Microstripline circuit are widely used in the telecommunications and microelectronics industry. The circuit is essentially a planar waveguide fabricated in a similar manner to an integrated circuit, or even printed onto a substrate. The electromagnetic energy propagates along the waveguide and into the surrounding dielectric media at microwave frequencies and cannot be analyzed using low-frequency current conduction principles. In order to analyze these devices ANSYS Multiphysics provides a finite element, frequency domain (FEFD) high-frequency solver that accounts for full electromagnetic free space wave propagation. The most basic form is a linear waveguide. More complex circuits are constructed by forming junctions at strategic distances from the ports that support particular modes / frequencies. In this way devices such as bandpass filters and antennas can be constructed. The following image shows a bandpass filter solid model created in ANSYS Multiphysics.
Microstrip devices can be analyzed in ANSYS Multiphysics in either 2-D or 3-D. Also symmetry can be employed to reduce model complexity where appropriate by using perfect electric (PEC) and perfect magnetic conductor (PMC) boundary conditions. Both harmonic and modal analysis are available.
ANSYS Multiphysics can perform modal high-frequency modal analysis in both 2-D and 3-D. A modal analysis is used to find the resonant frequencies and modal shapes for the electromagnetic field in a structure, as well as the quality (Q) factor if dielectric and surface losses are present. The following image shows the 3-D solid model of the basic stripline. A 2-D cross section was taken through this model and meshed using HF118 elements, which can then solve for either cut-off frequencies or propagating constants by setting the HFEIGOPT command. This approach is used for linear "extruded" structures such as this and also coaxial waveguides where a full 3-D analysis is not necessary. The substrate in this example has anisotropic material properties. Isotropic, orthotropic and fully anisotropic material properties are supported.
The following image shows a results contour plot for the sum of the electric field, (rollover the image with your mouse to see the mesh):
A harmonic analysis is used to understand the behavior/distribution of the electromagnetic field in a structure at specific frequencies, or to construct a frequency spectrum to understand the devices performance over a range of frequencies. The following image shows the harmonic results for the 3-D bandpass filter model shown above. Use your mouse to roll over the images to view differences in the field distribution at 5 GHz and 7.5 GHz. A spectrum is built up by performing sequential harmonic analyses.
Ernie Kim (University of San Diego)
Leo Maloratsky- Basics of Microstriplines (Rockwell-Collins)