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Before you can run an FDTD simulation, you have to define a source to excite your project’s physical structure. EM.Tempo offers a variety of excitation mechanisms for your physical structuredepending on your particular type of modeling problem or application:

# Ideal Source: A stand-alone localized voltage source with an internal resistance.# Lumped Source: An ideal source that must be place on a wire (a PEC line object) .# Distributed Source: A voltage source with a prescribed impressed field component that is defined on a rectangular region of space parallel to a principal plane.# Waveguide Source: A distributed source that must be placed across a hollow PEC box object.# Plane Wave Source: A distributed source with a plane wave profile defined using a virtual box object enclosing the entire physical structure . # Gaussian Beam Source: A distributed source with a complex-valued focused Gaussian beam profile defined using a virtual box object enclosing the entire physical structure . # Huygens Source: A distributed source defined based on know tangential electric and magnetic field components on the surface of a virtual box object.

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:

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 sinusoidal source does not have a finite energy and it does not decay as t → ∞. 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 does have has a bandpass frequency spectrum concentrated around your project's center frequency. For most practical problems, a modulated Gaussian pulse waveform provides an adequate performancewith EM. That is why this type of waveform is chosen by Tempo's default [[EM.Cubeparameters]] as your project's default waveformprovides an adequate performance.

When an FDTD simulation starts, your project's source starts pumping energy into the FDTD 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. If you use a Gaussian pulse or a modulated Gaussian pulse waveform to drive your FDTD source, after a certain number of time steps, the total energy of the computational domain drops to very negligible levels. At the point, you can consider your solution to have converged. If you drive your FDTD source by a sinusoidal waveform, the total energy of the computational domain will oscillate indefinitely, and you have to force the time loop to terminate after a certain number of time steps assuming a steady state have been reached.

An ideal source acts as a voltage source in series with a zero an internal resistance that can be placed between any two adjacent mesh grid nodes anywhere in the computational domain. To create a new ideal source, follow these steps: * Right click on the '''Ideal Sources''' item in the '''Sources''' section of the Navigation Tree and select '''Insert New Source...''' to open the Ideal Source Dialog.* You can change the default name of the source as well as its color. The ideal source is displayed as a small orange arrow in the Project Workspaceproject workspace.* By default, [[EM.CubeTempo]] creates a +Z-directed ideal source located at the origin of coordinates (0, 0, 0). You can change the location direction of the ideal source by setting new values for the X, Y and Z coordinates. When you use the spin buttons to increment or decrement the source coordinates, you can see the source moving in the project workspace. You can also change the '''Direction''' of the source from a dropdown list in the Source Location section of the dialog that contains ±X, ±Y and or ±Z options.* In the '''Source Properties''' section, you can specify the '''Source Amplitude''' in Volts and the '''Phase''' in Degrees.

A '''Lumped Source ''' lumped source is the most commonly used way of exciting a structure in [[EM.Cube]]'s [[FDTD ModuleTempo]]. A lumped source acts as a voltage source in series with is indeed an internal resistance ideal source that is must be placed between two adjacent mesh grid nodes on a line objectthat is parallel to one of the three principal axes and shows up as a small red arrow on the host line. Lumped sources are typically used to define ports and compute the port characteristics like S/Y/Z [[parameters]]. Lumped sources can also be place on line arrays. The property dialog of a lumped source has a drop-down list that contains the name of all the legitimate line object must be objects (i.e. lines that are parallel to one of the three principal axes) and line arrays. The '''Offset''' parameter of a lumped source is its distance from the start point of the host line. A lumped source by default is placed at the center of its host line. In other words, the default offset value is equal to half the length of the host line object.

{{Note|In order to create a lumped source, you must have at least one line object or line array in the project workspace.}} To create a new lumped source, follow these steps: * Right click on the '''Lumped Sources''' item in the '''Sources''' section of the Navigation Tree and select '''Insert New Source...''' to open the Lumped Source Dialog.* In the '''Source Location''' section of the dialog, you will find a list of all eligible line objects (i.e. lines that are parallel to one of the principal axes). Select the desired line object. The box labeled '''Direction''' shows the direction of the source with respect to the host line object. You have the option to select either the positive or negative direction for the source.* In the box labeled '''Offset''', enter the distance of the source from the start point of the line. A lumped source by default is placed at the center of the host line. In other words, the default offset value is equal to half the length of the host line object.* In the '''Source Properties''' section, you can specify the source '''Amplitude''' in Volts, '''Phase''' in Degrees and internal '''Resistance''' in Ohms.

Waveguide structures have many applications at microwave and millimeter wave frequencies. For example, a rectangular waveguide is used to feed a pyramidal horn antenna. A real waveguide structure is usually excited using some type of strategically located probe mechanism. This can be modeled using a lumped source placed on a wire structure made up of line objects. Alternatively, use can use [[EM.Cube]]'s Tempo also provides the '''Waveguide SourcesSource''', a special type of source that excites a prescribed TE₁₀ modal field distribution in a hollow rectangular waveguide structure. The scattering [[parameters]] are calculated from knowledge of incident and reflected fields at designated waveguide ports. Waveguide sources typically provide more accurate results for scattering [[parameters]] compared to lumped ports as they represent the actual dominant propagating modes at the transmission line ports. [[EM.Cube]] provides special waveguide sources that can excite either the TE_mn or TM_mn modes of a rectangular waveguide which is oriented along one of the three principal axes. In other words, the plane of the waveguide source must be parallel to one of the principal (XY, YZ or ZX) coordinate planes.

