Traveling Wave Tube Amplifier Research Articles

Traveling Wave Tube Eigenmode Solution for Beam-Loaded Slow Wave Structure Based on Particle-in-Cell Simulations

A scheme to characterize and model the dynamics of the electron beam–electromagnetic power exchange along a traveling-wave tube (TWT) is proposed. The method is based on defining a state vector at discrete periodic locations along the TWT and determining the transfer matrix of the unit cell of the “hot” (i.e., beam loaded) slow wave structure (SWS) that considers the small-signal interaction between the electromagnetic guided field and the electron beam via 3-D particle-in-cell (PIC) simulations. Once the estimate of the unit-cell transfer matrix is obtained, we show how to find the hot guided eigenmodes in the interactive system made of an electromagnetic wave in the SWS coupled to an electron beam by using Floquet theory. In particular, we show how to determine the complex-valued wavenumbers of the hot modes and the eigenvectors associated with them. We focus on finding the hot modes supported by a TWT amplifier with a serpentine SWS operating at millimeter waves. We show the dispersion relation of the modal complex-valued wavenumbers of the hot modes when varying frequency; near the synchronization point, the results are in agreement with the Pierce theory. Finally, we show how the proposed scheme is also useful to estimate the gain performance of a long TWT amplifier by cascading the transfer matrices estimated from PIC simulations of a shorter hot SWS. The results show that the gain calculated based on the proposed model very well matches the one calculated from 3-D PIC simulations of the whole structure. The technique is general and can be applied to any SWS geometry where electromagnetic modes interact with an electron beam. The model we proposed can be a very powerful tool to understand the physics of TWTs and can be used for optimization purposes.

In-Orbit Reliability Evaluation of Space TWTA Based on Copula Function and Bivariate Hybrid Stochastic Processes

Currently, it is still a challenge to study the degradation mechanisms of the space traveling wave tube amplifier (TWTA) with no failure and small sample tests. Given that the Copula functions are used to describe the correlation of multiple performance characteristics, this paper develops a bivariate hybrid stochastic degradation model to evaluate the in-orbit reliability of TWTA. Firstly, based on the impact analysis of the life of TWTA, helix current and anode voltage are selected as the performance degradation parameters. Secondly, stochastic processes with random effects based on the one-dimensional Wiener process and Gamma process are applied to describe the degradation of TWTA’s helix current and anode voltage, respectively, and the corresponding marginal distribution function is obtained. Then, the Copula function is utilized to describe the correlation between two different performance parameters of TWTA. Meanwhile, this paper also proposed a two-step method to estimate the reliability level of TWTA based on its in-orbit telemetry data through a two-step method, which contains a Markov Chain Monte Carlo (MCMC) algorithm and a maximum likelihood estimation (MLE) algorithm. Besides, the Bayes-Bootstrap sampling method is also used to improve the evaluation accuracy to overcome the defect of an in-orbit small sample of TWTA. Finally, a TWTA degradation case with a set of telemetry data is carried out, and the results show that the method proposed in this paper is more applicable and more accurate than other methods.

Experiments on a Disk-on-Rod Traveling Wave Tube Amplifier Driven by a Nonlinear Transmission Line Modulated Electron Beam

Experiments have demonstrated high-power amplification (100 MW class) from a traveling wave amplifier driven by a modulated electron beam. The modulated electron beam is generated by applying a modulated voltage pulse from a nonlinear transmission line (NLTL) to the cathode of the electron gun. The system showed that over the designed operating range of the disk-on-rod (DoR) TWT amplifier (1.2–1.4 GHz), the peak frequency of the amplifier output tracked the peak frequency of the NLTL to within ±2%; however, the output was not phase stable. Measured RF energy gain for this frequency regime ranged from 1.5 at 1.2 GHz up to 5.0 at 1.4 GHz).

