Angelos Alexanian

Multioctave spatial power combining in oversized coaxial waveguide

We describe a multioctave power-combiner structure using finline arrays in an oversized coaxial waveguide. The spectral-domain method (SDM) is used to compute the propagation constant in this structure, and is verified with HFSS simulations. The SDM method is then employed to synthesize broad-band tapered impedance transformers in finline for coupling energy to and from a set of monolithic microwave integrated circuit (MMIC) amplifiers. A modular assembly is described using a sectoral tray architecture. The concept is demonstrated for a 32-MMIC system using low-power traveling-wave amplifier MMICs, providing a 3-dB bandwidth of 13 GHz (3-16 GHz). An output combining loss of 1 dB is estimated from the small-signal measurements, suggestion a combining efficiency of /spl sim/75% for 32 MMICs.

Spectral transmittance of lossy printed resonant-grid terahertz bandpass filters

In this paper, we present terahertz bandpass filters composed of resonant arrays of crossed slots in lossy metal films deposited on dielectric membranes. The filters exhibit insertion loss as low as 1.9 dB at room temperature and 1.2 dB at 77 K at a center frequency of 2.2 THz. It is found that the dielectric substrate introduces a downward shift in frequency not predicted by standard mean dielectric-constant approximations. This shift is proportional to the permittivity and thickness of the substrate, and is accurately modeled for polyester, fused quartz and silicon substrates using a finite-difference time-domain (FDTD) model. It is also found that the insertion loss and Q-factors of the filters vary with the product of the thickness and conductivity of the metal film for lead and gold films, even in cases when the thickness is several skin depths at the center frequency. The FDTD theory presented here accounts for some of the conductor losses.

40-W CW broad-band spatial power combiner using dense finline arrays

This paper presents a broad-band spatial power-combining system based on tapered-slot antenna arrays integrated in a standard WR-90 waveguide environment. The system is designed using a modular tray architecture, providing full waveguide-band frequency coverage and an excellent thermal environment for a set of monolithic-microwave integrated-circuit (MMIC) amplifiers. The shape of the tapered-slot or finline structures was optimized to minimize return loss and provide a broad-band impedance transformation from the waveguide mode to the MMIC amplifiers. A prototype eight-element array using commercial GaAs MMIC power amplifiers yielded a maximum of 41 W output power (continuous wave) with a gain variation less than /spl plusmn/1.2 dB within the entire band of interest. The average combining efficiency over the operating band was estimated at 73%. The results suggest the efficacy of the design and a strong potential for higher powers by moving toward a greater number of MMICs per tray and a larger number of trays. Should the 100 W system be realized in the near future, our combiner system will become a promising candidate to challenge the dominant position currently claimed by the traveling-wave tube amplifiers.

Broad-band high-power amplifier using spatial power-combining technique

High power, broad bandwidth, high linearity, and low noise are among the most important features in amplifier design. The broad-band spatial power-combining technique addresses all these issues by combining the output power of a large quantity of microwave monolithic integrated circuit (MMIC) amplifiers in a broad-band coaxial waveguide environment, while maintaining good linearity and improving phase noise of the MMIC amplifiers. A coaxial waveguide was used as the host of the combining circuits for broader bandwidth and better uniformity by equally distributing the input power to each element. A new compact coaxial combiner with much smaller size is investigated. Broad-band slotline to microstrip-line transition is integrated for better compatibility with commercial MMIC amplifiers. Thermal simulations are performed and an improved thermal management scheme over previous designs is employed to improve the heat sinking in high-power application. A high-power amplifier using the compact combiner design is built and demonstrated to have a bandwidth from 6 to 17 GHz with 44-W maximum output power. Linearity measurement has shown a high third-order intercept point of 52 dBm. Analysis shows the amplifier has the ability to extend spurious-free dynamic range by N/sup 2/3/ times. The amplifier also has shown a residual phase floor close to -140 dBc at 10-kHz offset from the carrier with 5-6-dB reductions compared to a single MMIC amplifier it integrates.

