Simple Optimization Method for EMI Mesh Pattern ...

Simple Optimization Method for EMI Mesh Pattern Design Mehmet Erhan ALPMAN, Tolga SENGER ASELSAN Inc., MGEO Division, Cankiri Yolu 7.km Akyurt, Ankara, TURKEY

ABSTRACT Metallic mesh coatings are used on visible and infrared windows and domes widely to provide shielding from EMI (Electromagnetic Interference). In this paper, different EMI mesh geometries are compared with each other regarding various performance parameters. But to decide the best fitting EMI mesh geometry to particular optic system is a little bit complicated issue. Therefore, we try to find a simple optimization methodology to decide best EMI mesh geometry design that fits our particular high performance ISR (Intelligence, Surveillance and Reconnaissance) systems.

1. INTRODUCTION Previous works generally have been focused on performance variations between different EMI mesh geometries regarding diffraction pattern. Different EMI mesh geometries which have same SE (Shielding Efficiency) value are compared according to side diffraction lobe energy distribution pattern. Optical performance of different mesh geometries are observed and compared by modelling diffraction pattern of point source in software environment or real setup. Halman [3], Murray [5] and Osmer [6] have published papers about this issue. Some other works have been focused on how to model and calculate SE curves of EMI meshes. These works summarize how to calculate and models the SE curves of EMI meshes and compare experimental results with modelled ones theoretically. Noll [1], Liu [2], Jacoby [4],Halman[7], Xu[8] and Wang[9] have published papers about this issue. As a result, there are many works that are done up to now for different EMI mesh geometries. But each work done for EMI mesh patterns inspected performance effect from different aspects. Therefore, there is too much information about EMI meshes. But if you decide to optimize best EMI mesh fitting to your particular application, this information are a little bit scattered. In our study, we tried to bring together this valuable information. We tried to define simple optimization method for deciding best EMI mesh geometry for our particular ISR (Intelligence, Surveillance and Reconnaissance) system application. We touched on optimization issue in a different way for particular high performance very narrow field of view ISR system. Because of high image quality performance requirements, we developed simple optimization method inherent to this particular ISR system. We inspected MTF (Modulation Transfer Function) performance effect of EMI mesh geometries on image quality while inspecting well known performance effect of different EMI meshes. We compared MTF effect of two EMI grid geometries which have same duty cycle but different line spacing on image quality in software environment.

2. OPTIMIZATION METHOD Today, there are many airborne ISR systems on the market that have very narrow field of view value less than 1 degree. In this study we tried to define a simple optimization method for finding the best EMI mesh geometry that fits to our particular ISR (Intelligence, Surveillance and Reconnaissance) system which has 0.5 degree field of view. We tried to define the manner to decide EMI grid design parameters that will meet system requirements for both optical and shielding efficiency. Window and Dome Technologies and Materials XIV, edited by Brian J. Zelinski, Proc. of SPIE Vol. 9453, 94530L · © 2015 SPIE · CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2176820

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2.1. Decision of Duty Cycle Ratio At the beginning of the work, we have decided to priority of system parameters requirement for specific 0.5 degree VNFOV (very narrow field of view) of ISR system. In our case the highest priority system parameter was optical transmission value requirements of EMI mesh coating. This product was designed to be able to receive enough thermal radiation even under the worst weather conditions. Therefore at the beginning of our optimization study the most important system parameter was optical transmission parameter. We defined optical transmission value for EMI mesh coating as better than 95%. To meet this system requirement, duty cycle of EMI mesh should be around 0.01. The duty cycle of EMI mesh is defined as a ratio of an open area of EMI mesh to the total area of EMI mesh. For grid type EMI mesh, the duty cycle is defined as a ratio of line width to the line spacing. Duty cycle definition for grid type EMI mesh is shown as below: l: line width of EMI grids g: line spacing between grids DC: Duty Cycle =

= 0.01

(1)

Optical transmission of any EMI mesh is calculated by given formula below [3] Optical Transmission of EMI mesh = OT OT = (Open Area of EMI mesh / Total Area of EMI mesh) ² =

= [0.99] = 96%

(2)

As seen from above calculation, to achieve optical transmission value better than 95%, the duty cycle of EMI mesh should be around 0.01. This value will be enough to meet optical transmission value requirement for initial moment.

2.2. Decision of EMI mesh geometry Second step is to decide which EMI mesh geometry should be used for our application. There are many kind of EMI mesh geometries used on different products on the market like hub and spoke type, grid type, randomized ellipse or circles etc.

