Analysis and Solution for Excessive Radiated Emission with a Certain Type of Laser Scanner

HOU Yanchun,ZHANG Qian,YANG Xue,SHI Yanchao,LV Changchun

Beijing Institute of Astronautical Systems Engineering,Beijing 100076

Abstract:The problem of excessive radiated emission for a certain type of laser scanner is discussed in this paper.Firstly,the frequency characteristics of the laser scanner can be obtained by traditional test methods.According to the frequency and electronic equipment characteristics,the interference source can be estimated and established.Then based upon analysis,modified methods are proposed.By applying the above methods,the interference source can be identified and the amount of radiated emission from the scanner can be reduced effectively.Finally,the scanner can operate in a compatible manner with other electronic systems at the same time.The above methods can provide a reference for electronic equipment design ensuring electromagnetic compatibility and passing the radiated emission test.

Key words:radiated emission,modify,shielding,grounding,filter

1 INTRODUCTION

With the strong demand for high communications capacity,miniaturization and multi-tasking,high integration and multifunctional capability for electronic equipment is enhancing rapidly.Similarly the problems with electromagnetic interference (EMI)are appearing gradually.The laser scanner discussed in this paper is a new equipment using high power microwave to form an electronic system,belonging to flight vehicle platform equipment system.The laser scanner works with a large number of electronic equipment,so the electromagnetic environmental is complex.When the scanner works during ground testing,the positioning system cannot work as usual,and the flight vehicle is affected.In order to ensure the flight vehicle is normal and the positioning system will not be affected,the above incompatibility problem needed to be solved rapidly.Through multiple tests,the positioning signals the positioning system received are found being interfered with when the scanner works.In accordance with the three electromagnetic compatibility (EMC)elements (interference source,path and sensitive equipment)and the test results,the scanner can be estimated as an interference source,and the receiving equipment is interfered with by the radiated emission from the scanner.In order to solve the issue,the radiated emission characteristics of the scanner need to be measured and analyzed.Based upon the results,modified methods can be proposed.By applying the modified methods,the scanner will work with other electronic equipment in a compatible manner and the flight vehicle can fly normally.

2 GENERAL PLANNING

The interference phenomenon of radiated emission from a certain electronic system received by other electronic equipment is generally EMC.When developing an electronic equipment,the technology and functionality have to take into account the relevant standards covering electromagnetic emissions which are a necessary requirement in EMC testing[1,2].

In order to ensure that other equipment is not impacted when the scanner operates at the same time,the radiated emission information has to be acquired by testing first.Based upon the test results,including frequency and amplitude information,the main interference source can be identified and isolated.Targeting the interference source, modifications are proposed and verified to ensure that the scanner can work compatibly with other electronic systems[3].

The main steps are as follows:the radiated emission characteristics can be deduced from the appropriate EMC test;then the interference source of the scanning equipment can be identified by using a process of elimination with the auxiliary equipment,equipment under test (EUT) and the components of EUT; and on the basis of the original design,construction of the scanner cannot be altered much,the modified methods of shielding and etc.are proposed and applied.

The characteristics of the project presented in this paper are:

1) The interference source could be fast diagnosed.The main modification approach should then be selected.Naturally avoiding a follow on modification is essential hence care must be taken to ensure the nature of the interference is properly identified.

2) The validity of a single modification method should be verified after it is applied,and before other modification methods are considered.Modification methods are taken and verified one by one to reduce the inaccuracy of validity verification and repeat of modification methods.

3) The original design and construction could not be modified much as proposed in this paper and hence the financial impact is low.In addition,the modification approach is simple and practical.

3 OBTAINING THE RADIATED INTERFERENCE CHARACTERISTICS AND ISOLATING THE INTERFERENCE SOURCE

3.1 Obtaining the Radiated Interference Characteristics [4]

The scanner comprises a power unit and scanning equipment.The boost module is integrated in the power box,the actuator of the electrical equipment is installed in the scanning equipment.Under the test condition,the scanner was powered up using an external power source,and received its command from a test notebook PC,which is considered as an auxiliary equipment.

Based on the operation and frequency characteristics of the scanner and associated sensitive equipment plus the limitations of GJB151A/152A,a test frequency band of 2 MHz -1 GHz was selected.Under these constraints,using a receiving antenna and a receiver,the radiated emission test was conducted when operating in a normal state.The scanner was placed on a test bench,which was grounded.The scanner was powered by the Line Impedance Stabilization Network (LISN) which was connected to a stabilized voltage supply in the laboratory; the auxiliary test notebook PC was placed in a separate control room.The test configuration is shown in Figure 1.

Figure 1 Test configuration

Taking 30 -200 MHz,200 MHz -1 GHz as an example,the test results are shown in Figure 2 and Figure 3.

According to the test results,in the range 30 -200 MHz,the radiated emission from the scanner exceeded the limit line standard.The max.radiated emission is about 120 MHz; in the range 200 MHz -1 GHz,the radiated emission exceeded the limit line standard on multiple frequencies.The max.radiated emission is about 230 MHz.

