EMC radiation emission rectification - principle case analysis

Introduction

In the first article of this official account , Let's get to EMC And SI The relationship between , And mentioned EMC The basic principle of filter and EMC The basic principle of the receiver .

Today we will continue to discuss the theoretical model and method of radiation emission rectification , The theory and practice complement each other .

Before opening , Let's set up a model , The current and return current in the conductor will be distributed differently according to the frequency , Please look at the chart below. ：

As we can see from the picture above , Low frequency running everywhere , So if we need shielding , Then it is necessary to shield a wide area , Cover the reflow area , The high frequency return area is very small , Therefore, the area to be controlled is very small .

Just imagine , If in the place of high-frequency reflux （GND perhaps VDD） The plane leads out a long ground wire , also PCB The shielding effectiveness of the plane to the outgoing line is not enough , In this way, the lead wire has high radiation efficiency .

For low frequencies , Why single point grounding , There are many reasons , One is to control its own return path , Secondly, digital noise shall be prevented from invading through the above ground public impedance , Affect performance .

【 Phenomenon description 】

In this project , It's going on RE102 In the test , Frequency band 200MHz~1GHz in 200 many MHz Single point exceedance , And the high frequency has spurious single point , See the figure below for the data ：

【 Cause analysis 】

Rapid positioning ： The products are iron shell products , A lot of cables were pulled out at the laboratory site , Unplug the cables one by one , It is found that the interference comes from DVI Cable , take DVI Cable buckle magnetic ring , The interface is grounded and shielded , Found low frequency 200 many MHz Go down. , The middle goes up , It also exceeded the standard , Why is that , Please look at the chart below. ：

Further study of ： From the above measures, we can see , No matter how close we are to the ground, there will be different but similar frequency points exceeding the standard , It has nothing to do with the magnetic ring , Add it or not .

There are various signs that we have other sources of noise that have not been found , So we took the machine apart , Specially study whether the cable is properly grounded , We finally found out why , The external shielding layer of the cable at the grounding point isolates the plastic plug from the place where the machine appears . Remove the plug or wrap it with a conductive cloth ,RE Radiation immediate OK 了 , Please look at the chart below. ：

【 Rectification plan 】

Comprehensive product waterproof requirements , The plug cannot be removed , So make the plug into a conductive plastic plug . This scheme does not need to be revised , Just change the plastic stopper material , No impact on other performance .

Just say something

The model of the plug is like PCB equally , There is a big gap in the return path , Plug up or PCB Mend the cracks , The reflux is continuous . In addition, we can also consider the phase of the antenna here , Two points are connected , The potential phase is the same , No power lines , So there is no electromagnetic field , Of course, there is no electromagnetic wave .

This article comes from the Internet ：《EMC Radiation emission rectification , We can think of it this way !》