Shielding shells are commonly used to protect microwave printed circuit boards (PCBs). These enclosures not only provide physical protection but also help maintain the electrical performance of the circuits by shielding them from environmental interference. Understanding the effects of shielded enclosures and being able to predict these effects can significantly enhance the accuracy of modern computer-aided engineering (CAE) simulation tools. This article is divided into two parts, focusing on the techniques for accurately predicting the frequency, position, and intrinsic characteristics of resonant modes within a shielded shell. The first part outlines these key methods.
One of the critical factors in avoiding unwanted resonance is understanding the distribution of electric (E) and magnetic (H) fields, as well as 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 (Filter A) 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 TM110, TM210, and TM310 modes respectively.
In contrast, placing the second filter (Filter B) toward the bottom of the shield cavity results in a much weaker field strength, leading to reduced resonance effects. Simulations using Ansoft HFSS EM software confirmed that the resonance caused by Filter B was significantly smaller (see Figure 1).
Another example illustrates the issue of unintended coupling between circuits. In one setup, a grounded microstrip stub connects Port 1 to Port 2, while a step impedance low-pass filter connects Port 3 to Port 4. Both circuits are located in regions with high H-field strengths, which suggests potential resonance at frequencies of 4.2, 5.9, 7.2, 8.0, and 8.3 GHz. The energy transfer from Port 1 to Port 3, shown in Figure 3, highlights five distinct peaks at these predicted resonant frequencies.
When the same circuit is placed inside the shield (as seen in Figure 4), the grounded stub is positioned at the H-field zero point of the TM110 mode, which is expected to reduce the excitation of that mode (see Figure 4). However, the remaining peaks are still significant, as the H-field zero in the TM110 mode may correspond to a high or even maximum H-field in other modes.
This simple simulation demonstrates how the layout and routing of RF circuits inside a shield directly affect the degree of resonance. From the E-field and H-field graphs, it's clear that if higher-order modes are excited, the entire shield becomes very hot, highlighting the importance of careful design choices to minimize unwanted resonances.
It’s important to note that while proper circuit layout can reduce resonance effects, it cannot completely eliminate them. The only effective ways to fully suppress problematic resonances are to adjust the size of the shield, shift the resonant frequency away from the operating frequencies, or use RF absorbers to alter the internal electromagnetic environment.
This information serves as a general guide for addressing shield cavity resonance issues in RF design. The resonant frequency can be roughly estimated using the formulas provided. Identifying primary and secondary hot zones before the design phase helps avoid unnecessary mode excitations. Optimizing the shield size is also crucial for minimizing resonance effects. Additionally, this content can assist engineers in identifying and resolving resonance problems in existing designs, as well as determining where to place RF absorbers or metal support rods to further mitigate resonance.
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