A Novel Analog Predistortion Linearizer Based on a Schottky diode for Communication Applications

Background and Objectives: In order to compensate the nonlinearity, several linearization approaches such as feedback, feed-forward, and predistortion have been proposed. Among these linearization methods, the analog predistortion (APD) method is very suitable for the power amplifier and TWTA linearization, because it has a good compromise between complexity, linearity improvement and bandwidth. On the other hand, analog predistortion circuit can be added to existing amplifiers as a separate circuit. Many of the reported linearizer circuits are capable of linearizing power amplifiers with high gain and phase variations. However, these circuits are complex and bulky due to the use of distortion producer, phase shifter, adjustable attenuator, combiner/divider, and control unit. The objective of this paper is increasing the dynamic range of the predistorter by a simple proposed circuit. Hence, linearization capability of the linearizer is boosted.Methods: A new analog predistortion linearizer based on a coupler and a Schottky diode is presented. The proposed structure satisfies linearization requirements of the solid-state power amplifiers (SSPAs) and traveling wave tube amplifiers (TWTAs). Using a parallel combination of two Schottky diodes along with transmission line in port 2 and 3 of the coupler, the dynamic range of reflection coefficient of each port is increased compared to using a single diode in each branch. By increasing the range of reflection signal, the linearization capability has been increased to linearize power amplifiers and TWTAs. The AM/AM and AM/PM of the linearizer is controlled by changing the bias voltage of power supply.Results: The simulation results show that about 30° phase shift and 5 dB gain expansion can be compensated for power amplifiers and TWTAs. Because of utilizing two diodes, the linearization capability of the new linearizer is augmented compared to the conventional linearizer.Conclusion: A novel analog predistortion circuit based on the hybrid coupler and Schottky diodes is presented. The suggested topology increases the dynamic range of reflected power from each port. Using the proposed idea, AM/AM and AM/PM characteristics are expanded such that this characteristic can linearize the power amplifier with high linearity.

A NEW PRE-DISTORTION APPROACH TO TWTA COMPENSATION FOR WIRELESS OFDM SYSTEMS

Orthogonal Frequency Division Multiplexing (OFDM) has several desirable attributes which makes it a prime candidate for a number of emerging wireless communication standards. However, one of the major problems posed by OFDM is its high Peak-to-Average-Power ratio (PAPR), which seriously limits the power efficiency of the High Power Amplifier (HPA) because of the nonlinear distortion resulting from high PAPR. The present paper presents a new mixed computational/analytical approach for compensation of this nonlinear distortion for the case in which the HPA is a Traveling Wave Tube Amplifier (TWTA) with time-varying characteristic. TWTAs are used in wireless communication systems when high transmission power is required as in the case of the digital satellite channel. Compared to previous pre-distorter techniques based on LUT (Look-Up Table) or adaptive schemes, our approach relies on the analytical inversion of the Saleh TWTA model in combination with a nonlinear parameter estimation algorithm. This leads to a sparse and yet accurate representation of the pre-distorter, with the capability of tracking efficiently any rapidly time-varying behavior of the TWTA. Computer simulations results illustrate and validate the approach presented.

Performances of 2r16APSK and DVB-S2 16APSK Modulations over a Two-Link Satellite Channel

In this paper, the 2r16APSK modulation is contrasted with the 16APSK modulation adopted in the digital video broadcast standard, the DVB-S2, aiming at verifying if the 2r16APSK can be considered an alternative choice for the DVB-S2 and other alike communication systems when subjected to nonlinear distortions. To this end, the performances of both modulations are assessed in terms of the metrics: bit error rate (BER), constellation figure of merit (CFM), peak-to-average power ratio (PAPR), total degradation (TD) versus input back-off (IBO), and spectral regrowth, when the transmitted signal goes through a two-link satellite channel under memoryless nonlinear distortion produced by a traveling wave tube amplifier (TWTA), which is described by the Saleh model. The results show that the 2r16APSK modulation is indeed an alternative choice.

