For a fixed frequency <math>f_0</math> and a fixed incidence angle <math>\theta_0</math>, the resonant frequency is reduced to:
<!--Using "\mathbf" below because it tricks the wiki to rendering with PNG instead of html-->:<math> f_{res} = f_0 \mathbf{\sin } \theta_0 </math><!-->[[Image:FDTD102.png]]-->
The modulated Gaussian waveform must be chosen such that its effective bandwidth avoids the horizontal resonant frequency. Otherwise, the temporal response of the structure starts to oscillate, and the time marching loop will not converge. To avoid this problem, the modulation frequency and bandwidth of the waveform are chosen to satisfy the following condition: