LC TUTORIAL

1.1 Startup Window

Start LC by just typing the command `lc'. First the welcome window is displayed, which includes license information. This is important, because without a valid license, editing and analysis can be performed, but no simulations can be run.

1.2 Main Window

The next window displayed is the main window, which includes the main menubar along with a status message area and the block editing area.

2 Opening a Viewport

A viewport is a window which displays a three dimensional view of the current model. To create a viewport, use the Model Viewport selection of the View menu (or the control-m keyboard shortcut). Generally, actions in a viewport do not change the model. The exceptions are the Create and Paste pointer operations. Most model editing occurs in the main window, or in one of the Define menu dialogs.

Several viewports may be open at the same time, each giving a different view of the model.

At first, the viewport will be empty, because there is no model geometry yet. When the pointer is in the viewport, the cursor coordinates are shown. These coordinates are important feedback during pointer operations.

2.1 View Pointer Mode Dialog

Another useful dialog for editing is the Pointer Mode selection of the View menu (or control-p shortcut). This dialog has a set of toggle buttons to select the operation type for pointer (mouse) actions in the viewports.

3 Creating the Geometry

When creating the geometry blocks, which define the material properties within the model, the three most important items are the block position, size, and material. The properties of a newly created block are taken from the fields of the block editing area in the main window. If a block is created by a pointer operation, then the block position and size are defined by the pointer, and the other values come from the editing area.

Select the Create toggle from the Pointer Mode dialog. This will enable create mode, which allows a pointer drag in a viewport to create a new block.

3.1 Air

One of the first blocks usually created in a microstrip model is just a region of free space (referred to as material type air) to define the overall dimensions of the model.

Create a block of air by moving the pointer to coordinates (-10,10,0) in a viewport, then clicking and dragging to (10,-10,0). During the drag, a outline box is displayed, and when the button is released, a block is created. In this case it will be a large cyan-colored block of air.

The air can be given a more intuitive appearance by selecting a light pattern from the Fill option menu in the editing area of the main window. After selecting a light pattern, then use the Replace button to make the change take effect. The light fill pattern makes the air more transparent, so it looks more like air. Fill patterns and colors are entirely for visual effect, and don't change the simulation results at all.

What happens if something goes wrong, and the block isn't created correctly? There are two possible remedies. The Delete button in the editing area will remove the currently displayed (active) block, which in this case is the last block created. Another option is to modify the block fields in the editing area and then use the Replace button.

3.2 The Ground Plane

The second block to be created is a solid metal ground plane. First change the Fill option menu back to the original solid selection. Then select metal from the Material option menu in the main window editing area by clicking on the material arrow button and then selecting metal from the list. Now the default material is a dark blue colored conductor.

It's a good idea to give descriptive names to the blocks, so put a suitable name in the Name text box in the editing area, then click and drag from (-10,-9,0) to (10,-10,0) in the viewport. This will create a metallic ground plane on the bottom of the model.

Note that the ground plane overlaps the air block, and visually overwrites it. This spatial priority is why the air block was created first, and then the ground plane. The rule in effect is that newer blocks always overwrite older blocks, replacing the material properties within the overlapping region. This precedence is shown visually by the ground plane overwriting the air block.

3.3 The Dielectric Slab

3.3.1 Define Materials Dialog

The slab of dielectric material should be defined on top of the ground plane in a microstrip model. Only two materials are predefined: air and metal. A new material for the dielectric slab on top of the ground plane must be created.

Select the Materials dialog from the Define menu in the main window. The Define Materials dialog is displayed. Create a new material by entering a new name, assign the material a new color, and define it as a perfect dielectric with a permittivity of 2. Use the Add button to create the new material definition.

3.3.2 Creating the Slab

This operation only created a new material definition, but did not actually define a region within the model of that material. The new material automatically becomes the default material when it is created, so click and drag from (-10,-7,0) to (10,-9,0) to create a dielectric slab on top of the ground plane.

3.4 The Microstrip

Select metal from the Material selection list and then click and drag from (-2,-6,0) to (2,-7,0) to create the microstrip conductor. Don't forget to give it a name as well. Again the newer block overwrites the material of the older air block.

Select contrasting color such as yellow in the Color option menu and then press the Replace button to change the microstrip color.

