5.1    Coplanar Waveguide Analysis

Using Ansoft HFSS the first stage is to draw the structure using the 3D Modeler window. Fig 5.1 shows the HFSS draw screen displaying the Coplanar Waveguide transmission line structure.

Figure 5.1 Coplanar Waveguide Structure in 3D modeler HFSS window

Using Ansoft HFSS the first step is to draw the structure using the 3D modeler window Fig. 5.1 shows the HFSS draw screen displaying the Coplanar Waveguide TL Structure. Before drawing the model solution type need to be specified. There are three solution types in HFSS version 9, Driven modal, Driven Terminal and Eigenmode. For this particular structure we have used driven modal solution type. The second step is to specify the units of measurements for drawing geometric models. The units used here for this structure are millimeters (mm).

After inserting an HFSS design into the 3D modeler window the first step is to build the modal as a collection of 3D objects. Each material type is treated as a separate object. For this structure we have drawn a box by selecting two diagonally opposite corners of the base rectangle and then specify the height, immediately after this the properties dialog box appears automatically, enabling us to modify the object’s properties. Two boxes of 0.6mm thickness and 5.0mm width were drawn and named as ground planes; the central conductor is a box of 0.6mm thickness and 100mm width as shown in figure 5.1. Similarly the dielectric substrate of 0.56mm thickness were drawn beneath the conductor and ground planes and assigned Gallium Arsenide material properties. Lastly drawn an outer box, which is 4-times in height above the conductor and ground planes and 2-times deep below the substrate as shown in the figure 5.1.

Every object by default when drawn is assign material property of vacume. We can change then the desired material. For this structure we have assigned a gold material to the central conductor and to the ground planes. A GaAs material was assigned to the substrate and the outer box was set as an air box.

The boundaries for the structure must next be assigned and also the ports calibrated and defined. Figure 5.1 displays the location of various ports and boundaries for the Coplanar Waveguide structure. The structure has two ports, port 1 is for the signal to enter and port 2 is for the signal to exit the structure. Port 1 and port 2 are both 50W. Boundaries for the surface of the structure are defined, to let the simulator take into account the characteristics of the materials of which the structure is composed. Note that the structure under consideration has an inner and outer metal boundary (Perfect E) and is internally composed of air.

There are two recommendations for assigning waveports to the structure. First is the wave port size should not exceed lambda/2 in any dimension, to avoid permitting a rectangular waveguide modal excitation, 3(2g+w). As shown below in Fig. 5.2.  

Secondly the waveport width should be no less than 3´ the overall CPW width, or 3´(2g+w), and port height should be no less than 4´ the dielectric height, or 4h.

For port field display we need to assign integration line when defining waveport. An integration line is a vector that can represent a calibration line that   specifies the direction of the excitation field patterns at a port. Also for the computation of Zpi and Zvi port impedances, we need integration line. [80]

The system creates the background objects itself and is assigned the material characteristics of a perfect conductor. It is the region that surrounds the geometric model and fills any space not occupied by an object. There are many types of boundaries, e.g. perfect E, perfect H, finite conductivity, impedance, lumped RLC etc, but in this report we have used perfect E, and radiation boundaries as well where appropriate.

CPW E-field distributions are shown in the figure below, Fig.5.3.

·        Basic vector fields can be displayed in setup executive parameter/port impedances, by highlighting a port and using the port field’s button beneath the graphical window to adjust the window.

·        E-fields in CPW are symmetric to either side of the center trace, with E fields extending from each ground to the trace in phase. H-fields circulate around the center trace.

In figure 5.3 all the arrows in the plots are of different sizes and the length of the arrows vary. This simply indicates the direction and field’s strength respectively.

Mesh operations are optional mesh refinement setting in HFSS that provide HFSS with mesh construction guidance. This technique of guiding HFSS’s mesh construction is referred to as “seeding” the mesh. Seeding is performed using the mesh operations commands on the HFSS menu.

5.1.7  Specifying Solution Setup

HFSS computes a solution by adding a solution setup to the design. Each solution setup includes the general data information about the solution’s generation; adaptive mesh refinements parameters and frequency sweep parameters. Then a solution sweep can be added to the solution setup.

5.1.8  Setting up 3D Simulation

The Fast Frequency Sweep is preferred for the analysis of structures. This is because it is much faster to solve than the discreet frequency sweep. The fast frequency sweep saves time by solving problem at a single frequency. This is achieved by a rational function approximation for the frequency bandwidth of interest. The fast frequency sweep only calculates other frequencies in the specified bandwidth approximation. The software interpolates the sweep frequency points and uses additional frequencies to get a high degree of accuracy and other frequencies other than the single frequency specified.

In the simulations performed in this report, the fast frequency sweep was employed and all problems solved for the dominant or first order mode. All field solutions were saved for each analysis, which were then processed using the HFSS post-processing facility.

The following set-up conditions were used for CPW structure. 

                  Start frequency = 1GHz     

                              Stop frequency = 35 GHz

With 0.1 GHz step size and 0.01 maximum delta S per pass.

The initial mesh generated by Ansoft HFSS, solved by the finite element method, which was discussed earlier, was used to solve the structure three-dimensionally for each different value of dielectric constant. Using a fast frequency sweep from 1GHz to 35GHz and ensuring the port impedance was 50W, both for port 1 and port 2 the transmission line structure were analyzed. The simulation results for CPW are shown in the figure 5.5 below.

Figure 5.5       Magnitude S-parameter graph of CPW.

The red and green lines represent S12 and S21 respectively. While the black and blue lines represent S11 and S22 respectively. The S12 and S21 show the transmission behavior and S11, S22 predicts the reflection behavior at port1 and port2 respectively. The simulated results are in good agreement with the measured results for CPW.

5.2    Analysis of EBG Structure with two-rows of holes on each ground plane

We used the CPW structure and etched two rows of six holes on each ground plane and simulate the structure with the help of High Frequency Structure Simulator (HFSS) software. The simulated results are very similar to the measured results. The EBG structure with two rows of six holes on each ground plane is depicted in the figure (5.6) below.

Figure 5.6 EBG structure with two rows of holes on each ground plane.

The dimensions and materials, of the substrate, central conductor, ground planes and boundaries are given in the table (5.1) below.