Shielding shells are commonly used to protect microwave printed circuit boards (PCBs) from external interference and environmental factors. In addition to providing physical protection, these enclosures also help maintain the electrical performance of the circuit. Understanding the effects of shielded enclosures and being able to predict them can significantly improve the accuracy of modern computer-aided engineering (CAE) simulation tools. This article is divided into two parts, focusing on how to accurately predict the frequency, position, and intrinsic characteristics of resonant modes within a shielded shell. The first part outlines key techniques for minimizing shielding inaccuracies.
One of the critical aspects of avoiding unwanted resonance is understanding the distribution of electric (E) and magnetic (H) fields, along with their corresponding resonant frequencies. Careful placement and routing of the PCB components can greatly reduce the impact of these resonant modes. For example, placing two filters near the shield can demonstrate this effect. When the first filter (A filter) is positioned at the center of the shield, it creates an E-field hot zone at 4.1, 7.2, and 8.3 GHz, exciting the TM110, TM210, and TM310 modes.
In contrast, the second filter (B filter) is placed toward the bottom of the shield cavity, where the field strength is much lower. As a result, the resonance effect is expected to be significantly reduced. Simulations using Ansoft HFSS electromagnetic software confirmed that the resonance effect of the B filter was much smaller than that of the A filter.
Another example illustrates the issue of undesired coupling between circuits. One circuit connects port 1 to port 2 via a grounded microstrip stub, while another connects port 3 to port 4 as a step impedance low-pass filter. Both circuits are located in regions of high H fields, which means resonance is likely to occur at 4.2, 5.9, 7.2, 8.0, and 8.3 GHz. The energy transfer from port 1 to port 3 shows five distinct peaks at these frequencies.
When the same circuit is enclosed within a shield, the grounded stub is placed at the H field zero point of the TM110 mode, which should reduce the excitation of that mode. However, some residual peaks remain because the H field zero in one mode may correspond to a high H field in another mode.
This simple simulation highlights how the layout and routing of RF circuits inside a shield can influence the degree of resonance. From the E-field and H-field plots, it's clear that when higher-order modes are excited, the entire shield becomes "hot," making it crucial to choose the right circuit placement to minimize resonance effects.
It’s important to note that while effective layout can reduce resonance, it cannot completely eliminate it. To fully eliminate problematic resonances, you would need to change the size of the shield, shift the resonant frequency away from the design’s operating range, or use RF absorbers to alter the internal electromagnetic environment.
The information provided here serves as a general guide for addressing shield cavity resonance in RF design. Using the simple formula provided, you can estimate the resonant frequency. Identifying primary and secondary hot zones before design helps avoid unintended resonance. Choosing the optimal shield size and using this knowledge can help engineers identify and resolve resonance issues in existing designs. It also aids in determining where to place RF absorbers or metal support rods to suppress unwanted resonance modes effectively.
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