Evolution of Space Traveling-Wave Tube Amplifier Requirements and Specifications for Modern Communication Satellites

Complex nonconstant envelope high-order digital modulation schemes used in modern communication satellites bring new requirements for the linearity and power efficiency of the on-board traveling-wave tubes (TWTs). As a result, a reassessment of typical TWT specifications is required. A digital system-level figure-of-merit, the total degradation, is proposed. It accounts for the characteristics of any coded signaling scheme, as well as for the performance of the overall communications link, including cochannel and adjacent channel interference. This figure-of-merit allows for reoptimizing the operating point of currently used TWTs, comparing the end-to-end performance of different tubes, and revising the standard TWT linearity-versus-efficiency requirements based on carrier-to-third-order-intermodulation (IM) ratio (C/IM3), noise power ratio, and carrier-to-noise-plus-IM ratio [C/(N + I)].     

Nonlinear Memoryless Channel Modeling Based on Wavelet

1introductionSeveralnonlinearchannels(e.g.,satellitecommunicationchannels)I'lareequippedwithmemorylessnonlineardevicessuchasTWTamplifiers.Thesedevicesexhibittwokindsofnonlinearities,amplitudedistortion(AM/AMconversion)andphasedistortion(AM/PMconversion).SeveralanalyticalmodelssuchasBesselandrationalfunctionshavebeenproposedforthesenonlineardistortionsl2-41.However,thesemodelsonlysuiteTWTcharacteristicswithspecificasymptoticbehaviors,andachangeintheTWTcharacteristicscannotbetracked.Ref.…     

Development of linear traveling wave tubes for telecommunicationsapplications

Traveling wave tubes (TWTs) designed for telecommunications applications in multichannel power amplifiers (MCPAs) are required to have high linearity, low intermodulation distortion, high efficiency and high power outputs. The linearity of the TWT can be greatly improved by operation of the tube significantly backed off from saturation and by optimization of the design of the helix circuit. This results in two tone intermodulation products that are 25 to 40 dB below the carrier level, depending on the amount of back-off selected. However, operation backed off from saturation results in a greatly reduced efficiency of the TWT, which must be compensated for by optimal circuit and collector design. Several L-band and S-band Hughes TWTs have been developed for telecommunications applications and feature saturated power levels of up to 2 kW and average power of over 600 W with overall efficiencies of over 20% at 10 dB back-off and 40% at 6 dB back-off. These tubes provide high average power, high efficiency amplification with modest size and reduced cooling requirements compared to solid state amplifiers     

Traveling-wave amplifiers and backward-wave oscillators for VHF

An important aspect of the design of traveling-wave amplifiers and backward-wave oscillators for frequencies below 500 mc is the problem of obtaining a tube of reasonably small physical dimensions. Hollow beams of greater perveance than is obtainable with solid beams offer one method of reducing the size of such tubes by permitting operation at a lower voltage and greater gain per wavelength, for a specified beam power, than is possible in a solid beam tube. Some aspects of the design of minimum size hollow-beam forward-wave amplifiers using single helix circuits and backward-wave oscillators using bifilar helix circuits are presented. Several tubes of these types for operation below 500 mc have been built. Amplifier bandwidths and oscillator tuning ranges in excess of four to one in frequency have been obtained experimentally. Amplifier efficiencies in excess of 20 per cent and oscillator efficiencies in excess of 10 per cent have been achieved.     

螺旋线行波管中功率洞问题的研究

宽带螺旋线行波管在一较窄的频带内出现饱和输出功率凹陷的现象被称为功率洞。功率洞对于放大器所应用的系统,特别是通信系统的整体性能会产生严重影响。本论文分析了宽带螺旋线行波管中功率洞产生的原因,介绍了一种解决功率凹陷的办法—渐变磁场法,并应用二维大信号注波互作用程序进行仿真,研究结果表明：采用渐变磁场法可以在一定程度上有效地抑制功率凹陷,使宽带螺旋线行波管在工作频带内的饱和输出功率随频率变化的曲线较为平坦。     

Performance and Application of Slow Wave Circuits for Power TWT in Millimeter Wave

In millimeter wave range, high-power traveling wave tube (TWT) is a key device with broadband and high-power amplification. Basis of comparing traditional and new type slow wave circuits for high-power TWT in MMW, analysis and computation of propagating and RF performance for folded wave circuit are carried out. Theoretic calculation and experimental test show that folded wave circuit is an ideal slow wave structure for broadband and high-power TWT in MMW. There is extreme wide application for the circuit in high-power TWT. This development possesses important consult value to designer for MMW power amplifiers.     

