Broadband, Ultra-linear, Extremely High Frequency (EHF) Traveling Wave Tube Amplifier

Power amplifiers that provide both high linearity and high efficiency are essential for the transmission of high data-rate, bandwidth efficient signals used in MILSATCOM and avionics communications. Achieving this objective at millimeter-wave frequencies remains particularly challenging. Linearizer Technology, Inc. (LTI) in conjunction with L-3 Communications Corporation, Electron Devices Division (L-3 EDD) is proposing to develop a TWTA based transmitter including a block up-converter/linearizer (MPM-T) covering the frequency from 43.5 to 45.5 GHz with a linear output power>50 watts and an associated overall efficiency>30%. This transmitter will have a weight<15 lbs and a volume<0.5 cu. ft. when housed in an air-cooled, weather resistant package suitable for antenna mounting. Transmitter"s linear performance will be based on supporting Quadrature Amplitude Modulation (QAM). A transmitter goal will be an amplitude modulation/phase modulation ratio (AM/PM) of less than 1.5 degrees /dB from small signal up to saturation across the band. This work will be based on L3-EDD"s success in producing Q-band microwave power modules (MPMs) and LTI"s experience in producing linearizers for use at Q-band and with QAM at lower bands. In the first phase LTI designed a linearizer, optimized for operation with EDD"s EHF MPM"s TWT with QAM modulation. Simulation and testing showed this linearized MPM could achieve an AM/PM of<1 degrees /dB, and with 16QAM a spectral regrowth of<TBD at 2 dB OPBO and an error vector magnitude (EVM) of TBD at 2 dB OPBO. The L-3 EDD multi-collector miniature EHF TWTA produced for Phase I achieved a DC efficiency of>40%. BENEFIT: Modern communications systems use complex modulation schemes, such as QAM, to increase bandwidth utility (efficiency in bit/Hz). These bandwidth efficient signals have a non-constant envelope with high peak-to-average power ratios (PAR). Power amplifiers made for these modulations must be highly linear in order to minimize distortion that degrades adjacent channel signals and signal transmission quality (bit error rate). To achieve the required level of linearity, it is common practice to operate amplifiers at a reduced power level or high output power backoff (OPBO). The consequences of this approach are a very low efficiency, over-sized and heavy transmitters/amplifiers and power supplies, extra unnecessary heat generation, and higher costs. At millimeter-wave these factors can make a communication system impractical to implement, particularly for power-consumption-sensitive applications such as SATCOM and avionic/mobile communication platforms. This project will ultimately provide a compact EHF linear high power transmitter, which currently does not exist. It will result in greater millimeter-wave power with reduced size, weight, cost and prime power over that available with current technology. The benefits multiply with the need for linearity. The greater the linearity required, the greater the return. The target linearity of the proposed transmitter will improve upon current SATCOM specifications. Greater than a two to three times increase in effective efficiency over present millimeter-wave power amplifiers is anticipated. This technology also has value for both space-borne as well as ground-based systems. The resulting MPM-T will be of great value in all Q-band/EHF communication systems where bandwidth efficiency and power consumption is of concern. Portable, mobile and airborne communications systems will be prime candidates for integration of this technology.