[[Image:FDTD134.png|thumb|320px|EM.Tempo's Periodicity Settings dialog]]
[[Image|FDTD140(1).png|thumb|320px|Setting a custom plane wave source plane.]]
[[Image:FDTD139.png|theumb|320px|Placing a field probe above a periodic structure excited by an obliquely incident plane wave source.]]
===Setting Up A Periodic Unit Cell===
One of the pitfalls of the direct spectral FDTD method is the possibility of horizontal resonances, which may lead to indefinite oscillation or even divergence of field values during the time marching loop. This happens in the case of oblique plane wave incidence when θ > 0°. [[EM.Cube]]'s FDTD engine automatically detects such cases and avoids those resonances by shifting the modulation frequency of the modulated Gaussian pulse waveform away from the resonant frequency. However, in some cases, the size of oscillations may still remain large after a large number of time steps. Occasionally, a late-time diverging behavior may appear. To avoid situations like these, it is highly recommended that you place a time-domain field probe above your structure and monitor the temporal field behavior during the time marching loop as shown in the figure below.
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{{twoimg|FDTD140(1).png|Setting a custom plane wave source plane|FDTD139.png|Plane Wave visualization in the scene.}}
===Reflection & Transmission Characteristics===