The circuit component editor's properties tab includes a distributed option with checkbox that increases accuracy when a component is attached to a transmission line. When checked, XFdtd spreads the excitation across the width of the transmission line connected to the component. Remcom recommends checking this option in all applicable situations in order to produce more accurate results.
The following terminology is used to describe components:
- Distributed circuit component: when the distributed checkbox is checked, the component is spread over the area between the transmission line and ground plane.
- Lumped circuit component: when the checkbox is not checked, the component exists on a single cell edge and PEC leads are used to reach its endpoints.
The following criteria must be met in order to distribute a circuit component:
- One of the circuit component endpoints must be attached to an edge of the transmission line.
- The associated circuit component definition must be either a feed or passive load.
- The two endpoints must be aligned along one of the principle X, Y, or Z axes.
XF is particular about a component's attachment to an edge, so selection must be specific in order to determine the width of a distributed component. The picker tools (, , ) associated with a circuit component's endpoints enable users to select the vertices, edges, and faces that compose the geometry. When properly attached to an edge, the endpoint's text indicates that it is either on edge or center of edge.
If both endpoints are attached to an edge, the circuit component is distributed across the positive, (+), endpoint. By default, the second endpoint is positive.
When placing and viewing a distributed component in 3-D CAD view, a green box indicates the area over which the component is distributed. The box's width is equal to the length of the edge to which the component endpoint is connected. A lumped component is a green wire connecting the two endpoints.
In both cases, either a cone or cylinder in the center of the edge indicates a either feed or passive load circuit component definition, respectively.
A distributed component in the mesh view has either cones or cylinders placed on each cell edge in the area where the component is distributed. This indicates which cell edges are updated based on the associated circuit component definition.
A lumped component has either a single cone or cylinder, indicating that the component is applied to a single cell edge. If the lumped component spans multiple cell edges, green wires serve as PEC leads reaching the two endpoints.
Impact on Results
Generally, the finite-difference time-domain method computes electric and magnetic field values on each cell edge in the computation space, and then voltage and current results are derived from them. Distributed and lumped components compute voltage and current differently.
A distributed circuit component spreads the excitation or load across multiple cell edges. For excitations and results collection, XF assumes an analytic field distribution under the transmission line where the voltage and current density are evenly distributed across the component width. The portion of the signal matching that expected distribution is extracted and referred to as the output voltage and current. The portion not matching the expected distribution is accounted for by additional component loss, which could become significant when other structures cause the field distribution to deviate from XF's assumption. This can be reduced by moving the component to a more uniform section of the transmission line. XF accounts for components that are placed across inhomogeneous materials.
A lumped circuit component applies the excitation or load to the single cell edge at the component's center and uses perfect electrical conductor (PEC) leads on either side of it. The circuit component's output, including voltage and current, only considers the fields around the center cell edge. The PEC leads ensure connectivity between the component and geometry at the endpoints, but are not considered part of the component when computing output. The effect of the PEC leads can be seen on input impedance, where the imaginary part is artificially large due to the inductance of the wires.