RF Output Power Research Articles

Towards Advanced Fusion Gyrotrons: 2018 Update on Activities within EUROfusion

During the ongoing pre-concept phase (2014 – 2020) for a possible future European DEMOnstration Fusion Power Plant (DEMO) the activities within EUROfusion WP HCD EC Gyrotron R&D and Advanced Developments are focusing on options for near-term solutions, and, at the same time, on long-term even more advanced options. The near-term target for DEMO is to realize pulsed operation. According to the current baseline it will probably use an EC system operating at 170 GHz and 204 GHz is being assessed, whereas the long-term target aims for steady-state operation and frequencies for current drive up to 240 GHz. Common targets for both are an RF output power per unit of significantly above 1 MW (target: 2 MW) and a total gyrotron efficiency of significantly higher than 60 %. Frequency step-tunability and multi-purpose/multi-frequency operation have to be considered. Those targets shall be achieved by considering the coaxial-cavity gyrotron technology and advanced technologies for key components (e.g. CVD diamond-disk Brewster angle window). Advanced simulation and test tools are complementing the research and developments. Gyrotron development is additionally supported by a significant investment into a new multi-megawatt long-pulse gyrotron test stand which is under final installation at KIT currently.

Dynamic Wireless Power Transfer for Cost-Effective Wireless Sensor Networks Using Frequency-Scanned Beaming

A simple adaptive 1-D frequency-scanning method is proposed for radiative wireless power transfer (WPT) systems in low-power wireless sensor networks (WSNs). As a proof of concept, a directive leaky-wave antenna that scans 1-W output RF power in the angular range from ±10° at 2.4 GHz to ±37° at 2.5 GHz, is used to power a WSN covering an area of 1.2 m $\times1.2$ m. It is shown that, using a frequency-scanned antenna, a wider area than using a non-scanned directive antenna can be powered without additional expensive equipment. The WPT protocol is described, showing that any sensor in the WSN can select the optimum transmission channel in the 2.4-GHz band, based on the received signal strength indicator measurements, as the coordinator performs a scheduled frequency hopping phase. This maximizes the WPT beaming efficiency and the transferred dc power. The optimum channel selection can be performed periodically, which makes the system robust against channel state changes.

Theoretical Study on the Operation of the EU/KIT TE34,19-Mode Coaxial-Cavity Gyrotron at 170/204/238 GHz

The 170 GHz 2 MW TE34,19-mode coaxial-cavity modular short-pulse pre-prototype gyrotron at KIT was recently modified in order to verify the multi-megawatt coaxial-cavity technology at longer pulses. In parallel, theoretical investigations on a possibility to operate the 170 GHz TE34,19-mode coaxial-cavity prototype at multiple frequencies up to 238 GHz have been started, with a goal to find a configuration at which the tube could operate in the KIT FULGOR gyrotron test facility using the new 10.5 T SC magnet. This paper indicates which adjustments have to be made and show the feasibility of the multi-frequency operation. Small modifications at the gyrotron cavity will support an RF output power of more than 2 MW at 170/204 GHz. Furthermore, a new gyrotron launcher has been designed capable of producing a Gaussian microwave beam with a Gaussian mode content of more than 96% at these frequencies.

Planar 0.05–1.1 THz Laminate-Based Transition Designs for Integrating High-Frequency Photodiodes With Rectangular Waveguides

