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A novel method was proposed for the experimental investigation of wavefront distortion introduced to amplified radiation by pumped active elements in high-power laser amplifiers. The method is based on the simultaneous measurement of temperature distribution and the distribution of population density of the excited laser level in active elements. The underlying theory of the technique was presented; various factors affecting the accuracy of wavefront distortion determination were analyzed. The method was tested to study the wavefront distortion and the depolarization of radiation introduced by the Yb:YAG active element of a cryogenically cooled laser amplifier with high-power diode pumping. The focal length of the thermal lens was 0.40 ± 0.03 and 0.47 ± 0.05 m for the horizontal and vertical planes, respectively. The focal length of the electron lens was two orders of magnitude larger. The maximum value of losses induced by depolarization was 8.5%. [ABSTRACT FROM AUTHOR]

A novel method was proposed for the experimental investigation of wavefront distortion introduced to amplified radiation by pumped active elements in high-power laser amplifiers. The method is based on the simultaneous measurement of temperature distribution and the distribution of population density of the excited laser level in active elements. The underlying theory of the technique was presented; various factors affecting the accuracy of wavefront distortion determination were analyzed. The method was tested to study the wavefront distortion and the depolarization of radiation introduced by the Yb:YAG active element of a cryogenically cooled laser amplifier with high-power diode pumping. The focal length of the thermal lens was 0.40 ± 0.03 and 0.47 ± 0.05 m for the horizontal and vertical planes, respectively. The focal length of the electron lens was two orders of magnitude larger. The maximum value of losses induced by depolarization was 8.5%.

Dealing with high-power operation (i.e., >100 W) is extremely critical to power-amplifier designers due to the lack of accurate transistor models of multicell (i.e., powerbar) devices. The reason is twofold: from one side, it is extremely difficult to characterize high-power transistors (e.g., device instability, thermal issues, microwave instrumentation costs, and measurement uncertainty that drastically increases with the investigated power level); and from the other side, the scaling proprieties of the model, moving from the unit-cell device to the multicell one, are inherently poor due to the different passive access structures to the active-device area. In this article, for the first time, an accurate modeling technique oriented to multicell devices is described, which allows one to extract a compact model of a multicell transistor showing similar prediction accuracy of the unit-cell one. Our assumptions have been widely demonstrated in the manuscript by a comprehensive characterization campaign from small- to large-signal operations carried out on both the unit- and multicell devices. [ABSTRACT FROM AUTHOR]

There is increasing research interest in the development of solid-state devices to provide intelligent uniform heating, spot heating, and so on. However, the use of the solid-state devices to realize the same 1-kW output power as a conventional microwave oven requires configuration complexity that, for the moment, prevents its wider adoption. Although there is a demand for microwave generators with average output power of several tens to several hundreds of watts, existing solid-state solutions cannot produce this level of power. These problems prompted us to investigate, both by analysis and prototyping, small injection-locked oscillators that use high-efficiency microwave generators to provide several hundred watts of output power. Using an imbalanced coupling resonator in the feedback circuit, we were able to achieve an output power of 210 W and an efficiency of 51% at 2.45 GHz, when injecting a reference signal at a level of less than 1/10 000 of the output power. The high-power and high-efficiency solid-state injection-locked oscillators described in this paper have the advantages of long life, tunable frequency and phase, and low-voltage operation. These characteristics are potentially useful for realizing precise temperature control in chemical reactions, as well as spot and uniform heating by microwave ovens. [ABSTRACT FROM AUTHOR]

A high power gain magnetron amplifier with output phase control based on load–pull characterization has been developed. The equivalent resonant cavity method was first introduced to facilitate the proposed magnetron amplifier system as a basic injection model, and the system behavior was qualitatively evaluated based on Alder’s equation. It is the first time that a magnetron has been locked at a 58-dB gain. The magnetron amplifier shows an enhanced power-added efficiency of 72.0%, wide output phase control range, and 18.5-MHz bandwidth at a ratio of the output to the input power of 45 dB. This is the first time a successful magnetron output phase control based on load–pull at a power gain of 45 dB has been demonstrated. The magnetron output spectra were fully synchronized with a reference signal and the phase noise of the magnetron amplifier reached −106.3 dBc/Hz at an offset of 1 kHz. An output amplitude stability of 0.01 dB (peak to peak), a phase stability of 0.5° (peak to peak), and a high-efficiency dual 1-kW magnetron amplifier coherent-power combination based on the load–pull characterization was achieved. To the best of the authors’ knowledge, this is one of the highest power gains and phase noise performances ever reported for an S-band injection-locked magnetron with a power output of over 1 kW. [ABSTRACT FROM AUTHOR]

