On the physics of harmonic injection in a traveling wave tube

The physics of signal injection to shape the output spectrum in a traveling wave tube (TWT) is studied using an analytic solution to the approximate nonlinear S-MUSE model and with the large signal code LATTE. The results verify the long-standing conjecture that a frequency canceled by signal injection is composed of a component due to the injected signal and a component due to the nonlinearity of the TWT. Furthermore, the structures of the solutions are exploited to explain and predict behavior for various signal injection schemes. The scenarios studied include second harmonic injection to reduce the second harmonic and enhance the fundamental, multiple harmonic injection to eliminate the sensitive output power dependence on injected phase, second harmonic injection to reduce intermodulation distortion, and multiple signal injection to reduce the intermodulation spectrum. Insights are given regarding the effectiveness of an injection depending on whether the injected signal is within or outside the linear gain bandwidth of the TWT.