compared with other types of two-dimensional Bragg structures with a single conductor. Taking advantage of this feature, a two-dimensional coaxial Bragg structure can provide strong reflection with high mode purity under overmoded operation by suppressing the parasitic coupling using an optimized combination of corrugated outer and inner conductors. In this way, the structure has potential applications in the design of high-Q improved Bragg resonators for high-power microwave oscillators operating in the millimeter wave or terahertz range.

The generation of high power microwave radiation from a two-dimensional (2D) Bragg co-axial free electron maser (FEM) is reported. Two 2D Bragg structures positioned at the input and output of an oversized coaxial interaction region were used to define the cavity of the FEM. Measurements of the transmission coefficient of the 2D Bragg structures were conducted with experimental results compared with theoretical predictions. A 1 kA, 475 kV annular electron beam of a diameter of 7 cm was measured. The frequency of the microwave radiation generated, was measured to lie within the reflection zone of the 2D Bragg cavity.

By making use of the CST software, a comparative study is carried out for the effect of three ripple shapes (sinusoidal, rectangular and triangular ripples) on the frequency response in a coaxial Bragg structure. Results show that the rectangular ripple leads to the widest bandwidth while the triangular ripple narrows the bandwidth. Moreover, based on the mode-coupling model, numerical analysis is presented to demonstrate the influence of the ripple tapers. It is shown that the interval between the band gaps of the competing mode and the desired working mode is narrowed by the use of a positive taper, but is expanded if a negative taper is employed.

Band-gap overlap always occurs when an overmoded coaxial Bragg structure operates in the Terahertz frequency spectrum, which may cause serious competition of undesired modes with the operating mode at the operating frequency. In the present paper it is revealed that band-gap overlap of modes can be efficiently separated by setting the phase difference between the outer- and inner-conductor corrugations to be π. Moreover, the residual side-lobes interaction of the involved modes in a coaxial Bragg structure can be entirely eliminated by applying Hamming-window distribution to both the outer-conductor and inner-rod corrugations. These peculiarities provide potential advantage in constructing a coaxial Bragg cavity with high quality factor for single higher-order-mode operation of a high-power free-electron maser in the Terahertz frequency range.

A coaxial backward wave oscillator with a two-dimensional Bragg structure mounted at the exit from the interaction space is studied theoretically and experimentally. Azimuthal wave fluxes arising in this structure permit one to synchronize radiation of different fractions of a tubular relativistic electron beam with a large diameter. A narrowband radiation of the 8-mm range with a megawatt level of power has been obtained in the experimental prototype of the backward wave oscillator. The prototype was implemented on the basis of the Saturn accelerator (300 keV/200 A/1 μs), as the perimeter of the system constituted more than 15 wavelengths.

Investigators have repeatedly pointed out the importance of spectral information in the automated classification of white blood cells. In general, monochromatic images recorded through two or three color filters are used to extract this information. Although it has generally been thought that the use of narrow band filters provides "cleaner" color information than is obtainable through wide band filters, the choice has not been fully investigated and the question is far from being settled. The use of wide band filters has the clear practical advantage of increased light levels at the detector, resulting in higher signal-to-noise ratio with less demand on light source design. In order to investigate this issue, a series of 681 leukocytes of the most frequently occurring types were digitized by the use of both narrow (10 nm) and wide (90 nm) band filters. Parameters were extracted independently from both sets of images. These parameters were then used to develop a classifier for each set of images. The choice of features and classifier results indicate that there are no major performance differences between the two types of filters.

AlGaN/GaN high electron mobility transistors (HEMTs) have potential applications in the next generation high-power microwave and switching devices. They can maintain robust device characteristics under high temperature operation and high voltage conditions because of their superior material properties [1]. Sapphire and SiC are popularly used as substrate materials for GaN optoelectronic devices. Recently GaN on Si (111) technology has found applications in electronic devices because of its low cost and superior scalability of wafer size. In this study, 0.5µm gate length GaN on Si HEMT technology was proposed and demonstrated with field-plate technology. This GaN on Si HEMT process realized the high performance microwave and power electronics and circuits successfully.

5 - Couplers, hybrids and directional couplers

An Ultra-wide band filter is designed in this work. Microstrip lines along with passive components are used to create a wide band filter centered at 5GHz ranging between 3GHz - 9GHz. Shorted stubs are used to remove lower frequency. Detailed design procedure is given in paper.