From Frequency-Domain Physics-Based Simulation to Time-Domain Modeling of Traveling-Wave Tube Amplifiers for High Data-Rate Communication Applications

We report here on a methodology to use frequency-domain physics-based nonlinear simulations of traveling-wave tubes (TWTs) in block time-domain models. We present examples using an improved version of the large-signal code CHRISTINE and a two-block baseband time-dependent model with a feedback loop that takes into account reflections internal to the TWT. The novelty in this approach is that it enables us to relate digital performance to the physical characteristics of the device, and it has the potential to significantly impact the analysis of TWT amplifiers for high data-rate applications.     

Microfabrication and Characterization of a Selectively Metallized W-Band Meander-Line TWT Circuit

Vacuum electronic devices offer significant potential for increased power and performance at millimeter-wave frequencies. However, new approaches are required to reliably manufacture the miniature electromagnetic circuits used at these high frequencies. In this paper, we describe the design, fabrication, and testing of an innovative meander-line slow-wave structure for a W-band traveling-wave tube (TWT). The unique challenge of metallizing only the top of a high-aspect-ratio serpentine dielectric ridge using conventionally planar microfabrication techniques is overcome using a novel selective masking and metallization process. The procedure is demonstrated by fabricating a W-band meander-line circuit for a 10-W continuous-wave TWT. Cold-test S -parameter measurements are presented.     

Use of a simple external nonreciprocal attenuator in coupled-cavity TWT's

It is well known that use of a nonreciprocal sever in traveling-wave tube (TWT) design offers several advantages. Previous attempts to obtain these advantages involved incorporation of ferrite material within the envelope of helix TWT's. These attempts have encountered serious difficulties, and have had no apparent effect on TWT design practice. A different approach to realization of a nonreciprocal sever for a coupled cavity TWT is described. This approach involves readily available wave guide components external to the tube, and does not appear to require the solution of significant research and development problems. A computer simulation performed on the CTS 200 W TWT design indicates that a substantial improvement in efficiency could be obtained through use of this technique.     

Design of high-efficiency wide-bandwidth coupled-cavitytraveling-wave tube phase velocity tapers with simulated annealingalgorithms

The output circuit section of a traveling-wave tube (TWT) routinely contains an RF phase velocity taper for the purpose of increasing RF output power and efficiency. By slowing the RF phase velocity in approximate synchronism with the decelerating electron beam bunches, the taper increases power transfer from the beam to the RF wave. Recently, the computational optimization technique of simulated annealing was shown to be very effective in the design of an RF phase velocity taper that significantly increased computed RF power and efficiency of a coupled-cavity TWT. In this paper, two new broadband simulated annealing algorithms are presented that optimize (1) minimum saturated efficiency over a frequency bandwidth and (2) simultaneous bandwidth and minimum efficiency over the frequency band with constant input power. The algorithms were incorporated into the NASA 2.5-dimensional (2.5-D) coupled-cavity TWT computer model and used to design optimal phase velocity tapers using a 59-64 GHz coupled-cavity TWT as a baseline model. Compared to the baseline taper design, the computational results of the first broadband algorithm showed an improvement of 73.9% in minimum saturated efficiency. The second broadband algorithm indicates an improvement of 272.7% in minimum RF efficiency with constant input power drive and an increase in simultaneous bandwidth of 0.5 GHz over that calculated for the baseline TWT     

Neural networks for modeling nonlinear memoryless communicationchannels

This paper presents a neural network approach for modeling nonlinear memoryless communication channels. In particular, the paper studies the approximation of the nonlinear characteristics of traveling-wave tube (TWT) amplifiers used in satellite communications. The modeling is based upon multilayer neural networks, trained by the odd and even backpropagation (BP) algorithms. Simulation results demonstrate that neural network models fit the experimental data better than classical analytical TWT models,     

Experimental Investigation of a Novel Circuit for Millimeter-Wave TWTs

A novel folded waveguide circuit that features thick iron pole pieces with hollow centers was built as part of a periodic-permanent-magnet-focused W-band single-stage test-vehicle traveling-wave tube (TWT). These hollow centers, which comprise part of the slow wave circuit, increase the rms axial field and significantly reduce the unwanted transverse field imbalance. For this TWT, a tetrode gun that creates an ultralaminar 20-kV 0.25-A nominal electron beam was used. It was demonstrated that this gun and magnetic structure can provide greater than 97% beam transmission for peak beam power levels as high as 9.25 kW (25 kV, 0.37 A). The unplated circuit, operating around 91 GHz on the edge of a passband, exhibits between 10 dB and 12 dB gain that compares favorably with results of device modeling utilizing the 3D particle-in-cell code Magic3D. Using a feedback approach to characterize large-signal operation, the tube generated 40 W of regenerative oscillator power. Design-optimized versions of this circuit show promise of enabling W-band TWT amplifiers that provide up to 300 W of peak RF output power     

Design of Overmoded Interaction Circuit for 1-kW 95-GHz TWT

Advanced simulation tools were used to design major components for a 95-GHz traveling-wave tube (TWT) providing approximately 1-kW of average power. This output power would be challenging, if not impossible, using a fundamental mode circuit TWT design, because of beam-current density and thermal-mechanical-stability issues. We addressed these limitations using an overmoded interaction circuit. The program investigated the circuits' thermal and electrical performance, including the effect of multiple-propagating modes and spurious oscillations. We also completed the designs for the electron gun and periodic permanent magnet. The design results for the 1-kW 95-GHz TWT are promising.     

