Copyright Â© 2009 IEEE. Reprinted from Microwave Magazine - L-3 ...

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Most microwave engineersfail to appreciate the capability,efficiency, and reliability of vacuumelectronics technology.made possible by the advances in devices based onwide bandgap semiconductor material (GaN), as wellas more efficient power combining techniques [10],[11]. As a result, the power advantage of tube-basedamplifiers over SSPAs is no longer obvious at lowermicrowave frequencies, although they still have betterefficiency than SSPAs. In response to the challengesand market demand, the last decade has seen significantefforts by the VE industry to develop devices withAverage Power (W)10 510 410 310 210 1PPM FocusedHelix TWTMicrowavePowerPPM FocusedCC TWTModules1 Solid-StateThree Terminal10 –110 –2 0.1Single Device Limits1 10 100Frequency (GHz)Figure 3. Performance domain for the different high-poweramplifier technologies: Traveling-wave tubes (TWTs)including helix and coupled-cavity types, microwave powermodules (a solid-state and TWT hybrid) and solid-statepower amplifiers. Also shown is the common developmenttrend for the different technologies: higher power andhigher frequency.P(kW)f (GHz)Δf (GHz)1,000100101TWT-PPMTWT-SolenoidKlystron-PMMMPMSSPA0.128 30 32 34 36 38 40 42 44 46f (GHz)Figure 4. The figure of merit Pf 2 (Δf/f) for selectedmillimeter-wave power amplifiers including traveling-wavetube (TWT) amplifier, millimeter-wave power module(MMPM), klystron, and solid-state power amplifier (SSPA).higher power and greater bandwidth at the millimeter-waveband. More than 1 kW of millimeter-wavepower can now be produced with high efficiency in arelatively small package [12].Common to both defense and commercial applicationsis the need for increased signal power to achievethe required signal-to-noise ratios over large transmissiondistances and large-signal bandwidth (which, inturn, calls for higher operating frequencies) to accommodatemassive volumes of data. To measure the abilityof the devices to transmit information, the figure ofmerit Pf 2 1Df@f2, where 1Df@f2 is the fractional instantaneousbandwidth of an amplifier, is plotted for a numberof amplifiers at or above 30 GHz in Figure 4. Thehigh mark of a 100 kW GHz 2 is achieved by a 35 GHzhelix TWT with a periodic permanent magnet for beamfocusing. Presently, about 1 kW GHz 2 is achieved by a35 GHz SSPA. The larger value, about 500 kW GHz 2 , forsolenoid focusing devices is because of their ability totransmit more current than devices with periodic permanentmagnet. However, solenoid magnets are bulkyand heavy and require a separate power supply andcooling. As will be discussed later, the ongoing researcheffort to use a spatially distributed beam, such as a sheetbeam, would enable the achievement of greater than500 kW GHz 2 in a smaller permanent magnet.Next, we will give a brief overview of the commontube amplifiers used in high-power transmitters. Onlythree types of devices [TWTs including helix and coupled-cavitytypes, microwave power modules (MPMs),and klystrons] will be covered, with emphasis on therecent advance in the millimeter-wave band. Referencesto other tube devices can be found in the many proceedingsof the IEEE International Vacuum ElectronicsConference, as well as in the special issues on vacuumelectronics of IEEE Transactions on Electron Devices. Thesepublications contain a significant number of papersfrom countries such as Russia, China, France, Japan,South Korea, Italy, and the United Kingdom.Current Status of TWTAsIn the design of high-power transmitters for airborne,space, and mobile applications, a strong emphasisis placed on minimizing the power consumption,applied voltage, size, and weight of the amplifiers. Forsuch applications, where instantaneous bandwidth isalso a requirement, helix and coupled-cavity TWTs arethe device types of choice.The main reasons for the continued reliance onvacuum electronic amplifiers such as TWTAs in spacebasedtransponders and ground terminals are TWTAs’high output power and high efficiency. The typicalavailable powers for commercial satellite communicationTWTAs and SSPAs are compared in Figure 5.The power advantage of TWTAs over SSPAs is significant,especially at the higher frequency bands. At thelower frequency bands, on the other hand, the TWTAs’40 December 2009Authorized licensed use limited to: US Army Research Laboratory. Downloaded on December 2, 2009 at 13:24 from IEEE Xplore. Restrictions apply.
