A common source of EMI is when frequencies couple into and radiate from the power plane. If seeking CISPR-25 certification, this is especially critical as the antenna measurements are taken from the power cables of the device (Figure 3-3).

Figure 3-3 CISPR-25 Setup

Using decoupling capacitors is the primary method for creating a clean power in the circuit. Capacitors store energy when connected to a DC power source, so if the voltage fluctuates from the nominal value, the stored energy within the capacitor releases and is delivered to the load. This averages out the overall voltage, keeping the supply steady and reducing EMI spurs.

Common values for decoupling capacitors are 0.1uF and 1uF; however, these values are not capable of filtering out every frequency. Capacitors have finite internal resistance, with both resistive and inductive properties. The frequency at which resonance occurs and where the capacitive reactance and inductive reactance are equal is known as the self-resonance frequency (SRF). This frequency is where the impedance of the capacitor becomes zero. We require our decoupling capacitor to have the lowest impedance possible at the frequency we want to filter on our power supplies.

In the case of clocking, the CDC6C has a variety of rise time options. Automotive applications usually use clocks with much slower rise times due to the advantage of EMI effects being mitigated. The CDC6 Slow Mode 4 has a typical rise time of 2.7ns. This 2.7ns rise time can correspond to EMI spurs at 370MHz and the subsequent harmonics. In this case, a decoupling capacitor with an SRF of 370MHz can reduce the switching noise impact on the power and effectively discharge when the load current spikes.

Wurth Elektronik provides an EMI Filter Designer tool that can help choose the appropriate capacitor and ferrite bead values to target a specific frequency.