{{Note|Small values of inductance may result in the divergence of the FDTD numerical scheme. To avoid this problem, you need to increase the mesh resolution and adopt a higher mesh density. This, of course, may lead to a much longer computation time.}}
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== Defining Custom Waveforms ==
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[[Image:FDTD48A.png|thumb|700px|EM.Tempo's Excitation Waveform dialog showing a custom modulated Bessel temporal waveform.]]
In some time-domain applications, you may want to simulate the propagation of a certain kind of waveform in a circuit or structure. In addition to the default waveforms, [[EM.Tempo]] allows you to define custom waveforms by either time or frequency specifications for each individual source in your project. If you open up the property dialog of any source type in [[EM.Tempo]], you will see an {{key|Excitation Waveform...}} button located in the "Source Properties" section of the dialog. Clicking this button opens up [[EM.Tempo]]'s Excitation Waveform dialog. From this dialog, you can override [[EM.Tempo]]'s default waveform and customize your own temporal waveform. The Excitation Waveform dialog offers three different options for defining the waveform:
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* Automatically Generate Optimal Waveform
* Use Custom Frequency Domain Specifications
* Use Custom Time Domain Specifications
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The first option, which is also the default option, constructs an optimal modulated Gaussian pulse waveform based on your project's specified center frequency and bandwidth. This optimal waveform guarantees the most accurate frequency domain computations for your simulation. The second option gives you a choice of the three standard waveforms and lets you define their waveform parameters in terms of frequency domain characteristics like center frequency and bandwidth and spectral contents. The third option lets you define a completely arbitrary temporal waveform for your source.
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Select the third option of waveform definition and then choose the '''Custom''' option from the '''Waveform Type''' dropdown list. Enter a mathematical expression for your custom waveform a function of the time variable "T" or "t" in the box labeled '''Expression'''. You can use arithmetic operation, standard and library functions as well as you models and variables. For example, in the figure on the right, we have defined a modulated Bessel waveform in the form of "sin(2*pi*Fc*T-pi/4)*Jn(0,pi*Fc*T/4)", where Fc is the center frequency of the project and one of the project's default variables and Jn(n,x) denotes the Bessel function of the first kind and order n, which is one of [[EM.Cube]]'s library functions.
[[Image:Info_icon.png|40px]] Click here to see a '''[[List of Library Functions]]'''.
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[[Image:Info_icon.png|40px]] Click here to learn more about '''[[Parametric_Modeling,_Sweep_%26_Optimization#Creating_Custom_Functions_Using_Models | Creating Custom Functions Using Models]]'''.
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When you define a custom waveform in the Excitation Waveform dialog, make sure to click the {{key|Accept}} button of the dialog to make your changes effective. A graph of your custom waveform is plotted in the right panel of the dialog for your review. It is important to keep in mind that typical time scales in the FDTD simulation of RF structures are on the order of nanosecond or smaller. Using the variable "Fc" in the expression of your waveform definition usually takes care of this required scaling. Otherwise, you need to use scaling factors like 1e-9 explicitly in your expression.
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{{Note| If you define a custom excitation waveform for your source based on arbitrary time domain specifications, none of the standard frequency domain output data and parameters will be computed at the end of your FDTD simulation.}}
== Working with FDTD Simulation Data ==