CST – Computer Simulation Technology

Cross-SSN Analysis in Multilayer Printed Circuit Boards

As digital circuits became faster and more power is involved, direct coupling in multilayer Printed Circuit Boards (PCBs) among power (PWR) planes becomes a major concern for Signal Integrity (SI) and Electromagnetic Interference (EMI). CST MICROWAVE STUDIO® has been succesfully used for the analysis of the noise which propagates from a power plane to another power plane due to their proximity, named cross-simultaneous switching noise (X-SSN).

Figure 1: Test board with in evidence the SMA connector

Figure 1 represents a real view of the multilayer PCB, while Figure 2 illustrates a CST MWS simplified model (with only SMA A1 and B) and a cross section of the SMA connector with connecting pins.

Figure 2: CST MWS model and SMA connector detail

CST MWS results present a very good agreement with the measurements over the entire frequency range of interest (40MHz-5GHz), as illustrated in Fig.3....

Figure 3: Comparison between measured and simulated S-parameters between connectors A1 and B

To decrease the coupling between two consecutive PWR planes, a GND plane is interposed between them in the original stack-up and the updated model has been simulated by means of CST MWS. The comparison of |S21| magnitude, computed between A1 and B1 for the two stack-up is shown in Fig.4. It confirms a decrease of the coupling of around 50 dB up to 800 MHz and then a decrease of around 30 dB.

Figure 4: Comparison between |S21| of the original stack-up and the modified one

The use of CST MWS avoids the unecessary building-up a new PCB setup and hence allows easy and efficient prototyping rather than time-intensive and costly measurements. It provides design engineers with a tool for accurate Signal Integrity prediction and analysis.

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