1,721,155 research outputs found
Efficient power amplifiers for amplitude-tapered pulses with improved spectral confinement
No full textThis paper reviews various techniques that allow efficient amplification of amplitude-modulated radar pulses through some type of supply modulation. Examples at S and X-band with GaN PAs will be shown with efficiencies above 50% for 10-W transmitters and greatly reduced spectral sidelobes
X-Band GaN Multi-Level Chireix Outphasing PA with a Discrete Supply Modulator MMIC
No full textThis paper presents the characterization of a multi-level Chireix outphasing PA with a GaN discrete supply modulator MMIC at 9.7GHz. The internal PAs include class-F harmonic terminations, while the Chireix combiner determines the fundamental frequency load modulation. A power-DAC architecture provides 8 supply levels with 3 half-bridges, maintaining more than 85% efficiency. The combination of the two achieves a peak output power of 4.8W, and average total efficiencies for a 6dB PAR QPSK signal of 44.1% and 48.1% with and without considering the supply modulator consumption, respectively, demonstrating the feasibility of this PA architecture with a real discrete supply modulator
A 2.45-GHz Energy-Autonomous Wireless Power Relay Node
No full textThis paper describes the design and experimental characterization of a battery-less bidirectional 2.45-GHz circuit operating in oscillator mode as a wireless power transmitter or in rectifier mode as an energy harvester, with a measured efficiency greater than 50% in both operating states. The dc voltage harvested in rectifier mode provides the drain bias for the oscillator. The FET-gate self-bias mechanism is exploited in both functionalities, thus eliminating external gate bias. Bi-directionality is based on the time-reversal properties of a transistor oscillator. Energy autonomy is possible at received RF power levels as low as -4 dBm, by means of a bias-assisting feedback loop, consisting of a single matched low-power diode in shunt configuration. A hybrid prototype is demonstrated with the ability to operate as an energy-autonomous power relay node by switching between transmit and receive power modes
Bandwidth control of forbidden transmission gaps in compound structures with subwavelength slits
Full text linkPhase resonances in transmission compound structures with subwavelength slits produce sharp dips in the transmission response. For all equal slits, the wavelengths of these sharp transmission minima can be varied by changing the width or the length of all the slits. In this paper we show that the width of the dip, i.e., the frequency range of minimum transmittance, can be controlled by making at least one slit different from the rest within a compound unit cell. In particular, we investigate the effect that a change in the dielectric filling, or in the length of a single slit, produces in the transmission response. We also analyze the scan angle behavior of these structures by means of band diagrams and compare them with previous results for all-equal slit structures. © 2007 The American Physical Society.Fil: Skigin, Diana Carina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Loui, Hung. Sandia National Laboratories, New Mexico;Fil: Popovic, Zoya. University of Colorado; Estados UnidosFil: Kuester, Edward F.. University of Colorado; Estados Unido
Efficient Programmable Pulse Shaping for X-Band GaN MMIC Radar Power Amplifiers
Full text linkThis paper presents a supply modulated X-band 12-W peak power transmitter that maintains an average efficiency greater than 50% for various shapes of amplitude-modulated pulses. The main power amplifier is a two-stage GaN-on-SiC MMIC with a peak efficiency of 65%, while the pulse envelope modulator is a 95% efficient hybrid 3-b power DAC implemented with GaN-on-Si transistor switches. Envelope shaping of a pulsed waveform results in improved spectral confinement of greater than 15 dB for the first sideband compared with constant-envelope pulses, with over 20 points improvement in total efficiency. The combination of supply modulation and digital predistortion is shown to result in high composite (total) efficiency of over 55%, with simultaneous high dynamic range and with flexible digitally programmable pulse shaping
A Prepulsing Technique for the Characterization of GaN Power Amplifiers With Dynamic Supply Under Controlled Thermal and Trapping States
Full text linkThe asymmetry between capture and release time constants associated with charge-trapping phenomena observed in the electrical characteristics of microwave gallium-nitride (GaN) field-effect transistors (FETs) introduces distortion in GaN-based power amplifiers (PA). The PAs that operate with supply modulation to increase efficiency are particularly affected by this phenomenon, since the GaN FET trap state exhibits a nonlinear dependence on the voltage applied to the device terminals. In this paper, a measurement approach and the setup are presented for a large-signal characterization of GaN-based PAs operated with dynamic bias supply: a new prepulsing technique is introduced, which enables the characterization of the PA in controlled charge-trapping and thermal states. The characteristics obtained with this technique are shown to give an accurate description of the PA performance in the actual application working conditions. The proposed approach is validated by using the measured data for the direct computation of predistortion functions for the linearization of a 9.7-GHz envelope tracking 10-W GaN monolithic microwave integrated circuit PA for amplitude-modulated pulsed radar transmitters. Additional research on the PA trap-induced performance degradation is also presented and can be explored to predict the PA performance for different parameters of the operative regime or for the formulation of the PA behavioral models
