Resonance Prediction of Shielded Microwave PCB

　　Shielding cans are commonly used to protect microwave printed circuit boards (PCBs). While shielding cans protect the PCB from environmental influences, they can also change the electrical performance of the circuit. Understanding the effects of shielding cans and how to predict them can improve the accuracy of most modern computer-aided engineering (CAE) simulation tools. Techniques for minimizing the effects of shielding cans by accurately predicting the frequency, location, and nature of the shielding can's induced resonant modes are outlined in the first part of this two-part series.

　　One of the keys to avoiding unwanted resonant modes involves knowledge of the maximum electric (E) and magnetic (H) fields, and their corresponding resonant frequencies. Careful layout and routing of the PCB circuit can greatly reduce the effects of resonant modes. To demonstrate this approach, two filters were placed near a shield, with the first filter (Filter A) placed in the center of the shield, resulting in TM110, TM210, and TM310 modes with E-field hotspots and the expected field excitation at 4.1, 7.2, and 8.3 GHz.

　　In contrast, the second filter (B filter) is placed toward the bottom of the shielded cavity. In this area, the field strength is very small and the resonance effect is expected to be smaller than that produced by the first filter. Simulations using Ansoft HFSS electromagnetic (EM) software also predict that the resonance effect is much smaller for the placement of the B filter (see Figure 1).

 

　　Another example shows the effects of unwanted coupling of one circuit to another (Figure 2). The circuit connected from port 1 to port 2 consists of a grounded microstrip stub, and the circuit connected from port 3 to port 4 is a step impedance microstrip low-pass filter. Both circuits are in the vicinity of high H fields for all five modes described in the table. Therefore, we should expect resonant effects at 4.2, 5.9, 7.2, 8.0, and 8.3 GHz. A plot of the energy at port 1 appearing at port 3 is shown in Figure 3. Note that there are five fairly high transmission peaks at the predicted resonant frequencies.

　　If the same circuit is moved into a shielded case (see Figure 4), with a single grounded stub placed at the H-field zero of the TM110 mode, it is expected that the excitation of this mode should be reduced (see Figure 4).

 

　　The remaining peaks are still evident because the location of the H-field zero point in the TM110 mode is actually a high or even maximum H-field point inside the shield in the other mode.

 

　　The above simple simulation is used to prove that the layout and routing of the RF circuit inside the shield will affect the degree of excitation in the resonant mode. In addition, from the E-field and H-field curves above, it can be seen that if the high-order mode is excited, the entire shield will heat up very quickly, so the choice of circuit layout to reduce the excitation of the resonant mode may be quite controversial.

　　Finally, it cannot be overemphasized that effective layout of the circuit can only reduce the effects of resonances, but it cannot completely eliminate them. The only way to eliminate these problematic resonances is to change the size of the shield to move the resonant frequency away from any frequencies present in the design, or to use RF absorbers, which in effect changes the size of the shield.

　　The information presented here is a general summary for decomposing and resolving shield cavity resonances that can plague RF designs. The simple formulas presented here can be used to roughly estimate the frequencies of resonant modes. Major and minor mode hotspots should be determined prior to design to avoid the pitfall of exciting undesirable modes. Optimal shield dimensions should be determined to reduce the effects of shield resonances. In addition, the information presented here should help engineers find and troubleshoot shield resonances in existing designs and is a tool for identifying where to place RF absorbers or metal support rods to eliminate resonant modes.