元太电子

In some product design applications, we will encounter EMI problems with cable connections;

I compare the EMI-radiation theory of PCB and cable to compare:

A. PCB traces form radiation problems:

There are many signal loops on the PCB. There are differential mode current loops and common mode current loops. When calculating the radiation intensity, it can be equivalent to a loop antenna. The radiation intensity is calculated by the following formula:

E: electric field strength (V/m)

f: frequency of working current (MHZ)

A: Current loop area (cm^2)

I: current intensity (mA)

r: distance from the test point to the current loop (m)

B. Connection lines and cables form radiation problems:

When calculating the radiation intensity of a cable, it is equivalent to a monopole antenna, and its radiation intensity is calculated by:

E: electric field strength (V/m)

f: frequency of working current (MHZ)

L: cable length L (m)

I: current intensity (mA)

r: distance from the test point to the current loop (m)

Conclusion: It can be seen from the above two equations that the radiation efficiency of the cable is much larger than the radiation efficiency of the PCB! !

Case 1: The sound of the speaker cable is similar to that of the transmission test data; at this time, pay attention to the filter design of the product and the EMI problem of the audio cable! The EMI conduction test data is analyzed by the following products: EMI test case of Bluetooth audio plus lamp; EMI conduction data of product test is as follows:

Separately testing the power board and the control system does not exceed the standard problem. When the speaker cable is connected, the audio line interference will cause the EMI conduction test data to exceed the standard, as above test curve; at the same time, the filter and the magnetic ring are not effective;

2. Detecting the structural composition and circuit principle of the product

The product is powered by the switching power supply; the length of the speaker cable can not be shortened; the long connection line will have a larger coupling distribution capacitance; for the conduction problem, I will compare the 50R impedance of LISEN to the product structure for analysis:

From the equivalent analysis chart in the figure, we will clearly see the source of the EMI problem, and we can also implement a variety of solutions according to the illustrated path!

A. From the test curve to analyze the switching power supply itself is not much problem;

B. When the test cable has a length of about 1 meter and the power cable is not separated by a certain distance, there will be coupled audio interference on the power line; resulting in EMI exceeding the standard;

C. According to my suggestion, when the speaker cable and the power cord are pulled apart, the EMI over-standard point is basically improved by 5dB;

D. Due to the product's own use characteristics, the customer requires that the power board and device should not be modified for optimization; the best way is to change the speaker cable to shielded cable for test replacement!

3. Shielded cable In the test we should pay attention to the correct use method?

The above is the test connection diagram of the customer using the shielded cable and the obtained EMI test data;

Is the high frequency band exceeding the standard at the back end of the test data? Why?

When conducting the radiation disturbance test; the TV's HDMI interface is connected to the DVD, game equipment, etc. and performs data communication; in the case where the test data is horizontally polarized by the radiation receiving antenna, there is a point (frequency point 148.6Mhz) exceeding the CLASSB. Limitation requirements, and the high frequency single-noise noise is more than 74.3MHZ multiplier!

A. Analyze according to the test data shown in the figure: The transmission rate of the HDMI interface is very high. Periodic signals and harmonics of the signal will generate radiated disturbances through the transmission cable (the HDMI feature frequency is around 74MHZ).

B. Usually when the control chip and the interface chip are in data communication, the common mode voltage may be generated between the ground and the power source of the chip, which may cause noise problems!

The HDMI interface cable uses a shielded cable; when I checked the shielded cable, I found that the HDMI shielded cable shield for testing and the HDMI port metal connector used a "single-point grounding" connection! From this I can judge that the connection at the place needs to be optimized;

Change the connection mode of the shielded cable of the HDMI interface and the metal connector, that is, the shield layer and the metal shell of the connector are connected by a 360° ring! Test the EMI radiation data and pass the test; refer to the following:

Change the connection mode of the shielded cable shield to the metal connector, and cancel the original single-point grounding to achieve 360° overlap between the shield and the metal interface. Thereby improving the problem of EMI!

6. Summary

A. Connection line cable EMI conduction problem point - path analysis is critical:

B. The problem of single-point grounding of high-frequency digital signal shielding cable is equivalent to connecting a few tens of nH inductors in series on the shielding layer, which can generate a common mode voltage on the cable shielding layer of the interface due to the shielding current; As the frequency increases, the equivalent transfer impedance of the single-point ground connection will also increase rapidly, which will not only completely shield the shielded cable but also cause additional disturbances. Therefore, the shielding layer of the shielded cable and the connector are very important, and the application of high-frequency digital signals must ensure 360° overlap! !

From the product EMI test principle analysis: the main impact on the product EMI test results for common mode interference, therefore, the product's EMI problem is mainly related to common mode interference, for the EMI design of electronic products, the real need we focus on the common mode problem!

Therefore, for the actual application of electronic products, attention should be paid to the internal structure of the product and the problem of the cable of the cable; the radiation efficiency of the cable is much greater than the radiation efficiency of the PCB! The system should have priority when connecting cables and cables!