On the Joint Effects of HPA Nonlinearities and IQ Imbalance on Mixed RF/FSO Cooperative Systems

In this work, we provide a framework analysis of dual-hop hybrid Millimeter Wave Radio Frequency (RF)/Free Space Optical (FSO) MIMO relaying system. The source is equipped with multiple antennas and employs conjugate beamforming while the destination consists of multiple apertures with selection combining. The system also consists of a relay operating at amplify-and-forward mode. The RF channels are subject to Nakagami-m fading while the optical links experience the Málaga distribution. In addition, we introduce the impairments to the relay and receiver. In fact, the relay is impaired by the High Power Amplifier (HPA) nonlinearities while the receiver suffers from the In phase and Quadrature Imbalance. Moreover, we assume two types of HPA nonlinearities impairments called Soft Envelope Limiter (SEL) and Traveling Wave Tube Amplifier (TWTA). Closed-forms of the outage probability, the bit error probability, and the ergodic capacity are derived. Capitalizing on these performances, we derive the high SNR asymptotes to unpack insightful metrics such as the diversity gain. We also address the impacts of some key factors on the system performance such as the impairments, the interferers, the number of antennas and apertures and the pointing errors, etc. Finally, the analytical expressions are confirmed by Monte Carlo simulation.

Design and Stability Analysis of a High-Order Mode-Staggered Double Vane Traveling Wave Tube With Two Pencil Beams at G-Band

In this article, design of a high-order-mode (TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> -mode) staggered double vane (SDV) traveling wave tube amplifier (TWTA) with two pencil electron beams is presented. Since, the dimensions are larger when compared to fundamental mode TWTAs, the high-power capacity can be achieved. Meanwhile, several ways to suppress the fundamental mode competition and improve the stability of our high-order-mode TWTA are studied thoroughly, such as limiting the gain of each stage and using metal or wave-absorbing material columns. A three-stage TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> -mode SDV TWTA with aluminum nitride column at the center of vanes is simulated to get an over 145-W peak power with a corresponding gain of 32.5 dB. The frequency spectrum is pure at 220 GHz and the output signal is a stable TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> -mode signal. The bandwidth reaches about 38 GHz. The results show that high-order-mode operation can develop the high-power capacity vacuum electron devices at terahertz band.

Design and simulation of a W-band gyrotron traveling wave amplifier

The design of a W-band gyrotron traveling wave tube amplifier is presented. The device with a lossy ceramic interaction circuit operates in the fundamental harmonic TE01 circular electric mode. The 70-kV, 6-A electron beam is produced by a double anode magnetron injection gun with an average perpendicular-to-parallel velocity ratio of 1.2 and a parallel velocity spread of less than 3%. The beam-wave interaction is investigated by using a particle-in-cell code. The simulations predict that the amplifier can produce an output peak power of over 100 kW, 28% efficiency, 42 dB gain, and a 3 GHz bandwidth, respectively under the assumption of beam velocity spread 3%.

Dimensioning an FPGA for Real-Time Implementation of State of the Art Neural Network-Based HPA Predistorter

Orthogonal Frequency Division Multiplexing (OFDM) is one of the key modulations for current and novel broadband communications standards. For example, Multi-band Orthogonal Frequency Division Multiplexing (MB-OFDM) is an excellent choice for the ECMA-368 Ultra Wideband (UWB) wireless communication standard. Nevertheless, the high Peak to Average Power Ratio (PAPR) of MB-OFDM UWB signals reduces the power efficiency of the key element in mobile devices, the High Power Amplifier (HPA), due to non-linear distortion, known as the non-linear saturation of the HPA. In order to deal with this limiting problem, a new and efficient pre-distorter scheme using a Neural Networks (NN) is proposed and also implemented on Field Programmable Gate Array (FPGA). This solution based on the pre-distortion concept of HPA non-linearities offers a good trade-off between complexity and performance. Some tests and validation have been conducted on the two types of HPA: Travelling Wave Tube Amplifiers (TWTA) and Solid State Power Amplifiers (SSPA). The results show that the proposed pre-distorter design presents low complexity and low error rate. Indeed, the implemented architecture uses 10% of DSP (Digital Signal Processing) blocks and 1% of LUTs (Look up Table) in case of SSPA, whereas it only uses 1% of LUTs in case of TWTA. In addition, it allows us to conclude that advanced machine learning techniques can be efficiently implemented in hardware with the adequate design.

