Nonlinear space charge wave theory of distortion in a Klystron

We present a new view of nonlinear distortion in a klystron based on an analytically solvable nonlinear Eulerian model. The nonlinear contributions to the analytic solutions for the beam modulations are "nonlinear space charge waves" in the sense that they are produced by the nonlinear mixing of the linear space charge waves. For a single-frequency input, amplitude and phase distortion are shown to be results of "self-intermodulation" at the drive frequency, or mixing of harmonic distortions with the fundamental. The self-intermodulation contributions add out of phase with the linear space charge waves to produce gain compression and phase distortion. By comparing the results to a conventional large-signal Lagrangian model we find this physical picture is accurate for drive levels up to 1.2 dB of gain compression. For a two-frequency input we predict the third-order intermodulation distortion generation and suppression with the nonlinear space charge wave theory.     

Second- and third-order signal predistortion for nonlinear distortion suppression in a TWT

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 predistortion linearizers are based on third-order intermodulation (3IM) injection, this paper proposes using second-order (second-harmonic) signal injection in predistortion circuits. A detailed experimental investigation and comparison of second-order versus 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 signals amplitude, phase and the fundamental frequency.