Figure 2 The limit line and amplitude-frequency curve graph of test data for radiated emission(30 -200 MHz)

Figure 3 The limit line and amplitude-frequency curve graph of test data for radiated emission(200 MHz -1 GHz)

3.2 The Analysis and Confirmation of Interference

In accordance with above test results and the characteristics of constituent parts of the scanner,it was firstly necessary to determine whether or not the radiated emission was impacted by the auxiliary test equipment causing the limit line standard to be exceeded.Once this was confirmed,then the radiated emission characteristics of the power box,scanning equipment could be analyzed.

3.2.1 Radiated emission characteristics of the auxiliary test equipment

In order to confirm whether or not the interference was coming from the EUT or the auxiliary test equipment,the radiated emission from the auxiliary test equipment was observed and analyzed.

The radiated emission was measured when operating with only the auxiliary test notebook PC working at the band of 2 -30 MHz; then with only the auxiliary test notebook PC working at the band of 30 MHz -1 GHz.

The analysis of the test results is as follows.

At the band of 2 -30 MHz,the amount of radiated emission on multiple frequencies exceeded the limit standard for the scanner test in a normal operating state.The max.emission was about 25 MHz.

When only the auxiliary test notebook PC was working (EUT not work),the radiated emission exceeded the limit standard up to 25 MHz.

The test results are shown in Figure 4.

For working at the band of 30 MHz -1 GHz,the radiated emission from the auxiliary test notebook PC was within the limit standard.

According to the test results,the amount of radiated emission from the auxiliary test notebook PC exceeded the limit standard in the range of 20 -30 MHz.As the notebook PC was not part of EUT,an EMC harden notebook PC could be used as the auxiliary test equipment and the measurement repeated again.

3.2.2 The radiated emission characteristics from the power box and scanning equipment

Some test plugs and test apertures are utilized in the scanner.First,the test apertures were covered with copper foil and well grounded.The change in radiated emission could be observed.Then the scanning equipment was disconnected and the power box could be disconnected step by step.At each step the radiated emission could be observed and analyzed.Taking the range 30 -200 MHz as an example,the test results are shown in Table 1.

Figure 4 2 -30 MHz operating (red) and only auxiliary test notebook PC operating (black)

Table 1 Test results of radiated emission from power box and scanning equipment

The test results of radiated emission from scanning equipment and power box are as follows:

1) At the band of 30 -200 MHz,the amount of radiated emission was reduced (Figure 5,red curve) compared with that of in Figure 2;

2) When the power line plug of scanning equipment was disconnected,i.e.the scanning equipment is not operational,at the band of 30 -200 MHz,the amount of radiated emission was naturally reduced.Particularly at the band of 30 -100 MHz,the reduced amount is superior to 5 dB (compare blue curve with red curve in Figure 5);

3) When the power line plug of the power box is disconnected and the power line socket of the power box is covered with copper foil,i.e.the power box is also not operational,the reduction of radiated emission is not that obvious (compare black curve with blue curve in Figure 5).

Based on the above results,we can draw the initial conclusions:

1) The amount of radiated emission is reduced when the apertures of the scanner are covered with copper foil;

2) Comparing to the scanning equipment,the power box is not the main interference source in the range 30 -200 MHz.

Similarly,other frequency bands could be tested using the above approach and we can deduce that the scanning equipment is the main interference source.

4 MODIFICATION METHODS

In accordance with above results,some modifications could be proposed:

1) Cover the apertures on the scanner by copper foil,thus enhancing the shielding characteristics;

2) Place the scanning equipment in a shielded enclosure;

3) Change the auxiliary test notebook PC to an EMC hardened notebook PC.

In order to reduce the radiated emission further,other modifications also could be applied:

1) Add a filter to the boost module in the power box,and provide a shielded enclosure for the power box;

2) Add a shielding layer for the operational module and a grounded signal line.

4.1 Verification of Modification Methods

The radiation emission from the scanner was tested again after applying above modifications.The verification results are shown in Figure 6.

After applying above modifications the following was established:

Figure 5 30 -200 MHz testing

Figure 6 Operation before (red) and after (black) modification at the band of 2 MHz -1 GHz

1) At the band 2 MHz -1 GHz,the radiated emission of the scanner was notably reduced,and the modifications were effective;

2) With the exception of adding a filter to the boost module in the power box,the design and construction do not need to be changed by adopting the above modifications;

3) The tests validated the scanner could work compatibly with other electronic equipment subject to applying the modifications.

5 CONCLUSIONS

The problem of radiated interference for a scanner was presented in this paper.The typical test configuration could be chosen and the radiated data could be acquired; then the radiated frequency and the electronic equipment characteristics could be analyzed,the interference source could be distinguished from the auxiliary test equipment,power box and scanning equipment via a step by step process.To counter the effects of the interference source,modifications including shielding the apertures,adding a shielded enclosure and swapping the auxiliary test notebook PC with an EMC hardened notebook PC have been proposed.It was validated by tests that the radiated emission could be reduced effectively.In the range of 100 -280 MHz,the reduced amplitude was superior to 10 dB.In the range of 280 MHz -1 GHz,the radiated emission could meet the requirements of GJB151A/152A.Based upon this validation,the positioning system can receive signals normally when the laser scanner is operational,and the vehicle fly normally.

It should be noted that the modified methods discussed in this paper are universal,and would be suitable for other electronic products to ensure compatibility.