The JANUS (Jovis, Amorum ac Natorum Undique Scrutator) will be the on board camera of the ESA JUICE satellite dedicated to the study of Jupiter and its moons, in particular Ganymede and Europa. This optical channel will provide surface maps with plate scale of 15 microrad/pixel with both narrow and broad band filters in the spectral range between 0.35 and 1.05 micrometers over a Field of View 1.72 × 1.29 degrees². The current optical design is based on TMA design, with on-axis pupil and off-axis field of view. The optical stop is located at the secondary mirror providing an effective collecting area of 7854 mm² (100 mm entrance pupil diameter) and allowing a simple internal baffling for first order straylight rejection. The nominal optical performances are almost limited by the diffraction and assure a nominal MTF better than 63% all over the whole Field of View. We describe here the optical design of the camera adopted as baseline together with the trade-off that has led us to this solution.

By making use of coupled-wave theory, the transmission characteristics of a defected coaxial Bragg structure which operates in the vicinity of 100 GHz are investigated. Results reveal that the overmoded operation of the example structure excites serious interaction among the operating mode and its neighboring modes, which spoils the passband structure and causes hybrid-mode transmission. However, single-mode output can be ensured by applying Hamming-window-function distribution to the ripple amplitude of the example structure, and one or more desired narrow passbands can be generated and controlled by adjusting the localized ripple defect or/and the initial phase difference between the outer-wall and the inner-rod ripples. These peculiarities provide potential applications of defected coaxial Bragg waveguides in constructing high-Q open resonators and narrow passband filters in the sub-terahertz and terahertz frequency ranges.

The next generation of power electro nics will use high temperature Silicon Carbide semiconductors. To utilize the capabilities of these devices, high temperature magnetics and other passive components are required. A 300C ferrite material has been developed to operate at over 300 degree C c ombined ambient and rise. A 1.5 kW, 270 Vdc to 28 Vdc forward converter transformer has been built with this material and has been tested for 600 hours at > 290C ambient. Both test data and analysis are presented to show the material capability and techniq ues for high temperature windings. The test data and analysis includes a discussion of various winding and high temperature insulation techniques consistent with high voltage and power electronics high frequency magnetics issues

In this paper, the spoof surface plasmon polaritons (SSPPs) transmission line (TL) of periodical grooved bow-tie cells is proposed. The complex propagation constant and characteristic impedance of the SSPPs TLs and microstrip lines (MLs) are extracted using the analytical method of generalized lossy TL theory. The properties of the SSPPs TLs with different substrates and the same geometrical configuration are experimented. Then, for comparison, two ML counterparts are also experimented, which shows that the SSPPs TL is less sensitive to the thickness, dielectric constant and loss tangent of the chosen substrate below the cutoff frequency, compared with the ML ones. The single-conductor co-planar quasi-symmetry unequal power divider based on this SSPPs TL is presented in microwave frequencies. For experimental validation, the 0-dB, 2-dB, and 5-dB power dividers are designed, fabricated, and measured. Both simulated and measured results verify that the unequal power divider is a flexible option, which offers massive advantages including single-conductor co-planar quasi-symmetry structures, wide-band operation, and convenient implementations of different power-dividing ratios. Hence, it can be expected that the proposed unequal power dividers will inspire further researches on SSPPs for future design of novel planar passive and active microwave components, circuits and systems.

RF signals are radio waves that are very crucial for telecommunications. Quality communications require reliable materials which are appropriately processed to house high-powered semiconductors such as GaN and GaAs. It is equally crucial to accommodate passive electronic components along with these semiconductors into the housings for the Next Generation RF package or GenPack™. Evolving semiconductor technologies demand fail-safe packages to protect the devices in all applications and environmental conditions. Future aircraft, EV cars, spacecraft, augmented and/ or virtual reality headsets, medical devices and many more technologies require operating at a higher power and frequency with a proper thermal and power management platform. The higher speeds of telecommunications hinge on the performance of semiconductors and their packages. Therefore, the GenPack™ and packaging process are becoming crucial pieces of the supply chain. Rugged GenPack™ RF packages differ in several ways compared to the traditional air cavity packages. For instance, FR-4 or Flame-Retardant glass-reinforced epoxy laminate material can be replaced with Aluminum Nitride or Alumina Oxide, Silicon Nitride or even Sapphire. The switch offers mechanical bonding directly onto the thermal spreader. Traditional circuits created on printed circuit boards have limited ability to dissipate heat due to the low thermal conductivity of FR4. Circuits created on ceramic can dissipate higher amounts of heat. As in the organic FR4 boards, multiple vias can be formed in the ceramic substrates to create metal pads top the ceramic that are grounded to the flange beneath the ceramic. A metal flange can be positioned beneath the ceramic to provide a heat spreader and electrical ground that can be bolted onto a heatsink. Flanges can be made from a diamond metal matrix composite instead of lower thermal conductivity materials to provide much more efficient thermal transfer. Furthermore, narrow leads offer optimized impedance at RF and microwave frequencies. A range of ceramic and flange materials can be selected to optimize GenPack™ RF Packages with cost, quality, and performance in considerations for the respective applications.