=== Excitation Waveform & Frequency Domain Computations ===
When an FDTD simulation starts, your project's source starts pumping energy into the computational domain at t > 0. 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:
# Sinusoidal
# Arbitrary User-Defined Function
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.
The accuracy of the FDTD simulation results depends on the right choice of temporal waveform. [[EM.Tempo]]'s default waveform choice is a modulated Gaussian pulse. At the end of an FDTD simulation, the time domain field data are transformed into the frequency domain at your specified frequency or bandwidth to produce the desired observables.
{{Note|All of [[EM.Tempo]]'s excitation sources have a default modulated Gaussian pulse waveform unless you change them.}}
[[Image:Info_icon.png|30px]] Click here to learn more about [[EM.Tempo]]'s '''[[Basic_Principles_of_The_Finite_Difference_Time_Domain_Method#The_Relationship_Between_Excitation_Waveform_and_Frequency-Domain_Characteristics | Standard & Custom Waveforms and Discrete Fourier Transforms]]'''.
=== Defining Custom Waveforms in EM.Tempo ===