4 Adding the Source

So far the material properties have been defined everywhere within the model, and thus the geometry itself is fully defined. Before a simulation can be run, a source excitation must be added to provide input to the system.

Use the block type option menu in the editing area (which is currently displaying Geometry) to select the Source block type. Set the fields in the editing area to select a Gaussian Pulse Waveform, the +Y Direction, and Voltage Excitation. Define the Rise Time to be 1e-10 (0.1 nanosecond). Also set the Hard toggle. This defines the source to be a hard source, meaning that its values overwrite any others within the source region. Then click and drag from (-2,-7,0) to (2,-9,0) to create a voltage source between the microstrip and the ground plane.

Defining the source as a hard source is necessary because it is adjacent to the edge of the modelled space, and will interact with the boundary condition. The default type of source, the soft source, cannot be placed directly on the edge of the model.

So far all of the block definitions have been entirely two dimensional. The third dimension (Z in this case) has been defined by the depth limits in the viewport, which were (-10,10) by default. This default results in all of the two dimensional definitions being extended between Z=-10 and Z=10 automatically. The default has worked nicely so far, but the source region should be defined only under a small segment of the microstrip.

To make this change, update the Z coordinate Max value to -9 in the main window block editing area (this is the third input field of the (X,Y,Z) maximum coordinates), and then press the Replace button. This will modify the original source definition and restrict the excitation to a small region at one end of the microstrip.

5 Adding Probes

The last addition to the model necessary to make it complete is to define probes to measure, display, and save the interesting values from the simulation.

Rather than using a pointer drag to create the block, the probe can be made by modifying the source block displayed in the main window block editing area. Change from block type Source to Probe. Change the Direction to +Y to indicate that the voltage will be calculated from the lowest Y coordinate to the highest Y coordinate of the block. Then select the Plot toggle to display the probe values in an X-Y plot as the simulation runs. Change the Z Center and Size values in the main window block editing area to 0 (this is the third input field of the (X,Y,Z) center coordinates). This will place the probe in the middle of the microstrip. Give the probe a name by changing the Name input field, and then use the Add button to create a the probe block.

The green probe block will seem to overwrite the red source block in the viewport. Changing to a +Y View in the viewport will show how the model appears from on top. Now it is apparent that the source is at one end of the microstrip, while the probe is in the middle.

To get a three dimensional view of the geometry, click on the Rotate toggle in the Pointer Mode dialog, then click and drag in the viewport to rotate the model. The Zoom and Pan modes are also useful for looking at the model.

6 Saving the Model

Before running the model, it is a good idea to save the work done so far. Select the Save As dialog from the File menu. Enter a file name and save the model.

7 Running the Simulation

7.1 Run Simulation Dialog

Select Simulation from the Run menu to bring up the Run Simulation dialog. This dialog displays a lot of information about the simulation environment, plus has a set of buttons for controlling the simulation.

Particularly of interest to us is the Time Step value, which is the amount of time which passes for each time step of the simulation. The time step value is automatically set to the Courant limit (largest allowable value) based on the mesh cell size. Since each time step in this simulation is 1.668 picoseconds, 300 time steps will run the simulation for 500 picoseconds total. Recall that our source waveform rise time was 1e-10, or 100 picoseconds, so this is enough time for the pulse to propagate completely from the source and pass through the probe.

7.2 Run Start Dialog

To start the simulation, press the Start button. This will bring up the Run Start dialog. Enter 300 for the number of time steps and press Ok.

7.3 Probe Data Plot Window

A plot of the voltage probe data will be displayed as the simulation runs.

Notice that the measured waveform is not entirely smooth. This noise is caused by discretization error, an indication that the mesh cell size is not fine enough for the specified source rise time. This artificial ringing is caused by the loss of the high frequency components of the source waveform.

With a 100 picosecond rise time, the source has significant frequency content over 10 GHz. In the microstrip region, the relative permittivity is 2, so 10 GHz corresponds to a wavelength of 21 mm. Since our cell size is 1 mm, this is 21 cells per wavelength. As the number of cells per wavelength decreases below 20, the discretization error rises. At 20 GHz, the discretization is 10 cells per wavelength, enough to cause significant numerical error.