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To efficiently simulate modulated electromagnetic fields, the complex envelope (CE) concept has been adapted into the FDTD method. However, such formulated CE FDTD has been found inferior to the conventional FDTD method and of little practical values due to the CFL stability condition. Therefore, attempts have been made to adapt the CE concept to the unconditionally stable ADI-FDTD method. In this paper, the full-wave CE ADI-FDTD formulations are presented with the unconditional stability. Numerical dispersion analysis and numerical experiments are performed, showing that the CE ADI-FDTD is superior to the ADI-FDTD and is comparable to the conventional FDTD in terms of accuracy. Therefore, it offers an alternative time-domain modeling technique for RF/microwave structures especially for modulated signals.