This paper presents the design and measurement results of a single-chip front-end monolithic microwave integrated circuit (MMIC), incorporating a high-power amplifier, transmit–receive switch, low-noise amplifier, and calibration coupler, realized in 0.25 \mu \textm AlGaN/GaN-on-SiC MMIC technology of UMS (GH25-10). The MMIC is operating in C-band (5.2–5.6 GHz) and is targeting the next generation spaceborne synthetic aperture radar. The use of GaN technology has resulted in a design that is robust against antenna load variation in transmit as well as against high received power levels, without the need for an additional limiter. By including a transmit-receive switch on the MMIC there is no need for an external circulator, resulting in a significant size and weight reduction of the transmit–receive module. The measured output power in transmit is higher than 40 W with 36% PAE. The receive gain is higher than 31 dB with better than 2.4 dB noise figure. To the best of the author’s knowledge this is the first time such performance has been demonstrated for a single-chip implementation of a C-band transmit–receive front-end. [ABSTRACT FROM PUBLISHER]

The long-term memory effects of gallium nitride (GaN) transistors have prevented its use in situations where the modulated envelope signal has a wide amplitude variation over time, such as in time division duplex systems. These long-term memory effects are generally attributed to electron trapping in GaN high electron-mobility transistors (HEMT), which have shown to be very difficult to compensate, especially in cellular base station transmitters known to be subjected to highly restrictive linearity specifications. On top of the electron trapping effects, we show that thermal effects can also induce long-term memory behaviors, which should also be accounted for when linearizing these devices. Because the conventional behavioral modeling approach has been incapable to compensate these long-term memory effects on GaN HEMT-based power amplifiers (PAs), we started by investigating the physical mechanisms responsible for these semiconductor impairments in GaN devices. This physics-based knowledge was then used to design new predistorter models that could effectively compensate those PAs subjected to GaN trapping and thermal effects. In this paper, we describe the new predistortion models for PA linearization, as well as the characterization methods used to determine their parameters. To validate the linearization effectiveness of the proposed model, several high power GaN-based PAs are tested with multicarrier GSM signals, and their linearization results are compared against other state-of-the-art models, evidencing a clear and significant improvement. In fact, to the authors’ knowledge, the proposed approach is the first one to reduce the PA distortion effects due to GaN long-term memory effects to such low levels, allowing a comfortable compliance with the imposed linearity masks. [ABSTRACT FROM AUTHOR]

In order to realize a high-power (100 s of kilowatts at the W-band) wideband microwave amplifier, we suggest to use a cascade of two gyrotron traveling-wave tubes (gyro-TWTs), one of which possesses a relatively high gain (40–50 dB) while the other provides a high output power at a moderate (15–20 dB) gain. Both gyro-TWTs are assumed to use helically corrugated waveguides and operate at the second cyclotron harmonic. A principal scheme of such an amplifier chain is discussed, including issues on the microwave input, output, and transmission between the tubes. Computer modeling of the beam-wave interaction using CST Particle Studio PIC Solver shows the possibility of achieving of 200–370-kW output power with 8–10-GHz bandwidth when driving by a 50-mW input source at the W-band. [ABSTRACT FROM AUTHOR]

