In general, there are various ways of evaluating the multi-tone behavior of a TWT. Time-domain simulation tools such as CST Particle Studio's particle-in-cell solver, impose no estrictions on the input signals and, thus, allow to study, e.g..memory effects and stability. However, they are very slow compared to frequency-domain methods and therefore only suitable for validation. An alternative proprietary time-domain ool [2] specialized on TWTs is currently under development at Thales. While it is significantly faster than general purpose tools, a full simulation of, e.g., realistic QPSK modulated signals on multiple carriers would still take many hours.