Efficiency Enhancement Of Helix Traveling Wave Tube Based On εMulti-Objective Evolutionary Algorithm

A two-dimensional non-linear helix traveling wave tube (TWT) theory in frequency-domain is described in this article and the theory is integrated with ε Multi-Objective Evolutionary Algorithm(MOEA). The efficiency optimization of a helix TWT with phase taper has been done. The optimization results was verified by a three dimensional nonlinear model. The results show the feasibility and reliability of the optimization algorithm and the two-dimensional nonlinear theory. The integrated program can provide calculable parameters for the design of helix TWT. It can also accelerate development process and reduce test costs.     

UC3901在行波管收集极电源中的应用

介绍了行波管的性能,阐述了收集极电源的特点,论述了UC3901在行波管收集极电源中的应用。     

Traveling-wave tube efficiency improvement with textured pyrolytic graphite multistage depressed collector electrodes

The improved efficiency of a multistage depressed collector (MDC) with ion-beam textured pyrolytic graphite electrode surfaces is demonstrated by comparison with the performance of other MDC's tested with the same microwave rf amplifier traveling-wave tube (TWT). Because of the sensitivity of overall TWT efficiency to collector efficiency, significant improvements in TWT performance are possible with the use of textured pyrolytic graphite MDC's.     

Ultra-high speed modulation-doped field-effect transistors: atutorial review

A tutorial review on the modulation-doped field-effect transistor (MODFET) and its application to ultra-low-noise, medium-power, and ultra-wide-band traveling-wave amplifiers as well as ultra-high-speed digital logic circuits is presented. It is believed that with further advances in material growth and device scaling significant improvements in cutoff frequencies, switching speed, noise, and power will be achieved in the near future     

Modern multistage depressed collectors—A review

Well performing multistage depressed collectors (MDC's) have been developed for traveling-wave tubes (TWT) in an sizes fitting the requirements of wide-band countermeasures, radars, and ground and space communications. In the case of NASA-type symmetric collectors, rigorous analytical tools were developed that permit accurate a priori designs of MDC's for any given helix-type tube. Although only few verified cases of MDC's augmenting communications klystrons are known, the resulting performance and available theories indicate significant gains in reducing power consumption by TV and other klystron amplifiers without the deleterious effects of backstreaming.     

A high-power wideband traveling-wave tube with stepwise changes in the diameter of the drift channel

Results of the design of a high-power wideband (in the band 8–18 GHz) traveling-wave tube (TWT) with stepwise changes in the diameter of the drift channel are presented. The TWT is stable against self-excitation by a backward wave at high values of the accelerating voltage. It is shown that the output section of the device can be increased by a factor of 1.5 and the operating current can be raised by a factor of 1.3. These increases are attained owing to the 1.27-GHz diversity of the π-type frequencies of sections with different values of the channel diameter and owing to growth of the starting current corresponding to self-excitation by the backward wave. Application of the TWT with the interaction space that has steps of the channel diameter improved the electron efficiency by a factor of 1.2 and increased the output power by a factor of 1.6.     

Ultra-High Power and Efficiency Space Traveling-Wave Tube Amplifier Power Combiner With Reduced Size and Mass for NASA Missions

In the 2008 IEEE Microwave Theory and Techniques Society International Microwave Symposium Digest version of our paper, recent advances in high power and efficiency space traveling-wave tube amplifiers for NASA's space-to-Earth communications are presented. The RF power and efficiency of a new $K$-band amplifier are 40 W and 50% and that of a new $Ka$-band amplifier are 200 W and 60%. An important figure-of-merit, which is defined as the ratio of the RF power output to the mass (W/kg) of a traveling-wave tube (TWT), has improved by a factor of 10 over the previous generation $Ka$-band devices. In this paper, a high power high efficiency $Ka$ -band combiner for multiple TWTs, based on a novel hybrid magic-T waveguide circuit design, is presented. The measured combiner efficiency is as high as 90%. In addition, at the design frequency of 32.05 GHz, error-free uncoded binary phase-shift keying/quadrature phase-shift keying (QPSK) data transmission at 8 Mb/s, which is typical for deep-space communications, is demonstrated. Furthermore, QPSK data transmission at 622 Mb/s is demonstrated with a low bit error rate of ${hbox{2.4}}times {hbox{10}}^{-8}$, which exceeds the deep-space state-of-the-art data rate transmission capability by more than two orders of magnitude. A potential application of the TWT combiner is in deep-space communication systems for planetary exploration requiring transmitter power on the order of a kilowatt or higher.