=== Excitation Waveform & Frequency Domain Computations ===
[[Image:FDTD48A.png|thumb|600px|EM.Tempo's Excitation Waveform dialog.]]
When an FDTD simulation starts, your project's source starts pumping energy into the computational domain at t > 0. [[Maxwell's Equations|Maxwell's equations]] are solved in all cells at every time step until the solution converges, or the maximum number of time steps is reached. A physical source has a zero value at t = 0, but it rises from zero at t > 0 according to a specified waveform. [[EM.Tempo]] currently offers four types of temporal waveform:
# Arbitrary User-Defined Function
[[Image:FDTD48A.png|thumb|650px|EM.Tempo's Excitation Waveform dialog.]]
A sinusoidal waveform is single-tone and periodic. Its spectrum is concentrated around a single frequency, which is equal to your project's center frequency. A Gaussian pulse decays exponentially as t → ∞, but it has a lowpass frequency spectrum which is concentrated around f = 0. A modulated Gaussian pulse decays exponentially as t → ∞, and it has a bandpass frequency spectrum concentrated around your project's center frequency. For most practical problems, a modulated Gaussian pulse waveform with EM.Tempo's default [[parameters]] provides an adequate performance.