Compact planar laminate-based transitions for integrating high-frequency photodiodes (PDs) with rectangular waveguides (WRs) are presented for the WR15 to WR1 standard waveguide bands, i.e., from 0.05 THz up to 1.1 THz. The transitions couple the optically generated (e.g., via heterodyning) millimeter-wave or terahertz signals from the grounded coplanar waveguide (GCPW) output of the PD chip to the WR. To our knowledge, this is the first scalable integration concept that enables hermetic photodiode packaging up to the terahertz frequency range. For the WR15-WR6 bands, all transitions are designed on ultrathin microfiber reinforced PTFE composites (127-μm Rogers RT/duroid 5880 laminate). For the WR5-WR2.2 and the WR1.5-WR1 bands, liquid crystalline polymer ULTRALAM 3850 laminates are used with a thickness of 50 and 25 μm, respectively. The proposed GCPW-WR transition design is based on a double-slot antenna structure and enables the development of fully hermetic photonic packages, which is required to improve the durability of the PD chip. The transition designs are optimized by systematic EM-wave propagation modeling for achieving a wide operational bandwidth of up to 30% of the respective center frequency for each WR band. The optimized transitions exhibit an average insertion loss of about 1.5 dB and a return loss of about 10 dB for all waveguide bands (WR15-WR1). Based upon the systematic numerical modeling, generic guidelines are developed that allow designing a specific transition for any given waveguide band to 1.1 THz. In addition to the numerical analysis, GCPW-WR12 transitions for E-band (60-90 GHz) operation are fabricated and integrated with InP-based balanced-PD chips and GaAs HEMT MMIC medium power amplifiers in a fully hermetic package. Furthermore, hermetic WR12 coherent photonic mixer (CPX) modules are developed. The packaged WR12-type CPX allows direct optical-to-wireless conversion of optical baseband or IF-band signals, e.g., for radio-over-fiber fronthauling in mobile communications. The WR12-CPX module provides RF output power of up to +15 dB·m at 77.5 GHz.

RF Watt-Level Low-Insertion-Loss High-Bandwidth SOI CMOS Switches

CMOS RF switches support watt-level RF output power while enabling signal processing with high speed. Typically, RF FET switches offer a tradeoff between a power handling and modulation bandwidth. This paper investigates the cause of compression in a FET switch, and the analysis suggests the optimization of the gate driving impedance for a stacked-FET switch. A design methodology is presented to realize watt-level power handling with high fractional bandwidth (FBW). The measurements demonstrate a BPSK modulator that can handle up to 10 W with wide FBW. The high-power, low-insertion loss, and high-modulation bandwidth demonstrate record performance.

A low phase noise oscillator based on substrate integrated coaxial line technology

In this article, a miniaturized resonator based on substrate integrated coaxial line technology (SICL) is presented. The SICL resonator consists of a hairpin half-wavelength open-ended coaxial line element and two symmetrical parallel coaxial feed lines. The gap between the feed lines and the hairpin element can be adjusted properly to obtain a steep phase change, which can result in a higher group delay peak and achieve a superior phase noise performance. Based on the proposed SICL resonator, an X-band oscillator has been designed and demonstrated by using low-cost printed circuit boards (PCBs). Great consistency between the measured and simulated results is verified. According to the measured results, the oscillation frequency of the SICL oscillator is 10.0625 GHz with −1.81 dBm RF output power, and the phase noise is −138.6 dBc/Hz@1 MHz. The statistic figure of merit (FOM) is −206.1 dBc/Hz@1 MHz.

Temperature Stabilization of the Klystron Cooling Water at the KOMAC

A total of nine klystrons each with a high peak power are used to deliver RF power to the 100 MeV proton linac at the Korea Multi-purpose Accelerator Complex (KOMAC). We found that the temperature of the cooling water to the klystron cavity was deeply related to the RF power output from the klystron when the klystron was operated either in the open loop mode or at a saturation power level. To stabilize the RF power from the klystron, we need to stabilize the cooling water’s temperature as much as possible. To keep the cooling water’s temperature constant, we used an inverter-driven cooling fan at the cooling tower with proportional integral derivative (PID) controller. Through a proper tuning of the control parameters, we could suppress the temperature fluctuation to within ±0.2 °C and stabilize the output RF power from the klystron, regardless of the ambient temperature. Details on the dependency of the output RF power on the coolant temperature and stabilization method will be given in this paper.

Stability Study in Dielectric Ring Loaded Periodic Interaction Structure for a Megawatt Class Gyrotwystron

In this paper, the stability and design studies of an X-band gyrotwystron using a distributed loss technique at its output waveguide section are presented. The periodic dielectric loading (PDL) in the output section of the amplifier suppresses the self-generated backward-wave parasites including TE11, TE21, and second harmonic TE02 modes. These parasites are investigated using the small-signal linear theory. The PDL technique improves not only the stability of the amplifier but also its gain. The PDL waveguide provides a transmission loss of 0.63 dB/cm to the desired TE01 operating mode at 10 GHz that is good enough to stabilize the operation of gyrotwystron against parasites. The nonlinear simulation predicted an RF output power of ~ 25 MW in the TE01 mode for the dc beam of 220 A, 440 kV.