In this paper, we present a design of a broadband powerful $W$ -band gyroamplifier with an interaction circuit in the form of a helically corrugated waveguide operating at the third cyclotron harmonic. An analysis of stability of the gyrotron traveling wave tube (gyro-TWT) with respect to parasitic wave-beam interactions at the operating and nonoperating cyclotron harmonics has been performed, and the range of parameters at which the gyro-TWT is zero-drive stable is found. Moreover, a comparison with a traditional smooth-waveguide gyro-TWT revealed that the helical-waveguide gyro-TWT possess superior stability characteristics at higher cyclotron harmonics. For a set of parameters attractive for possible applications, the 3-D particle-in-cell simulations were performed, according to which the proposed gyro-TWT yielded a 6% amplification bandwidth with the maximum output power of 80 kW and gain of 25 dB for a 10-A, 70-kV electron beam. [ABSTRACT FROM PUBLISHER]

The results of experimental investigation of two Ka-band gyrotron traveling-wave tube (gyro-TWT) amplifiers with helically corrugated waveguides are presented. The first tube produces pulsed output power of 130–160 kW within the frequency range of 33.1–35.5 GHz and is capable of operating with a 10% duty factor. Reliability of its major components in the high average power operation regime (about 10 kW) was proven in a continuous-wave (CW) experiment. The second gyro-TWT amplifier delivered CW power of up to 7.7 kW with −3-dB bandwidth of 2.6 GHz and −1-dB bandwidth of 2.1 GHz. Effective implementation of single-stage depressed collectors (to the best of our knowledge, for the first time for gyro-TWTs) enabled the electron efficiencies as high as 36% for the pulsed tube and 33% for the CW tube to be achieved at operation at the second cyclotron harmonic. [ABSTRACT FROM AUTHOR]

A methodology to design high-power amplifiers composed of several amplifier blocks and passive combiners/dividers is proposed.The optimum matching condition between amplifiers and combiners has been found using the mixed-mode S-parameters approach. Measurements and computer-aided design simulations have been successfully used to validate the proposed approach. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1500-1504, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27600 [ABSTRACT FROM AUTHOR]

We build up a finite element modeling (FEM) approach to analyze the thermal performance of collector-up (C-up) heterojunction bipolar transistor (HBTs) with a heat-dissipation via configuration. Highly compact heat-dissipation packaging structures of GaInP/GaAs C-up HBTs have been designed and evaluated systematically. In this work, we devise the 2-D and 3-D models to simulate the actual devices and to investigate the temperature distribution behavior. Results from 2-D model indicate that the large heat-dissipation via configuration can be further reduced by 29% to meet the requirement of HBT-based small high-power amplifiers (HPAs) for the cellular phones. Furthermore, the demonstrated results show that the maximum temperature within the collector calculated from 3-D model is lower than that from 2-D model. In the 3-D analysis, it is revealed that the configuration can be reduced by 32%. Therefore, thinning the heat-dissipation via constructed underneath the GaInP/GaAs C-up HBT should be helpful for miniaturization of HBT-based HPAs in future mobile communication systems. Copyright © 2009 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

A previously proposed method, which enables feeding and extraction of microwave radiation to and from gyrotron traveling wave tubes (gyro-TWTs) using a single highly oversized barrier window has been experimentally proven. A Ka-band gyro-TWT using a helically corrugated waveguide bounded by a reflector at the cathode side and a polarizer at the collector side and also having a polarization separator at the beginning of the output transmission line has produced 6 kW CW power with $-$3-dB bandwidth of 2.1 GHz in the zero-drive-drive stable regime and about 9 kW power in the frequency locking regime. [ABSTRACT FROM PUBLISHER]

We present the design and simulation results of a high-power K-band coaxial frequency-doubling gyroklystron. The six-cavity gyroklystron tube has three cavities operating at the fundamental frequency and the last three cavities operating at twice the fundamental frequency at 22.848 GHz. Numerical simulations predict an efficiency of 37.2% and a gain of 78.3 dB at an average perpendicular-to-axial velocity ratio of 1.4. Start oscillation curves show that the tube is stable to self-oscillations up to an average velocity ratio of 1.4. [ABSTRACT FROM AUTHOR]