grip on power is being challenged by SSPAs. For example,at C-band, commercially available modular SSPAsare capable of producing 1.5 kW of output power, and3.0 kW by phase-combining two systems [13]. However,these SSPAs are still relatively inefficient, withefficiency at less than 20%. Compared to a linearizedTWTA (LTWTA) producing similar RF power,the above SSPA would cost US$14,000 more in annualelectricity cost [14].Collector depression is a common technique forimproving the efficiency of TWTs [1]. Using a depressedcollector, one can convert some of the kinetic energy inthe spent electron beam (i.e., after its interaction withthe slow-wave circuit) into the potential energy of thepower supplies and, therefore, increase the overall efficiencyof the device [1, ch. 14]. Additional improvementcan be achieved by employing a multistage depressedcollector (MSDC) that can better match the energydistribution of the spent beam. The total efficiency ofstate-of-the-art space TWTs with a MSDC is over 70%[15] with typical TWTA efficiency between 50–60%. Thetypical SSPA’s efficiency is about 25–30% [16].The linearity of a power amplifier is always of greatimportance. For communication applications, theTWTA has long been considered as a device with poorlinearity as compared to the SSPA. The general belief isthat a TWTA must be backed off 3–4 dB from saturationto achieve the same level of linearity as an SSPA. This isan incomplete statement. It was pointed out in [16] thatthe power consumed by a TWTA is usually less thanthat of an SSPA with only half of the rated RF power. Asa result, for two amplifiers with the same total powerconsumption, a TWTA generally has more available linearpower than an SSPA for most of the frequency band.The two amplifiers have the same linearity performancefor lower-power and lower-frequency amplifiers only.Furthermore, the linearity of a TWTA can be improvedmuch more by use of linearization than that of an SSPA.As a result, more of the TWTA’s RF power that is lostdue to output backoff is available as linear power.Predistortion linearization is a simple and effectivetechnique for improving the performance of bothSSPAs and TWTAs [17]. Its effectiveness on TWTAs ismore significant because of the slow approach to saturationand the higher nonlinearity of the TWTA. It hasbeen shown that by applying fifth-order linearization,it is possible to achieve less than 1 dB overall gain compressionat saturation so that the combined transfercurve approaches that of an ideal linear limiter [18].Consequently, the need for backoff from saturation canbe greatly reduced to take advantage of the higher efficiencynear saturation.The effect of reduced efficiency by operating significantlybelow saturation to obtain linearity can bemitigated by reoptimizing the helix circuit for reducedgain compression and phase shift. It can also be mitigatedby reoptimizing the MSDC (both the collectorOnly vacuum electronic devices meetmany of the demanding requirementsfor reliable performance.Power (W)3,0002,0001,0000C X Ku DBSFrequency Banddesign and depressed voltages) to better match thespent beam distribution at large backoff operation.This will improve the collector efficiency and, therefore,the total efficiency. L-band and S-band TWTs at200–300 W output with 25–40% efficiency and eighttonecarrier-to-intermodulation ratio levels of 270 dBchave been produced with this technique. This performanceis a significant improvement over solidstatesystems having the same fidelity and twice theefficiency of SSPA [19].Recently, a new unexplored regime of TWT operation,based on the transverse interaction betweenan electron beam and a circularly polarized circuitwas investigated [20]. It was predicted that aC-band transverse TWT could be more efficient andmore linear—12 dB smaller value for the carrier-tointermodulationratio—than a conventional longitudinalTWT with a comparable saturated power. SuchTWTAs, if implemented, could meet the demandingrequirement for high-data-rate communication.The last decade has also seen a leap in the performanceof helix TWTs. These devices have criticalapplications in high-data-rate communications,high-resolution radar, and broadband electronic warfare.For example, in [21], a record helix TWT performancehas recently been reported at C- and X-band,with 25 kW and 8 kW peak power for radar applications.Compact (around 3.5 lb) helix TWTs at Ka-band(30 GHz) with 500 W continuous wave (CW) power,and 55% efficiency and at Q-band (44 GHz), with 230W CW power and 43% efficiency [22] have been demonstratedfor communication applications. A majorcontributing factor for the millimeter-wave TWTKaTWTASSPAKPAFigure 5. Typical power of commercial satellitecommunication high-power amplifiers including travelingwavetube amplifier (TWTA), klystron power amplifier(KPA) and solid-state power amplifier (SSPA).QDecember 2009 41Authorized licensed use limited to: US Army Research Laboratory. Downloaded on December 2, 2009 at 13:24 from IEEE Xplore. Restrictions apply.
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