Reduction of Amplitude-to-Phase Conversion in Charge-Compensated Modified Unitraveling Carrier Photodiodes

The amplitude-to-phase (AM-to-PM) conversion in photodetectors is one the primary noise sources in the generation of ultralow noise microwave signals with stabilized optical frequency combs. We have designed charge-compensated modified unitraveling carrier (CC-MUTC) photodiodes to reduce the AM-to-PM conversion. The photocurrent-induced phase change at 15 GHz is reduced by >3 times and the AM-to-PM is greatly suppressed, especially at large photocurrent. For 10-μm devices, the 3-dB bandwidth at −6 V is 35 GHz and the maximum RF output power is 13.7 dBm at 15 GHz. Compared with the original CC-MUTC structure, the maximum output power is increased by 2.7 dB.

An electrical equivalent circuit to simulate the output power of an AlGaAs/GaAs planar gunn diode oscillator

AbstractThe planar Gunn diode offers the potential of microwave, milli‐metric and THz‐based oscillators, which can be fabricated as part of a microwave monolithic integrated circuit (mmic). The paper develops an electrical representation of an aluminum gallium arsenide (AlGaAs) based planar Gunn diode with an active channel length of approximately 4 μm and width of 120 μm, embedded in an oscillator circuit. The work indicates a maximum simulated RF output power of +5 dBm compared with published experimental results of −19 dBm for similar diodes.

A wireless powered electroactive polymer using magnetic resonant coupling

This paper reports an ionic polymer-metal composite (IPMC) soft actuator that is controlled and powered in a wireless manner. IPMC is an ionic electroactive polymer (EAP) that can be actuated at a low voltage, fast response and miniaturized, therefore, attracts significant interest that have led to extensive investigations, especially for biomedical application. The device design utilizes magnetic resonant coupling as the wireless powering scheme. A planar based LC receiver circuit and DC rectifier circuit are fabricated using double sided and single sided copper-clad flexible polyimide sheets, respectively. The fabricated IPMC cantilever is bonded to the DC rectifier circuit and performs unimorph motion when the exposed magnetic field (generated by the transmitter circuit) matches the resonant frequency of the LC receiver circuit, ∼25 MHz. Experiment wireless operation of the fabricated LC receiver circuit shows a maximum induced DC voltage of ∼2.3 Vdc. The cantilever deflection is measured as 66 μm at ON-OFF activation cycle with 0.6 W RF output power.

(Invited) A Revolutionary GaN-on-SiC Heteroepitaxy for High-Frequency Power Transistors

GaN HEMTs have a great potential to be well suited for the forthcoming 5G communication infrastructures and other millimeter wave applications, where high-power and high-efficient devices are required. For GaN HEMT heterostructures used in high-frequency applications (> 8 GHz), semi-insulating (SI) SiC is the best choice of substrates, owning to its high thermal conductivity and high resistivity, and the maturity in large-wafer mass production (up to 150 mm). Nevertheless, the typical thickness of a GaN layer grown on SiC substrates has to be at least 1.5~2.0 µm to obtain a structural quality manufacturable for device fabrication, due to the large in-plane lattice mismatch with GaN (3.4%). As a result, the thick GaN layer must be doped with acceptor-like impurities like carbon or iron [1] during the epitaxial growth to increase the resistivity, preventing a parallel conduction in the device. However, these impurities introduce deep traps, which capture electrons during the RF operation, rendering a depletion of the channel electrons thus reducing the device current/power density. We report the realization of a revolutionary heterostructure of III-nitride materials grown on SiC substrates by a unique MOCVD method. Our success in the material growth enabled a new design of HEMT heterostructures requiring no intentional Fe- or C-doping. Excellent DC and RF performances of the highly-scaled GaN-based high electron mobility transistors (HEMTs) using the proposed new heterostructures are demonstrated. The new HEMT heterostructures also open up new opportunities in material growth schemes to further reduce the charge trapping effects that have been prevailing in the field over that last two decades. Consequently, a high RF output power of ~4 W/mm was obtained at a fundamental frequency of 30 GHz in the HEMT devices with a gate length (Lg) of 0.1 µm, biased at VDS= 30 volt. We also found that the thermal resistance of the new HEMT devices exhibits a substantially lower thermal resistance (RTH) of 4.1 oC mm/W, as compared with that of the conventional ones, which is typically around 9~10 oC mm/W [2] The details of the new heterostructures and device characteristics will be presented. Our breakthrough in the material growth has brought the GaN HEMT technology to a new height and will serve as a new route for further improvement. IV.

