How EMC test data analyzes and improves radiation

In the EMC test of electromagnetic compatibility radiation emission, the most common is the clock radiation exceeding the standard (as shown in the figure). It can be said that 70% of the excessive radiation is caused by clock problems. Since the periodic signal whose clock kinetic energy is concentrated in the time domain corresponds to the discrete frequency band in the frequency domain, the performance of clock kinetic energy in the frequency band is that the kinetic energy of the clock frequency point (the basic frequency of the clock and its frequency doubling) is very high, rather than that the clock frequency point has no kinetic energy basically.

So how do we analyze data to test and improve radiation?

2、 Test data analysis

Before analyzing the radiation data, let's briefly understand the most basic feature of the clock: frequency doubling. As shown in the figure, the common trapezoidal wave in the frequency domain has many spacing rules, such as the 24MHz camera clock, so that f0=24MHz, every 24 frequency domain MHz will have a single kinetic energy very high (24) MHz, 48MHz, 72MHz, 96MHz, 120MHz...).

Therefore, if you want to analyze the root cause of the clock from the EMC test, you need to calculate the single interval △ f of each adjacent clock, f0=△ f. For example, the data test above:

f5-f3=875.2470-750.1083≈125MHz

f3-f2=750.1083-625.0780≈125MHz

f2-f1=625.0780-501.1790≈125MHz

It can be seen from the numerical value that point 1, point 2, point 3 and point 5 are 125 MHz clock multiplication (the noise of the 833MHz mobile phone number 4 in the laboratory can be ignored)

3、 Reasons for excessive radiation

By analyzing the clock frequency exceeding the standard, you can easily analyze the clock frequency pcb according to the spectrometer or schematic diagram to find out which clock signal comes from which chip. However, this does not mean that we can handle the clock problem well. Due to the diversity of clock radiation paths, it may include differential mode and common mode radiation. Even if we find the way of radiation, we may not be able to reduce the radiation. Because the clock is a useful signal required by our electronic equipment, many filtering methods will affect the integrity of the signal, resulting in abnormal effects or reduced stability and consistency. This requires us to balance electromagnetic compatibility and product performance, which makes many engineers older and older.

Next, let's look at several common reasons for clock radiation exceeding the standard:

1. Generally, CPU has reference clock. If there is a long line around the reference clock crystal, the clock noise can easily be combined with the long line and then radiated. Therefore, the wiring should be far away from the crystal and should be wrapped around the crystal.

2. Many data lines also have clock noise (④ and ⑥). Generally, the working RC filtering is done at the source end of the data line. The value of R is 22 or 33 Ohms (the magnetic beads can also be replaced to improve the filtering effect), and the value of C is within 33 pf.

3. Flat cable is an antenna with good clock radiation, such as screen flat cable, camera flat cable, speaker flat cable, etc. Many flat cables do not have clock lines themselves, but they will be combined with the noise of the clock. Therefore, we should pay attention to whether it is to input flat cables (①) or export flat cables (②). We should take filtering measures at the connection. If necessary, it is better to add a magnetic ring. In addition, shielding grounding is very important. Shielded and grounded cables can reduce 10-30 db radiation.

4. The clock wiring itself has the strongest radiation (⑤ and ⑦), so we must cover the grounding wire throughout the process. The less wiring, the better. The best choice of the source end filter is the magnetic bead capacitor. The magnetic bead is 60-600 Ω @ 100MHz, and the capacitance is 33pf. Because the clock exceeds the standard, it is generally 100-1000MHz. The magnetic bead is used to reduce 100-500MHz noise (there is a good filtering effect above 500Mhz), and the capacitance is used to reduce 500-1000MHz noise.

5. The long wiring (⑧) in the PCB sometimes has strong clock noise, which may be due to the principle problem of the internal circuit of the chip, or the principle problem of the internal circuit of the chip. If there is a problem in the wiring design of the PCB, the clock noise will be generated. For long lines, use the spectrometer probe to check whether there is noise, and then filter. If the speed of this line is very low or the signal integrity is very low, the filtering parameters can be added, so the processing is very good.

6. Close to the clock line (⑨) is easy to be coupled to noise, so when the clock line does not cover the ground, other lines should be away from high-speed signals such as clocks to reduce the degree of coupling.

7. Because the filtering parameters of the clock cannot be too large, many clocks can do nothing even if they know where the problem is. At this time, you can choose to add spread frequency IC on crystal (⑩), which can effectively reduce radiation. The following figure shows the data test before and after increasing the display frequency.

4、 Conclusion

The clock radiation problem can be analyzed to find the root cause of the clock through EMC testing, but sometimes it may not be able to solve the problem reasonably. Because there are many reasons for exceeding the standard, we can't keep an eye on the clock. At this time, we need to carefully analyze, peel off the silk and cocoon, and remove the clouds to see the light.