Broadband, Ultralinear, Extremely High Frequency TWTA


Achieving high power, efficiency, and linearity in airborne amplifiers in the microwave and millimeter wave frequency bands is important to the future of many Air Force programs, particularly for communications applications. RF power levels required for these high frequencies, high data rate link budgets have not been effectively satisfied by the use of solid state amplifiers. The Traveling Wave Tube (TWT) due to its simultaneous frequency-bandwidth-power characteristics has been the answer to many military and commercial data link requirements. As military and commercial data links become more sophisticated, complex modulations (such as Quadrature Amplitude Modulation QAM and others) are desired for a variety of reasons. The amplification of these modulated signals demands that the amplifier be operated in a highly linear fashion in order to preserve phase and amplitude content without distortion, an increase in Bit Error Rate (BER) and/or adjacent channel interference. A Typical TWT has 6 dB gain compression and about 40 degrees of phase change at saturated power out. Thus, in order to operate in a fully linear fashion, a TWT would have to be operated at or near the small signal region. An output power back off (OPBO) from saturation of 6 dB would not be uncommon. This is a huge disadvantage in term of device efficiency and link power. The most recent answer to this dilemma has been to create linearized TWT Amplifiers (TWTAs). Linearization can be accomplished by several methods, both internal and external to the TWT. One method is to utilize a pre-distortion linearizer as a pre-amplifier to the TWT. The linearizer is constructed with gain and phase distortion complementary to the distortion of the TWT. The combined performance effectively increases the linear operating range of the amplifier. Up until now there have been few, if any, highly linear amplifiers in the range of 43.5 to 45.5 GHz and at this medium power level. The three major thrusts of this project are directed at answering that need. The development of a basic TWT-type vacuum device which is capable of producing 50 watts at 43.5 to 45.5 GHz. A method to overcome the inevitable linearity deficits of the TWT. A Power Supply and associated support elements required to create a workable ultra-linear TWTA.  Bottom of Form


* Acknowledgements: This work has been supported by Triton Electron Technology Division INC./Air Force Research Lab (AFRL).