1.0 THz GaN IMPATT Source: Effect of Parasitic Series Resistance

The degradation of high-frequency characteristics of a 1.0-THz double-drift region (DDR) impact avalanche transit time (IMPATT) diode based on wurtzite gallium nitride (Wz-GaN), due to the influence of parasitic series resistance, has been investigated. A two-dimensional (2-D) large-signal (L-S) simulation method based on a non-sinusoidal voltage excitation (NSVE) model has been used for this purpose. A comprehensive model of series resistance has been developed by considering the influence of skin effect, and the said model has been incorporated in the 2-D L-S simulation for studying the effect of RF power output and DC to RF conversion efficiency of the device. Results indicate 24.2–35.9% reduction in power output and efficiency due to the RF power dissipation in the positive series resistance. However, the device can still deliver 191.7–202.9 mW peak RF power to the load at 1.0 THz with 8.48–6.41% conversion efficiency. GaN IMPATT diodes are capable of generating higher RF power at around 1 THz than conventional diodes, but the effect of parasitic series resistance causes havoc reduction in power output and efficiency. The nature of the parasitic resistance is studied here in the level of device fabrication and optimization, which to our knowledge is not available at present.

A four channel phased array transmitter using an active RF phase shifter for 5G wireless systems

This paper presents a four channel phased array transmitter at 15 GHz aimed for the upcoming 5G wireless systems. The circuit is designed and fabricated using 45 nm CMOS silicon on insulator technology. The design is programmable with exhaustive digital controls available for parameters such as bias voltage, resonance frequency, and gain. The phase shift required for the phased array is provided at RF using an IQ vector modulator (IQVM) topology, which provides both amplitude and phase control. Based on the measurement results, the IQVM provides 360° of phase shift and 15 dB of gain variation. Both phase and amplitude information are encoded in a 10 bit control word. The mean angular separation provided by the IQVM is 3° at optimum amplitude levels. Active area occupied is 2.88 square millimeter. Total DC power consumed by one transmit channel from 1 and 2.6 V supply is 268 mW. The maximum RF output power from one transmit channel is 1dBm. Measured EVM for a 256 QAM modulated signal is as low as 2.0%. All results include the impact of printed circuit board traces and pad parasitics. Based on the achieved results, the proposed architecture is well suited for the next generation of the wireless systems.

A 24-GHz Low-Power RTD-Based ON–OFF Keying Oscillator With an RTD Pair Configuration

In this letter, an ON–OFF keying (OOK) oscillator that uses a resonant tunneling diode (RTD) pair configuration has been designed and fabricated. To achieve improved RF output power characteristics, the RTD pair topology was utilized in the RTD OOK oscillator design to realize an increased negative differential conductance voltage span. The fabricated IC showed high-speed switching capability of 5 Gb/s with a good phase noise characteristic of −118.17 dBc/Hz at a 1-MHz offset. The maximum RF output power was determined to be −1.76 dBm at a carrier frequency of 24.1 GHz. It consumes 12.5 mW of power, resulting in a good figure of merit value of 2.5 pJ/bit. These results demonstrate its potential as a high-performance ON–OFF mode oscillator for high-data-rate OOK transceiver systems.

Design and Simulation of a T-Vane Relativistic Inverted Magnetron

A design for a high-power relativistic inverted magnetron oscillator (IMO) is presented and modeled numerically using the massively parallel electromagnetic particle-in-cell code, improved concurrent electromagnetic particle in cell (ICEPIC). The inverted magnetron presented here is designed to operate in the $L$ -band for magnetic fields ranging from 0.07 to 0.1 T with voltages ranging from ~250 to 480 kV. Simulations in that voltage/magnetic field range demonstrate that the inverted magnetron is capable of fast startup, $\pi $ -mode dominance with little-to-no mode competition, as well as high output power, exceeding 2.5 GW at the high end of voltages examined. End-loss current, a common source of energy loss in relativistic magnetrons, is not present for this IMO design. The IMO radiates RF energy axially through the use of a novel dual-ring radiator that excites the TM01 mode in the downstream waveguide. This approach yields nominal RF output power efficiencies near 20%. ICEPIC simulations predict that the above features combined with the IMO’s stable, robust, and reliable performance in the desired mode over a large voltage range yield a class of high-power microwave source notable for its inverted geometry and T-vane slow-wave structure.