A comparative study among several nonlinear high-power amplifier (HPA) models using real measurements is carried out. The analysis is focused on specific models for wideband OFDM signals, which are known to be very sensitive to nonlinear distortion. Moreover, unlike conventional techniques, which typically use a single-tone test signal and power measurements, in this study the models are fitted using subsampled time-domain data. The in-band and out-of-band (spectral regrowth) performances of the following models are evaluated and compared: Saleh's model, envelope polynomial model (EPM), Volterra model, the multilayer perceptron (MLP) model, and the smoothed piecewise-linear (SPWL) model. The study shows that the SPWL model provides the best in-band characterization of the HPA. On the other hand, the Volterra model provides a good trade-off between model complexity (number of parameters) and performance. [ABSTRACT FROM AUTHOR]

Active phased arrays will play an important role in future satellite communications, because of the system flexibility and potential reconfigurability that they may provide. Therefore, an investigation of the relationship existing between payload characteristics and the synthesis parameters of this kind of antenna may constitute a helpful tool. To this end, a parametric analysis of the on-board non-linear circuitry of the transmitting system (power and up-converter sections) has been carried out, establishing the dependence of the mass and DC power demand on the number of beams and feeds of the transmitting phased array. The parametric investigation has been, in particular, applied to an active imaging phased array and an active direct radiating phased array whose suitability to provide a multibeam coverage of Western Europe at Ku band has been demonstrated within an ESA/ESTEC contract. This paper describes the parametric approach followed in the investigation of the on-board non-linear subsystems of the active transmitting phased arrays. The assumed circuitry configurations are widely commented, and the parametric curves for the case of imaging and direct radiating phased arrays are reported. [ABSTRACT FROM AUTHOR]

We present electromagnetic cold-test measurements of BeO ceramic pillbox vacuum windows for a 220-GHz traveling-wave tube amplifier. Transmission and reflection measurements show better than 20 dB return loss over a 25 GHz bandwidth, with band centers in the range of 212–225 GHz. We observe tuning of the window response as the circular waveguide length is changed. High-power testing is performed at 2.5 W, 100% duty at 218 GHz. [ABSTRACT FROM AUTHOR]

The paper proposes a maximum likelihood sequence estimator (MLSE) receiver for satellite communications. The satellite channel model is composed of a nonlinear traveling wave tube (TWT) amplifier followed by a multipath propagation channel. The receiver is composed of a neural network channel estimator (NNCE) and a Viterbi detector. The natural gradient (NG) descent is used for training. Computer simulations show that the performance of our receiver is close to the ideal MLSE receiver in which the channel is perfectly known.

A comparative study among several nonlinear high-power amplifier (HPA) models using real measurements is carried out. The analysis is focused on specific models for wideband OFDM signals, which are known to be very sensitive to nonlinear distortion. Moreover, unlike conventional techniques, which typically use a single-tone test signal and power measurements, in this study the models are fitted using subsampled time-domain data. The in-band and out-of-band (spectral regrowth) performances of the following models are evaluated and compared: Saleh's model, envelope polynomial model (EPM), Volterra model, the multilayer perceptron (MLP) model, and the smoothed piecewise-linear (SPWL) model. The study shows that the SPWL model provides the best in-band characterization of the HPA. On the other hand, the Volterra model provides a good trade-off between model complexity (number of parameters) and performance.

Modern communications systems that involve nonlinear, high-power amplifiers, and linearizers typically have many specifications on linearity, including nonlinear phase shift, amplitude modulation-to-phase modulation (AM-to-PM) conversion, carrier-to-third-order-intermodulation (C/3IM) separation, noise power ratio (NPR), and AM-to-PM transfer. For large (≥10 dB) carrier separations and high gain linearity, we demonstrate that AM-to-PM transfer in traveling-wave tube amplifiers is approximately proportional to the second derivative of the RF phase versus input drive curve (phase concavity), with a negligible contribution from phase slope. The validity of this approximation improves as the amplifier transfer characteristic approaches that of an ideal limiter. This result allows insight into the AM-to-PM transfer behavior and prediction of the metric with minimal computation. This also allows one to more easily understand the trades involved when optimizing between AM-to-PM transfer and other linearity figures of merit (such as C/3IM) [ABSTRACT FROM AUTHOR]