Traveling-wave tube amplifier performance evaluation and design optimization for applications in digital communications with multilevel modulations

In this paper, we demonstrate the use of a power margin as a figure-of-merit for evaluating the performance and optimizing the design of traveling-wave tube amplifiers (TWTAs) used in digital communication applications with multilevel modulations. The power margin is a system-level measure that balances both device efficiency and nonlinear distortion and provides a more direct prediction of the system-level performance of power amplifiers than device-level measures such as device efficiency or error-vector-magnitude. We calculate the power margin for M quadrature amplitude modulation for an existing TWTA to demonstrate the setting of an optimal amplifier operating drive level according to the criterion of the maximum power margin. The power margin can also be used to compare the performance of different traveling-wave tube (TWT) configurations. We compare the calculated power-margin performance for helix TWT circuits optimized with different optimization goal functions using the helix TWT design code CHRISTINE. The goal functions used in the optimization of the TWT circuits include AM/PM optimization, complex gain optimization, efficiency optimization, and a new digital goal function optimization. The digital goal function is shown to provide an enhanced power margin compared to the other three goal functions and demonstrates the potential of TWT device design optimization from a system perspective. Index Terms--Communication systems, microwave circuit optimization, microwave power amplifiers (PAs), nonlinear distortion, satellite communication, traveling-wave tubes (TWTs).