RF Spatial Combining and Field Control in Dielectric Media
Full text linkRecent developments in higher static magnetic field MRIs have allowed for increased sensitivity and improved imaging compared to current clinical MRIs due to the inherent low SNR of nuclear magnetic resonance. Ultra-high field MRI promises to improve medical imaging technology with higher SNR, increased parallel imaging performance, and higher spectral dispersion. With the decrease in wavelength inside of the imaging volume, standing waves develop and decrease the high homogeneity required for accurate diagnosis of medical problems. Additionally, at higher operating frequencies the bore becomes a loaded waveguide which supports propagating modes. This thesis focuses on the development of circularly polarized patch probes for traveling wave MRI at small bore 16.4 T, wide-bore 7T and 10.5T systems with boundary modification structures to improve field coupling and homogeneity. The attenuation of the wave in the unloaded portion of the waveguide becomes an issue with available RF systems as the excitation is placed away from the imaging volume. Additionally, boundary modifying structures are developed to improve coupling and SNR inside of the imaging volume. These boundary modifying structures placed around the imaging volume can be relatively simple and provide an increase in field homogeneity. To improve the relatively poor received spatial coverage of the single patch probe, an additional array is developed with interdigitated capacitor probes at 10.5 T. Multiple excitations can be combined with various relative magnitude and phase excitations, known as B1 shimming. These two techniques can be combined to drastically increase the field coverage inside of an imaging volume. The techniques proposed for cavity excitations in MRIs can be translated to microwave heating systems that convert waste to carbon-rich fuel. Recent military interest in the development of waste-to-fuel techniques have used pyrolysis and incineration, however, these approaches are relatively large due to the necessary supporting infrastructure to operate them
Supply Modulation of a Broadband Load Modulated Balanced Amplifier
No full textThis work presents multi-level supply modulation of a load modulated balanced amplifier (LMBA). The RF-input LMBA employs dynamic active matching for efficient operation over an octave bandwidth. Supply modulation is a natural candidate to further enhance the back-off efficiency of this broadband amplifier due to its independence on RF carrier frequency. In the ET-LMBA prototype demonstrated here, an efficient hybrid multi-level converter (power DAC) produces supply levels from 10 to 30 V to modulate the drain of the balanced amplifier. Digital predistortion is used to linearize the ET-LMBA behavior at discrete steps and over frequency. This system is demonstrated at 2.1, 2.7 and 3.5 GHz with a 20-MHz, 10-dB PAPR signal showing composite efficiency improvement between 9 and 12 percentage points when supply modulation is employed, while maintaining linearity
Multilevel Supply-Modulated Chireix Outphasing With Continuous Input Modulation
Full text linkThis paper presents a dynamic characterization of a multilevel (ML) Chireix outphasing (ML-CO) power amplifier (PA) with modulated signals. The ML-CO technique combines the advantages of envelope tracking and outphasing architectures by limiting the supply modulation to discrete levels with an efficient power-DAC modulator and using outphasing for fine amplitude control. We describe an experimental test bench that supplies the required phase- and time-aligned modulated signals for outphasing and supply modulation simultaneously. Pulsed characterization is used to design an ML memoryless polynomial DPD. The linearized ML-CO GaN X-band MMIC PA is demonstrated with fixed input drive levels for 9.3-dB peak-to-average power ratio (PAPR), 1.4-MHz and 11.3-dB PAPR, 10-MHz LTE signals with a 9.7-GHz carrier. For both signals, the average total power consumption is reduced by a factor of two when supply modulation is used. An investigation of control signal generation strategies is presented, including combinations of discrete supply and continuous input modulation. A simple exponential function tracks the optimal efficiency trajectory of the ML-CO PA, enabling overall efficiency improvements within 9 and 27 percentage points. This control strategy is applied to radar pulses with amplitude shaping and frequency chirp, with the efficiency improvement of up to 11.8 percentage points compared with constant-supply operation, with 29-dB improvement in spectral confinement
Pre-pulsing characterization of GaN PAs with dynamic supply
No full textNonlinear charge-trapping observed in the electrical characteristics of GaN FETs can introduce distortion in GaN-based power amplifiers (PA), especially in supply-modulated (envelope tracking) transmitters. A measurement approach is developed for large signal characterization of GaN-based PAs operated with dynamic bias supplies for efficiency enhancement. A new pre-pulsing technique is introduced which forces the active device to operate in trapping and thermal states close to those found in the actual application. The characteristics obtained with this technique are shown to give an accurate description of the PA performance. The measured data are used for the direct computation of pre-distortion functions for the linearization of a 10-GHz Envelope Tracking (ET) 12-W GaN MMIC PA for amplitude-modulated pulsed radar transmitters. The demonstrated measurement method can be also exploited for the identification of PA behavioral models, which take into account trapping effects
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