The development of low-noise pulse traveling wave tube amplifier for radar scatterometer

The development of low-noise pulse traveling wave tube amplifier for radar scatterometer

High Linear Power E-Band Traveling-Wave Tube for Communication Applications

A low-voltage and high linear power E-band traveling-wave tube amplifier is designed to meet the demand of high-data-rate wireless communication. The integrated design, including the dispersion combination of folded waveguide slow wave structure, broadband diamond pillbox window, and AlN-SiC lumped attenuator, ensures the success of the prototype tube with satisfying performance. The test results show that the amplifier is capable of providing more than 45.2 W output power, 38.8 dB saturated gain, 14.3% total efficiency, and 45.6 dB small-signal gain in 71-76 GHz band with a low voltage of 12.88 kV and a current of 80 mA. The 1 dB compression point exceeding 20 W verifies the feasibility of dispersion combination technology for high linear power design.

Study on linearization of Ka-band wideband traveling-wave tube amplifer

Linearizer is a key component in the millimeter-wave communication system, it plays an important role in improving the linearity performance of amplifier and communication quality. At present, the development of traveling-wave tube amplifier (TWTA) linearization technology cannot meet the application requirements of communication technology, therefore, the research of linearization technology is very important. In this paper we propose a kind of a new wide-band analog pre-distortion structure used to improve the nonlinear characteristics of Ka-band TWTA. The simulation results show that when the input power changes from −20 to 10 dBm in the frequency range of 26−30 GHz, the gain expansion of the linearizer is greater than 5.08 dB, and the phase expansion exceeds 64.81°. The linearizer and the TWTA are cascaeded for testing. The test results show that the gain compression and phase compression of center frequence is less than 3.12 dB and 2.31° respectively, and the third-order intermodulation (IMD3) improves significantly.

Wideband Output Brewster Window for Terahertz TWT Amplifiers Application

A wideband terahertz (THz) output network consisting of a pyramidal horn and a diamond Brewster window has been developed for 0.7-1.0-THz traveling-wave tube (TWT) amplifier and is measured at 0.325-0.5 THz owing to the limitation of the measurement facility. A pyramidal horn is designed to convert the waveguide mode into a high-purity quasi-plane polarized mode to meet the demand of the Brewster window. A diamond Brewster window plate with low THz transmission loss is fabricated via chemical vapor deposition (CVD). The plate is directly placed on the pyramidal horn with a Brewster angle. A circular window plate supporter is intended to balance the stress distribution during diamond-metal brazing. The measurement shows transmission coefficient is better than 0.85 within a wide bandwidth of 0.325-0.5 THz. This letter provides a practical way for a single diamond window structure for THz TWT amplifiers.

Complete analysis and design for a Q/V band (46‐52 GHz ) wideband analog predistorter

This article shows the complete analysis and design for a Q/V (46-52 GHz) band wideband analog predistorter (APD). First, the reflection coefficient for hybrid coupler's through port with complex impedance is corrected. Second, the acceptable error region is introduced to analyze the influence of different nonlinear loads. Levenberg-Marquardt algorithm is then used to solve the circuit parameters of the hybrid T matching network. Finally, a Q/V band (46-52 GHz) wideband APD is fabricated and measured. The measurement results are in good agreement with the analysis results. Furthermore, the linearized traveling-wave tube amplifiers (TWTA) gain compression is less than 0.5 dB, and the phase change of TWTA after linearization is less than 4°. The C/IM3 shows more than 10 dB improvement.

Meander-Line Slow-Wave Structure on Dielectric Substrate for a Millimeter-Band Traveling-Wave Tube

A novel planar microstrip meander-line slow-wave structure (SWS) on dielectric substrate for a miniature low-voltage millimeter-band traveling-wave tube (TWT) with a high-aspect-ratio sheet electron beam is proposed. Main electromagnetic parameters of the SWS were studied. Using of such a slow-wave structure may lead to increase of the gain and output power of the TWT amplifier.

