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

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The power advantage of travelingwavetubes over solid-state poweramplifiers is significant, especiallyat the higher frequency bands.achievements is the use of a simulation-based designmethodology. This design methodology relies heavilyon the suite of recently developed codes for vacuumelectron devices [23]–[25].A coupled-cavity TWT uses a slow-wave circuitthat is mechanically and thermally more robust thana helix. A coupled-cavity structure is usually largerin size than a helix structure at the same frequency.It can, therefore, provide higher power at the samefrequency or operate at much higher frequencies thana helix TWT. One example is Communication PowerIndustries, Inc.’s Millitron coupled-cavity TWT [26]. Itis capable of producing 3 kW peak and 300 W averagepower at W-band.Current Status of Microwaveand Millimeter-Wave Power ModulesThe MPM [27], [28] is an example of what RobertSymons calls appropriate technology [29]. In the caseof a low noise, high efficiency compact microwaveor millimeter-wave power amplifier, the appropriatetechnology to use is solid state for the input amplificationand vacuum electronics for the output. Thisgain partitioning between solid state and vacuumelectronics is what defines the MPM. When marriedwith a miniaturized electronic power conditioner,a high-power-density RF amplifier module results,providing 100 W and higher in the microwavebands and 50 W and higher in the millimeter-wavebands. The combined performance in size, weight,High-VoltageModuleSSA2.9-mmInputControl28-VdcInputWRD-180OutputLow-VoltageBoardTWTFigure 6. 40 W Ka-band microwave power module withcomponents/subassemblies identified. SSA: solid stateamplifier. TWT: traveling wave tube. Microwave powermodule cover has been removed.efficiency, bandwidth, and noise level is unattainableusing either constituent technology alone. Dueto its small size and high efficiency, the MPM is particularlywell-suited for use in resource-constrainedapplications, including communication (wireless),radar, and electronic warfare.A millimeter-wave power module (MMPM), as thename suggests, is simply a MPM designed to operatein the millimeter-wave frequency bands. MMPMshave been developed at both Ka- and Q-band. Aphotograph of a first-generation Ka-band MMPM isshown in Figure 6. In the unit shown, a nominal 100mW solid-state gallium arsenide (GaAs) amplifierdrives a low-voltage (about 7 kV) compact vacuumpower booster helix TWT. The MMPM provides anoutput power of 40 W from 26 to 40 GHz (with 50 W inthe 30–31 GHz communication band) in a conductioncooledpackage of 7.5 in 3 8.5 in 3 1.25 in, weighing6 lbs. The module displayed operates from 28 V dcinput power, requiring 300 W (max); however, otherprime power formats, including 270 V dc and threephase115 V ac, are possible. All high voltage is selfcontainedwithin the module.Since its initial development in the 1990s, theMPM has made significant strides in both performanceand functionality. Second-generation Ka- andQ-band MMPMs currently under development atL-3 Communications Electron Devices provide twicethe output power (100 W) at nearly double the efficiency(33%) over the first-generation designs. Theseadvances are the result of the wholesale applicationof advanced modeling and simulation in componentdesign [23], [24], as well as the steady progressionin miniature multistage TWT collector technologyand incremental improvements in power conditioningefficiency. Interestingly, although the power of thesecond-generation MPMs and MMPMs has doubled,their size and weight have not increased accordingly.The result is an increase in RF power density, 1.4 3power density (W/volume) and 1.6 3 specific powerdensity (W/mass), over first-generation performance.The addition of a miniature predistortion linearizer[30] in the MPM RF chain results in an increasein rated linear power of two and one-half times(over operation without a linearizer) with a concomitantincrease in rated efficiency by a factor oftwo. Such improvements in linear performance areimportant for digital and multicarrier communicationapplications.Future directions in MPM and MMPM developmentpoint to the continued evolution of MPM performanceand functionality. In the power-frequencyrealm, wideband dual-mode operation covering theKa- and Q-communication bands in a single MPMwith a single TWT is envisioned. While movinghigher in frequency, compact W-band MPMs providing100 W for radar and communication are seen as42 December 2009Authorized licensed use limited to: US Army Research Laboratory. Downloaded on December 2, 2009 at 13:24 from IEEE Xplore. Restrictions apply.
