RF Filter with Shield


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RF Filter with Shield


Contents


Introduction

Abstract:The impact of a 3D shield on RF filter performance is characterized using FEM simulation in ADS.RF/MW circuit designs can be enclosed with mechanical shields to increase isolation and reduce radiation. However, these shields also introduce unwanted parasitics that affect circuit performance. In order to take the parasitics into account, 3D EM simulations are required. This example shows how a full 3D EM simulation of a 3D mechanical shield combined with a microwave filter layout characterizes the change in the filter performance due to the shield.

Design Challenges

Adding an RF shield to a microwave filter design can shift the filter's frequency response, introduce unwanted resonances, and cause the filter to fail to meet specifications.

Changes in filter performance due to the shield can be characterized using 3D EM simulation. Traditionally, designers had to export circuit designs to a separate 3D EM tool and combine it with the mechanical structure for full 3D EM simulations. This approach can be error prone and tedious.

In this example, the 3D EM simulation is performed in the ADS environment without the need to export the layout design to a separate 3D EM environment. This can be done by placing a 3D model of the shield on the filter layout in ADS and then running FEM simulations on the combination of the 3D shield and the filter layout.

The 3D shield is first created in the EMPro 3D modeling environment. The shield can be completely parameterized in EMPro, and the parameter values can be changed even after the shield is placed in the ADS layout. This way, multiple simulations can be run in ADS to determine the optimum shield structure without having to go back and change the structure in the EMPro environment. This design flow is shown in Figure 1.


Figure 1: A parameterized 3D shield component is created in EMPro, then placed on a filter layout in ADS, where a full 3D simulation is performed

Once the shield is placed in the filter layout, a 3D view can be generated in ADS.


Figure 2: Microwave filter layout in ADS, and 3D view of the filter combined with a 3D shield in ADS

Results

The filter was simulated with 3D FEM technology in ADS. The FEM mesh and E-field visualization in ADS are shown below.


Figure 3: FEM mesh and E-field visualization of the filter and shield

Without the shield, the filter passband is 5-6 GHz as shown in Figure 4. A second simulation is run with the shield added and we can see a shift in the frequency response and a parasitic resonance introduced. The shield grounding was improved and a third simulation was run, and the parasitic resonance was eliminated.


Figure 4: Simulation results of the filter with and without the shield, and after the shield grounding is improved

Conclusion

The effects of shields, packaging, connectors and other 3D components on layout designs can now be analyzed in ADS with the use of fully parameterized 3D components. This example demonstrated how a 3D mechanical shield can be created in EMPro, placed in an ADS filter layout, and then simulated using 3D FEM technology. Simulations showed that the 3D shield introduced and unwanted frequency shift and parasitic resonance. After improving the grounding design in ADS, the FEM simulation was repeated and the filter performance verified.

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