Second- and Third-order Signal Predistortion for Nonlinear Distortion Suppression in a Traveling Wave Tube

The nonlinearity inherent in the Traveling Wave Tube (TWT) amplifier distorts amplified signals and results in reduced efficiency and bandwidth, limiting its use in communication and electronic countermeasure applications. Signal predistortion is an effective technique for suppressing nonlinear distortion in TWTs that provides high suppression and requires simple circuits for implementation. While conventional presitortion linearizers are based on third-order intermodulation (3IM) injection, this paper proposes using second-order (secondharmonic) signal injection in predistortion circuits. A detailed experimental investigation and comparison of second-order vs. third-order signal predistortion is presented. It is observed that both schemes result in suppression of up to 30 dB (55 dBc) for the 3IM distortion products. However, experimental results indicate that second-harmonic signal injection performs better than 3IM in suppressing higher-order products. The paper also investigates spatial evolution of the wave spectrum along the TWT axis with and without injection, and sensitivity of the suppression to injected signal’s amplitude, phase and the fundamental frequency.