Systematic Design Study of the Elliptical Sheet Beam Electron Gun for Terahertz TWT Applications

The design methodology of a sheet beam electron gun has been discussed and, subsequently, used for the design of a gun intended for use in a 0.3 THz traveling wave tube amplifier. The initial gun design parameters have been calculated analytically using a theoretical approach. Then, starting with these calculated parameters, the designed gun is then simulated using a commercial 3-D simulation code “CST Particle Studio” and optimized to get the target beam parameters. The circular-shaped cathode with an elliptically shaped beam-focusing electrode and elliptically shaped anode has been used to produce the good laminar elliptical sheet beam. The effect of varying the several parameters, such as height, width, and distance of focusing electrode, cathode-anode distance, and anode height have been studied to investigate their sensitivity on the target beam parameters. The simulation results predict that the designed sheet beam electron gun produces an elliptical beam with a desired beam cross section of 0.33 mm × 0.066 mm at a beam-waist position of 6.61 mm from the curved cathode emission surface corresponding to the beam parameters of 12 kV beam voltage and 55 mA beam current.

Broadband linearizer based on equivalent power‐dependent impedance function of diode and load match network

AbstractThis paper presents a broadband linearizer based on equivalent power‐dependent impedance function (PDIF) of diode and load match network (LMN). A method to measure the PDIF is proposed, then the broadband LMN can be synthesized by using the PDIF and the transfer characteristic of the amplifier to realize a broadband linearizer. To verify the proposed method, a linearizer for a 100 W traveling wave tube amplifier (TWTA) is designed and measured from 29 to 31 GHz. By using the linearizer, the output 1‐dB compression point (OP1dB) has increased by more than 1.2 dB, and phase error has reduced by more than 16.9°. Then a two‐tone signal with 100 MHz spacing is used for test, the results show that 12.2 to 14.6 dB suppression of third‐order intermodulation distortion (IMD3) is achieved over the working frequency range without bias tuning.

The Development of broadband millimeter-wave and terahertz gyro-TWAs

The gyrotron travelling wave tube amplifiers (gyro-TWAs) presented in this paper can operate with high efficiency (30%), huge powers and wide bandwidths at high frequencies that no other amplifier can provide. In principle, this is a technology that can be scaled to &gt;1 THz and operate with 20% bandwidths. Resonant coupling of two dispersive waveguide modes in a helically corrugated interaction region (HCIR) can give rise to a non-dispersive eigenwave over a wide frequency band. The synchronism between the ideal wave and an electron cyclotron mode, either fundamental or harmonic, of a large orbit electron beam contributes to the broadband amplification. An electron beam of 55 keV, 1.5 A with a velocity pitch angle of ~1 generated by a thermionic cusp gun is used in our 100 GHz gyro-TWA experiment, which achieves an unsaturated output power of 3.4 kW and gain of 36–38 dB. The design and experimental results of the many components making the gyro-TWA will be presented individually and then the whole system will be introduced. The amplification of a swept signal by the W-band gyro-TWA is demonstrated showing its capabilities in the field of telecommunications. Furthermore, the design studies of a cusp electron gun in the triode configuration and the realization of a 3-fold HCIR operating at 372 GHz will also be displayed.

A reconfigurable analog predistorter using tunable impendence matching network

A reconfigurable analog predistoter (APD) based on tunable impedance matching network (TIMN) is proposed in this brief. Firstly, a design method to specific gain and phase conversions is introduced. The equivalent circuit parameters of the impedance matching network (IMN) can be computed for the desired gain and phase conversions. To simplify the design, these parameters can be transformed to the terminal impedance of the IMN. Then the complicated nonlinear circuits design can be transformed to the IMN design. Secondly, a reconfigurable APD using the proposed method is presented for high tuning capability. By designing the coverage area of the TIMN, an experimental reconfigurable APD for traveling-wave tube amplifiers (TWTAs) & solid-state power amplifiers (SSPAs) is designed and fabricated. The fabricated APD shows more than 21 dB tuning range of gain conversion and 114° tuning range of phase conversion in 5.7–6.3 GHz. Good linearity improvement has been obtained for a 45 W C-band TWTA and a 5 W SSPA only by changing the APD bias voltage.