{{Note|In order to create define a waveguide source, you must have at least one "Hollow" Box hollow box object with no caps or only one end cap or a hollow box array in your project.}}

To create A waveguide source must be placed across a new rectangular waveguide sourcewhich is oriented along one of the three principal axes. In other words, follow these steps: * Right click on the '''Waveguide Sources''' item in the '''Sources''' section plane of the Navigation Tree and select '''Insert New Source...''' waveguide source must be parallel to open one of the Waveguide Source Dialogprincipal (XY, YZ or ZX) coordinate planes.* In the '''Source Location''' section The property dialog of the dialog, you will find waveguide source provides a drop-down list containing the name of all the eligible legitimate box objects. These are all the hollow boxes in the project workspace whose longitudinal axis is parallel to one of the principal axes and have at least one unchecked '''Cap Ends''' or box in their property dialog. Select the desired box objectarrays. The box labeled '''Direction''' shows the direction of the waveguide source is displayed as an orange rectangle with respect to a cross and a perpendicular small orange arrow across the host box object. You have the option to select either the positive or negative direction for the source.* In the box labeled The '''Offset''', enter the distance parameter of the a waveguide source plane is its distance from the base of the host box object. A waveguide source by default is placed at the center of the its host box. In other words, the default offset value is equal to half the length longitudinal dimension of the host box object.* The default waveguide more to be excited is TE₁₀. You can select '''TE''' or '''TM''' mode types with arbitrary "m" and "n" modal indices.* In the '''Source Properties''' section, you can specify the source '''Amplitude''' in Volts, and the '''Phase''' in Degrees.

Waveguide sources are a special case of distributed sources in [[EM.Cube]]'s [[FDTD ModuleTempo]]. A Distributed Source is defined in a rectangular plane of finite extents, parallel to one of the three principal coordinate planes. An impressed electric field component is assumed across the specified rectangular area, which pumps energy into the computational domain. The current version of [[EM.CubeTempo]] provides three spatial field profiles for a distributed source:

To create a new distributed source, follow these steps: * Right click on the '''Distributed Sources''' item in the '''Sources''' section of the Navigation Tree and select '''Insert New Source...''' to open the Distributed Source Dialog.* In the '''Excitation Plane''' section of the dialog, first you have to select the orientation of the source plane. The dropdown list labeled '''Direction''' gives three options: '''X, Y''' and '''Z''', which create planes parallel to the YZ, ZX and XY principal planes, respectively. Depending on the choice of the plane orientation, another dropdown list labeled '''Field Dir''' gives four options for the direction of the source field component. For example, the default plane orientation is X (parallel to the YZ-Plane) and the available field directions are +Y, -Y, +Z and -Z. Next, you have to enter the coordinates of two opposite corners of the source plane: the lower left and upper right corners. You can type in values for the X, Y, Z coordinates or you can use the spin buttons to slide the default source planes in the Project Workspace.* In the '''Source Properties''' section, you can select the '''Profile''' from three options: '''Uniform''', '''Sinusoidal''' and '''Edge-Singular'''. You can also specify the source '''Amplitude''' in Volts, and '''Phase''' in Degrees and the source's internal '''Resistance '''in Ohms project workspace.

In [[EM.Cube]]'s [[FDTD ModuleTempo]] you can define simple lumped elements such as resistors, inductors, capacitors as well as nonlinear diodes. Although lumped loads are not sources and do not excite a structure, their properties are similar to lumped sources. Lumped Loads are incorporated into the FDTD grid across two adjacent nodes in a similar manner to lumped sources. Likewise, lumped loads are defined on Line objects. In order to create a four lumped load, you must have at least one line object in your project.types:

To create # '''Resistor''' with a new lumped loadResistance value (R) in Ohms.# '''Capacitor''' with a Capacitance value (C) in pF.# '''Inductor''' with an Inductance value (L) in nH. # '''Nonlinear Diode''' with a Saturation Current (I_s)in fA, follow ambient temperature (T) in degree Kelvin, and a dimensionless ideality factor (n). The default values of these steps:[[parameters]] are 100fA, 300°K and 1, respectively.

* Right click on the '''Although lumped loads are not sources and do not excite a structure, their properties are similar to lumped sources. Lumped Loads''' item are incorporated into the FDTD grid across two adjacent nodes in the '''Sources''' section of the Navigation Tree and select '''Insert New Sourcea similar manner to lumped sources.Likewise, lumped loads are defined on Line objects..''' In order to open the Lumped Load Dialog.* You can change the name of the create a lumped load as well as its color using the '''Color''' button of the dialog and selecting the desired color from the color palette.* In the Lumped Element Location section of the dialog, you will find a list of all eligible line objects (i.e. lines that are parallel to must have at least one of the principal axes). Select the desired line objectin your project. The box labeled '''Direction''' shows the direction of the lumped load with respect to the Lumped loads show up as small yellow arrows on their host line object. Note that for Similar to lumped a passive lumped loadsource, the direction does not make any difference. But it matters for the case of a diode.* In the box labeled '''Offset''', enter the distance of the lumped element from the start point of the line. A lumped load by default is placed at the center of the host line. In other words, the default has an offset value is equal to half the length of parameter that determines its location on the host line object.* In the Lumped Element Properties Section, you can set the type of the load. In the dropdown list you have four options: '''Resistor''', '''Inductor''', '''Capacitor''' and '''Diode'''. The resistance is expressed in Ohms (O), inductance in Nano-Henry (nH) and capacitance in Pico-Farad (pF). In the case of a diode, you have to specify the '''Saturation Current''' in femto-Ampere (fA), the ambient '''Temperature''' in Degree Kelvin and also the diode's '''Ideality Factor''', which is usually a number between 1 and 2.