[Repost] Most EMI problems are related to clock signals

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[Repost] Most EMI problems are related to clock signals [Copy link]

With the development of technology, the clock frequency of digital signals is getting higher and higher, and the circuit system has put forward higher and higher requirements for signal establishment, hold time, clock jitter and other factors. EMI, or electromagnetic interference, refers to the impact of the circuit system on the surrounding circuit system through conduction or radiation. EMI will cause the performance of the circuit to degrade, and in severe cases, it may cause the entire system to fail. In actual operation, relevant agencies have issued electromagnetic compatibility specifications to ensure that the electronic products on the market meet the requirements of the specifications.

The clock signal is often the signal with the highest frequency and the steepest edge in the circuit system, and most EMI problems are related to the clock signal.

There are many ways to reduce EMI, including shielding, filtering, isolation, ferrite magnetic rings, signal edge control, and adding power and GND layers to the PCB. The above methods can be used flexibly in applications. Among them, shielding is a relatively simple mechanical method with high cost and is not suitable for handheld and portable devices; filtering and signal edge control are effective for low-frequency signals and are not suitable for high-speed signals that are currently widely used. In addition, the use of passive components such as EMI/RFI filters will increase costs; reducing EMI through layout techniques is obviously time-consuming, and the methods vary depending on the design. Spread spectrum clocking is another effective way to reduce EMI. This article will briefly describe how the spread spectrum clock generator (SSCG) reduces EMI. Overview Spread spectrum clocking disperses the energy concentrated in a narrow frequency band to a set wide frequency band through frequency modulation, and reduces the amplitude (energy) of the clock at the fundamental frequency and odd harmonic frequencies to achieve the purpose of reducing the peak value of the system's electromagnetic radiation. Generally, digital clocks have a high Q value, that is, all energy is concentrated in a very narrow frequency range, which is manifested as a relatively high energy peak. It is easy to see in the spectrum that there is a high peak at the intermediate frequency and a lower peak at the odd harmonic position; SSCG reduces the peak energy and the Q value of the clock by increasing the clock bandwidth. Figure 1 shows the working principle of SSCG.

Clock spread spectrum modulates the original clock signal in a specific way. Linear and Hershey Kiss (not Hershey Kiss chocolate) are commonly used modulation methods.

Application characteristics

SSCG is an active and low-cost solution to EMI problems. It can deal with EMI problems in a wider frequency range while ensuring the integrity of the clock signal. Compared with the traditional use of Ferrite Beads and RF Chokes to suppress EMI, SSCG achieves the purpose of suppressing EMI peaks by modulating the frequency of the clock's internal integrated circuit. SSCG not only modulates the clock source, but also other data, address and control signals synchronized with the clock source are modulated at the same time as the clock is spread. The overall EMI peak will be reduced, so clock spread is a system-level solution. This is the biggest advantage of SSCG compared to other EMI suppression measures.

The SSCG function can be configured by the user in different ways, such as ON or OFF, and with different modulation ranges.

Passive EMI suppression devices usually also integrate ESD protection functions and do not consume power. SSCG consumes energy because it uses integrated circuit functions such as spread spectrum.