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

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 frequency, 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. Triton’s preferred 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. One successful Ku Band amplifier was able to increase the linear operating range to within 2 dB of saturation, a 2½ times increase in usable (linear) power. Distortion levels of AM/PM ≤ 2º/dB and AM/AM ≤1.2 dB/dB were obtained at 70 watts of output power and below. 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 proposal 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. Triton Services Inc. has had over 40 years of experience in manufacturing TWTs, TWTAs, MPMs, and other related sub-systems. Based on our many decades of experience and previous millimeter-wave developments, we propose to create an ultra-linear EHF TWTA combining a pre-distortion linearizer, a specially designed TWT using advanced fabrication techniques and a custom power conditioner which feeds the active components. During the Phase I, we will design and demonstrate the design elements necessary to produce the ultra-linear EHF TWTA. Preliminary calculations based on TWT theory and various design codes will be employed to determine optimum design parameters for the EHF TWT. Several possible mechanical configurations and thermal attachments for the TWT RF circuit will be evaluated for manufacturability cost effectiveness and ability to remove heat from the beam interaction region. The linearizer requirements will be developed working in concert with our long-time business partner, Linearizer Technology Inc. (LTI). LTI has been the premier supplier of advanced linearizers to Triton, and the entire TWT and Satcom communities. Recent advances in the state of the art at LTI have resulted in several linearizers in Q Band, including the 43.5 to 45.5 GHz segment. Phase I will define a working linearizer specification based on the modeled complex gain parameters of the EHF TWT. Initial costs for the linearizer would also be obtained. Based on the operating voltages and currents predicted for the EHF TWT and the linearizer, a new Power Conditioner specification will be developed. This specification will be reviewed with our various power supply vendors; 5-Star Associates, Brandt Electronics or DB Control, for feasibility and cost. While the exact final TWT operating voltages may change as the design progresses, this will most likely represent only minor impact to the overall power conditioner. Cost and technical content of the power conditioner should be quite similar to a final design . In Phase II, we will develop and manufacture a fully functioning ultra-linear EHF TWTA prototype. Based on the EHF TWT design work and construction technique investigations, necessary high-precision tooling will be designed and procured. The circuit assembly method and precise alignment fixturing will be developed and verified. Several TWT iterations are expected. Using test data from a successful EHF TWT, specifications for the power conditioner and linearizer will be finalized and initial units procured. The assembled components will then be optimized and tested to characterize and verify linear performance by a variety of methods. BENEFIT: Severe crowding of the radio frequency spectrum can be mitigated and bandwidth can be increased to satisfy DOD requirements by utilizing operating frequencies between 20 – 50 GHz. Significantly increased data delivery rates and transmission of high resolution imagery can be achieved in tactically useful time to multiple users when operating in the EHF region of the spectrum. These benefits will be enjoyed in applications such as bandwidth intensive communications and high speed data links, imagery, intelligence information, missile warning systems, mapping and geodesy data, meteorological and oceanographic data. It is prudent to note, Triton ETD design techniques and mechanical configurations are easily adaptable to adjacent frequency bands between 18 – 50 GHz. Thus, as interest in using other regions of the EHF band increase, a “tunable” product will already be available. Other potential applications are as follows: Additional near-by bands TWTA and MPM implementations. Jamming – higher freq + techniques requiring linear behavior Small Radars UAV mm wave jamming – adaptable to wide band for ECM