enabling the exploitation of this region of the electromagneticapplication space.MPM functionality will also continue to increase.Most of this increase will come through higher levelsof integration, such as that exhibited by the MPMtransmitter (MPM-T). The MPM-T unit is a MPMassembly providing integral cooling, reverse andforward power sampling, linearization, harmonicand electromagnetic interference (EMI) filtering,serial interface control, and optional input signalup-conversion. The MPM-T, therefore, provides thesystem user a drop-in transmitter subassembly whilestill retaining all the desirable performance attributesof the MPM. A comparison of 75–100 W solidstate and MPM forced air-cooled Ka-band amplifiersshows the MPM-T to be a factor of two times smaller,lighter, and more efficient than an equivalent SSPA.Lastly, while it may come as a surprise to some, thereliability of the MPM-T assembly is expected to beon par with, if not better than, that of an SSPA. Theoutput power of some sample MPMs and MMPMs isshown in Figure 7.Current Status of Klystron AmplifiersKlystrons operate from UHF to millimeter-wave frequencies.They possess high gain, dynamic range,power, efficiency, and low noise, albeit with relativelynarrower bandwidths compared to TWTAs.Among the many applications for klystrons is theirlong use as the workhorse for high-power transmittersin terrestrial broadcast and satellite communications.Continued technical improvementshave further reaffirmed their dominance. In [31], byincorporating MSDC, the saturation efficiency for a2.4 kW klystron designed for direct broadcast satellite(DBS) band was raised from 24% to 40%. This im -provement can result in savings of over US$10,000in energy costs per year. This MSDC technology hasled Communications & Power Industries's (CPI’s)GEN IV klystron to capture 95% of the satellite communicationsuplink klystron power amplifier (KPA)market [32]. The typical available power for commercialsatellite communications klystron amplifiersis also shown in Figure 5.For many applications, a klystron is constrained byhigh voltage, limited bandwidth, and declining powerat high frequency. Two noteworthy technologies havebeen developed to address these issues: the extendedinteractionklystron (EIK) and the multiple-beam klystron(MBK).In the EIK, one or more of the klystron cavitiesare replaced by structures containing more thanone interaction gap. Compared to conventional klystrons,EIKs have a wider bandwidth and a higherpower level because of the distributed interaction.The most interesting development in the EIKhas been at the millimeter-wave frequencies [33].Future directions in power moduledevelopment point to the continuedevolution of microwave power moduleperformance and functionality.Power (W)2502001501005001.5 Octave0 10 20 30 40 50 60Frequency (GHz)Figure 7. Average output power of sample microwavepower modules (MPMs). The diamonds are MPMs withless than 0.5 octave bandwidth; the squares are MPMs withone octave bandwith; the triangles are MPMs with greaterthan 1.5 octave bandwidth.EIKs are available at frequencies from Ka-band toG-band. For example, an EIK is capable of deliveringan average power of 400 W at 95 GHz and 9 W (CW)at 218 GHz [34].MBKs offer the advantages of reduced beam voltage,reduced size and weight, and increased bandwidthwhen compared with single-beam klystrons.Multiple-beam devices have been developed extensivelyin Russia [35]. The highest level in compactnessmay well have been achieved by ISTOK (Russia),which developed a Ku-band MBK capable of delivering0.4 kW of peak power (133 W average) weighingonly 400 g, including the magnet.Advances in Wideband TWTA LinearizationDuring the past five years, there has been great progressin the linearization of TWTAs. Linearization isstill primarily by means of predistortion because ofthe wider bandwidth and higher efficiencies achievableby this form of linearization. Both analog anddigital (digital-signal-processing based) linearizationare now being applied to TWTAs. Digital linearizationoffers the advantage of near ideal transfer responsecorrection. A two-tone carrier-to-intermodulation ratioof greater than 50 dB can be achieved with a TWTA at3 dB output power backoff from saturation.The major constraint of digital-based linearizationis bandwidth. Although it is now possible to digitallyprocess signals over greater than 1 GHz of bandwidth,December 2009 43Authorized 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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