8.2.2.19 Frequency Dithering for Conducted Emissions Control

The international standard CISPR 22 (and adopted versions) are often used as a requirement for conducted emissions. Ethernet cables are covered as a telecommunication port in section 5.2 for conducted emissions. Meeting EMI requirements is often a challenge, with the lower limits of Class B being especially hard. Circuit board layout, filtering, and snubbing various nodes in the power circuit are the first layer of control techniques. A more detailed discussion of EMI control is presented in Practical Guidelines to Designing an EMI Compliant PoE Powered Device With Isolated Flyback, (SLUA469). Additionally, IEEE802.3at sections 33.3 and 33.4 have requirements for noise injected onto the Ethernet cable based on compatibility with data transmission.

Occasionally, a technique referred to as frequency dithering is used to provide additional EMI measurement reduction. The switching frequency is modulated to spread the narrowband individual harmonics across a wider bandwidth, thus lowering peak measurements. The circuit of Figure 36 modulates the switching frequency by feeding a small AC signal into the FRS pin. These values may be adapted to suit individual needs.

Figure 36. Frequency Dithering