Broadband waveguide-based spatial combiners

An array of tapered slotlines is inserted between rectangular waveguides. Results for a 2/spl times/4 active array at X-band are presented, indicating good combining efficiency and thermal properties as well as excellent bandwidth. To increase the device packing density and remove the lower cutoff frequency of the rectangular waveguide a coaxial combiner is also proposed. A radial arrangement of tapered slotlines is placed between two flared coaxial lines. 64 elements are combined with low combining loss over the 5 to 20 GHz band.

Broadband spatially combined amplifier array using tapered slot transitions in waveguide

Most reported spatially combined or quasioptical amplifier arrays exhibit resonant narrowband performance (<10%) and have not addressed thermal management issues. We report a waveguide-based spatial combining scheme using broadband tapered-slot transitions, capable of realizing full waveguide band coverage (40% fractional bandwidth) with good thermal properties. An X-band prototype using eight medium-power GaAs monolithic microwave integrated circuits (MMICs) produced an output power of 2.4 W and 9-dB power gain at 1-dB compression, with a combining efficiency of 68% and </spl plusmn/1-dB gain variation over the full waveguide band (8-12 GHz).

Three-dimensional FDTD analysis of quasi-optical arrays using Floquet boundary conditions and Berenger's PML

Infinite periodic grid structures excited by normally incident beams are analyzed using finite-difference time-domain (FDTD), with Berengers PML (perfectly matched layer) absorbing boundary condition used to terminate the computation domain along the beam axis. Floquet boundary conditions are used to handle arbitrarily shaped unit cells. Restriction to normal incidence permits using a Gaussian pulsed excitation to generate the wideband frequency response. The technique is used to model a previously reported multilayer quasioptical rotator array, with excellent agreement to the measurements obtained in the 26.5-40 GHz hand in a lens-focused test setup.

20 watt spatial power combiner in waveguide

In this paper, we present the continued effort in the development of broadband waveguide-based spatial combiners. A 20 W result at X-band is reported. The combiner was implemented by using eight commercial GaAs MMIC amplifiers in a rectangular waveguide environment. A new combiner design is proposed to alleviate, if not eliminate, various technical problems and further improve the power performance. Resulting simplification in analysis also enable us to better characterize the combining circuits. The preliminary result suggests promising outlook in performance improvement.

Quasi-optical traveling wave amplifiers

Previously reported quasi-optical amplifier arrays have limited bandwidth and suffer from poor input/output isolation. These problems can be solved by using traveling wave antennas and distributed amplifier techniques. FDTD simulations of a linearly tapered slot array topology demonstrate very broadband quasi-optical transitions are feasible with small unit-cell aperture. Experiments using a single-element low frequency prototype exhibit a 50% fractional bandwidth at 3.5 GHz using Vivaldi-type slots and a hybrid microstrip MESFET TWA circuit.

6 to 17 GHz broadband high power amplifier using spatial power combining technique

High power, broad bandwidth, high linearity and low noise are among the most important features in amplifier design. Broadband spatial power combining techniques address all these issues by combining the output power of a large quantity of microwave monolithic integrated circuit (MMIC) amplifiers in a broadband coaxial waveguide environment, while maintaining good linearity and improving phase noise of the MMIC amplifiers. A coaxial waveguide was used as the host of the combining circuits for broader bandwidth and better uniformity by equally distributing the input power to each element. A new compact coaxial combiner with much smaller size is investigated. Broadband slotline to microstrip line transitions are integrated for better compatibility with commercial MMIC amplifiers. Thermal simulations are performed and a new thermal management scheme is employed to improve the heat sinking in high power applications. A high power amplifier, using the compact combiner design, is built and demonstrated to have a bandwidth from 6 to 17 GHz with 44 W maximum output power. Linearity measurement has shown a high IP3 of 52 dBm.