CST – Computer Simulation Technology

Bandpass Waveguide Filter with Dual Mode Cavities

Waveguide Filters are widely used in High Power Applications because of their low-loss performance. This particular design is a waveguide bandpass iris filter for standard X-band military satellite communications which is used by most countries throughout the world. The filter is a receive-reject filter and is intended for the output of the transmitter source to filter out any intermodulation products the source may generate within the receive band. This Bandpass Filter has been built to have a passband between 7.9 and 8.4 GHz and a high rejection below 7.75 GHz. To achieve high rejection in the stop band, a dual mode design has been used. The design consists of 8 cavities. Two of the centre cavities operate with dual modes.

Figure 1: Geometry of the Waveguide Bandpass Filter

Figure 1 shows the geometry of the filter. The entire design has been defined in a parametric way in order to allow optimization and sensitivity studies. The structure has been simulated using the Tetrahedral Mesh Frequency Domain Solver of CST MWS. The starting mesh consisted of about 1300 Tetrahedrals. The convergence criterion was set to be a S-Parameter Error of 1% which was reached after 15 adaptive passes. The final mesh has about 65000 Tetrahedrals. The entire mesh adaptation takes less then 12 minutes whereas the last solver run takes about 60 seconds. All calculations have been performed on a 1.8 GHz Pentium M with 1 GByte of RAM. The peak memory usage was less than 400 MByte....

Figure 2: Final Mesh after Adaptive Mesh Refinement

Figure 2 shows the final mesh after 15 adaptive passes. Broadband S-Parameters have been calculated by using the interpolating S-Parameter scheme of CST MWS. Broadband S-Parameter Convergence has been reached after calculating 22 frequency samples. The total calculation for mesh adaptation and frequency sweep was less then 30 minutes.

Figure 3: S-Parameter of the Filter

Figure 3 shows the S-Parameters from 7 to 8.5 GHz with the transmission zeros at around 7.78 GHz.

Figure 4: Averaged Electric Field in the Pass Band

Figure 4 and 5 show the averaged E-Field distribution inside the Filter at 8.25 and 7.5 GHz respectively. The energy passes through the filter at 8.25GHz whereas it is rejected at 7.5GHz.

Figure 5: Averaged Electric Field in the Stop Band

CST MWS is the first commercial code to offer the advantages of both hexahedral and tetrahedral meshing in one 3D EM simulator. The User now has the ability to choose the method (Method on demand™) and the mesh (Mesh on demand™) best suited to a particular structure. As shown in this article, the Frequency Domain Solver in combination with the tetrahedral mesh is well suited to High-Q Filter Structures. The iterative memory efficient solver runs even demanding problems with more then 200 000 Tetrahedral on a 2 GByte Computer within minutes per frequency point. Using the advanced interpolating frequency sweep, only a few frequency points need to be calculated to obtain the broadband response of structures.

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