In Finite-Difference Time-Domain (FDTD) simulations, source injection needs to be smoothly done in order to suppress the undesired high frequency components excited during turn on. To visualize this problem, we provide two cases with and without turn-on source injection. On the right simulation, total field/scattered field injection is done using the cosine excitation without any smooth turn-on which effectively mimics the step function operation. This results in the injection of high frequency components creating the ripples and fluctuations in the signal propagating in the domain. On the contrary, the left simulation has a smooth turn of by using a half Hanning window ramp-up which effectively acts as low pass filter for the injected source. As a result, the signal propagating in the domain is free of high frequency components, hence no ripples and fluctuations.
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