With the development of technology, the clock frequency of the digital signal is getting higher and higher, and the circuit system puts forward higher and higher requirements for the signal establishment, holding time, clock jitter and other elements. EMI, that is, electromagnetic interference, refers to the impact of the circuit system on the surrounding circuit system through conduction or radiation. EMI will cause degradation of circuit performance, and in severe cases, it may cause the entire system to fail. In actual operation, relevant agencies promulgate electromagnetic compatibility specifications to ensure that 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. Most EMI problems are related to the clock signal.
There are many ways to reduce EMI, including shielding, filtering, isolation, ferrite ring, signal edge control, and adding power and GND layers in the PCB. The above methods can be used flexibly in applications, among which 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 the cost; reducing EMI through the LAYOUT technique is obviously time-consuming, and the means are different due to different designs.
Spread Spectrum Clocking (Spread Spectrum Clocking) is another effective way to reduce EMI. This article will briefly describe how the Spread Spectrum Clock Generator (SSCG) reduces EMI.
OverviewClock spreading spreads the energy concentrated in a narrow frequency band to a set wide frequency range by means of frequency modulation, and reduces the amplitude (energy) of the clock at the fundamental frequency and odd harmonic frequencies to reduce the peak of electromagnetic radiation of the system. the goal of.
Generally, digital clocks have a very high Q value, that is, all energy is concentrated in a very narrow frequency range, which manifests as a relatively high energy peak. On the spectrogram, it is easy to see that there are very high peaks at the intermediate frequency and lower peaks at the position of odd harmonics; SSCG reduces the peak energy by increasing the clock bandwidth and reduces the Q value of the clock. Figure 1 illustrates the working principle of SSCG.
The clock spread spectrum modulates the original clock signal in a specific way. Linear and Hershey Kiss (not Hershey's Kiss Chocolate) are commonly used modulation methods.
Application characteristicsSSCG is an active and low-cost solution to EMI problems, which can deal with EMI problems in a wider frequency range on the basis of 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 internal integrated circuit of the clock. SSCG not only modulates the clock source, but other data, address and control signals synchronized with the clock source are modulated at the same time as the clock is spread, and the overall EMI peak value will be reduced accordingly. Therefore, the clock spread is system level. s solution. This is the biggest advantage of SSCG over other EMI suppression measures.
The SSCG function can be selected by the user in different configurations, ON or OFF, and different modulation ranges.
Passive EMI suppression devices usually also integrate ESD protection functions and do not consume power. SSCG consumes energy due to the use of integrated circuit functions such as spread spectrum.
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