A Silicon Aperiodically Distributed Traveling-Wave Photodetector With Enhanced RF Output Power

We propose an aperiodic loading technique to enhance the RF output power of a silicon distributed traveling-wave photodetector (TWPD), which can be used to handle high optical powers in analog optical links. This technique contains a traveling wave electrode whose input terminal is open circuit so as to avoid shunting any photocurrent from the photodetector (PD) array. As a result, the RF output to the load impedance that locates at the other terminal is enhanced. To mitigate the bandwidth reduction caused by the strong RF reflection at the open input terminal, the spacing between adjacent PD units gradually increases from the input to the load terminals. By breaking the balanced distribution of the photocurrent from each PD among the forward and the backward propagation directions, this arrangement weakens the strength of the interference between the forward and the reflected backward RF waves, and hence improves the bandwidth. In order to validate this operation principle, an analytical model is developed to calculate the frequency response of the silicon distributed TWPD. The model then is used to optimize both periodically and aperiodically distributed TWPDs on silicon. After the optimization, the 3-dB bandwidth of a four-stage aperiodic TWPD is 25.7 GHz, which is only 0.8 GHz lower than that of a four-stage periodic TWPD. However, since the aperiodic TWPD eliminates the matching impedance at its input terminal, its RF output power on the load is four times as strong as that of the periodic TWPD from 1 to 24.7 GHz.

Analysis, Design, and 3-D Simulation of a High-Efficiency Overmoded Nonuniform Relativistic Backward-Wave Oscillator

A high-efficiency X-band overmoded relativistic backward-wave oscillator (RBWO) using a nonuniform slow-wave structure (SWS) and cavity resonant reflector (RR) is designed in order to increase the beam–wave interaction efficiency. The electrodynamic SWS is a sinusoidally corrugated and moderately overmoded (D/ $\lambda$ > 1) structure that decreases the strength of electric field per unit area at its surface and RR. This reduces the probability of occurrence of RF breakdown, hence the pulse shortening. The nonuniform SWS enhances the beam–wave interaction efficiency by providing variable coupling impedance between −1st harmonic of backward fundamental mode and slow space-charge wave associated with the electron beam. The 3-D particle-in-cell simulation of the designed RBWO generated an RF output power of ~700 MW at 9.24 GHz, while driving by a trapezoidal voltage pulse of 600 kV. The axial guiding magnetic field is 2.5 T that restricts the velocity of electron beam in the axial direction only. The output signal has a high mode purity because the higher order modes are cutoff by the structure. The frequency tuning is obtained by varying the distance between RR and SWS.

High-Power Photodiodes With 65 GHz Bandwidth Heterogeneously Integrated Onto Silicon-on-Insulator Nano-Waveguides

High-speed InP-based modified uni-traveling carrier photodiodes with top p-contact on silicon-on-insulator nanowaveguides are reported. The photodiodes have low dark current (1 nA), high bandwidth (65 GHz), and deliver record-high RF output power at 70 GHz (–2 dBm at 20 mA). Balanced and traveling-wave photodiodes of this type have bandwidths of 20 and 35 GHz, respectively.

Fiber dispersion compensation in single side band optical communication system using ideal fiber Bragg grating and chirped fiber Bragg grating without DCF

In this paper, a 10 Gbps, single side band optical communication system is implemented with different dispersion compensation schemes. This paper proposes that dispersion compensation is fruitful with a linear chirped fiber Bragg grating, without a dispersion compensation fiber at higher input powers. Its performance is compared with fiber Bragg grating and DCF. Same global parameters are used for all simulations. The simulated transmission system has been analyzed by different performance parameters like Q factor, output RF power and eye diagram extinction ratio. The Q factor, output RF power and extinction ratio can be improved about by 21 dB, 3.5 dB, and 20 dB respectively. The simulations are conducted for higher input power from 0 dBm to 8 dBm and for fiber length from 20 km to 80 km. The BER in CFBG is 15 dB lesser than FBG. The CFBG shows better performance than conventional FBG. The signal delay is also the lowest in the method using CFBG. The Q factor and signal delay are also compared with CFBG, FBG, DCF and uncompensated system.