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Before deciding which EMI mesh geometry to be used we have to review some theoretical information about physical optics. Therefore we have to inspect effect of EMI mesh geometry on point source diffraction pattern. EMI mesh diffraction pattern depends on mesh geometry and these patterns give us information about selected mesh geometry effect on optical system performance. Many works focused on diffraction pattern of different EMI mesh geometries to be able to see diffraction side lobes effect on neighbor pixels. These studies showed that main lobe energy density distribution is independent from the selected EMI mesh geometry .The main lobe energy density loss is same as mesh optical reflecting value. Therefore these studies focused on side lobes energy distribution densities because the energy of side lobes is coupling to neighbor pixels and inducing unwanted noise signals. Different geometries were compared according to side lobe energy densities distribution patterns and energy peak levels. These studies showed that optimum mesh geometry is hub and spoke design for such systems to minimize side lobe energy induced noise on neighbor pixels. The disadvantage of hub and spoke geometry is pattern complexity. This complexity causes implementation to be harder than other geometry. Besides, manufacturing mask of such geometry is complex issue. So we can investigate if it is possible to use alternative mesh geometry like grid pattern for our particular ISR system. First, we have to review side lobes pattern distribution theory. After the diffraction of point source, the closest lobe to main lobe is secondary lobe. The secondary diffraction lobes are separated from main lobes by ΔØ is calculated by formula given in reference [3], where ΔØ = λ/d

(3)

Where λ= wavelength d= line spacing. If we calculate ΔØ for the IR system working at wavelength λ= 5µm and grid line spacing d= 200µm we find that second lobes diffraction angle as: ΔØ = 0.025rad = 1.43 deg.

(3)

As seen from the above result for most of the airborne optic equipment which have narrow fields of view, the diffraction angle value of the second and higher order lobes will be out of the system field of view. This will eliminate side lobe energy induced noise on neighbor pixels for very narrow field of view systems if proper grid line spacing is selected according to field of view. Therefore, while analyzing this kind of specific system, if the system field of view is very small compared to the second lobes diffraction angle we can use any type of mesh geometry that meets optical transmission requirements of mesh, or in other words duty cycle requirements. In case of our particular ISR system, the FOV value is 0.5 degree so we will not consider second and higher order lobes distribution around diffraction pattern. So we can select any EMI mesh geometry for our application. Because of simplicity, we decided to use grid mesh geometry for our application. The grid type mask fabrication and application is easier than other geometry types such as hub and spoke.

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2.3. Decision of grid parameters Next step is to decide optimum g and l values. How to decide g and l values to achieve optimum system performance? We have to decide g and l value for best SE (Shielding Efficiency) performance. Before deciding these values we have to review the formulation and modelling approach for SE curve calculation versus frequency. Noll [1] and Liu [2] have studies that show SE modelling and calculations for grid type shielding geometries. These works theoretically and experimentally show that to keep grid line spacing as small as possible will improve shielding efficiency at higher frequencies. Therefore, in our work we will try to keep line spacing as small as possible. In previous step we calculated the duty cycle of grid pattern according to the optical transmission requirements. Hence, after this point to be able to decide smallest line width and spacing value we have to consider the physical limitation of mask and coating manufacturing process. The physical limitation of mask and coating manufacturing process will limit our possible smallest line spacing decision. This value differs for selected manufacturing process for both mask and coating. After collecting this information we can make decision about line width and spacing easily. 2.4. Effect on system MTF performance After decided possible smallest line width and spacing parameter we have to inspect the effect of decreasing line space on system MTF (Modulation Transfer Function) performance. We have to be sure that decreasing line spacing will induce the system MTF response drop or not. To analyze this phenomenon we compared MTF function of two type’s grids which have the same duty cycle as an example. By this way we hope to observe line space decreasing effect on the system overall MTF performance. For this purpose two types of grid geometry named as Model#1 and Model#2 is compared. These two grid geometry have same duty cycle value as 1%. But the line spacing value is different. For better understanding the effect of line spacing on MTF function, the ratio of line spacing between Model#1 and Model#2 is exaggerated and the ratio is selected as an 100. MTF of each model of grid is calculated on software environment then these results are compared. For Model#1 grid line width g=1um and line spacing l=100um is selected. For Model#2 grid line width g=100um and line spacing l=10000um is selected. MTF function of these two grid model is simulated for 20cm rectangular aperture. Calculated MTF result for both types of grid shown in figure1 below.

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FIGURE 1 MTF result of both models

Observed MTF results for both type of grid pattern are very close to each other, so from this figure it is not easy to see slight diferences between to MTF curves.

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FIGURE 2 Shows area on curve for magnifying

To be able to see slight differences between two MTF curves, the area which shown in Figure 2 inside the circle is magnified.

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FIGURE 3 Magnified sketch of MTF function The difference between two MTF function results is seen well in Figure 3. As seen in the Figure 3, the red line curve belongs to Model#2 while the blue one belongs to Model#1. Spikes on MTF function curve of Model#1 is more than Model#2. Since the line spacing is less in Model#1 the spikes are repeating itself in Model#1 more than Model#2. For each hundred spikes of Model#1 there is one wider spike on MTF curve of Model#2. These results show us there are no differences between two models on image quality. The MTF curves are very close to each others. To check the result again and to be sure these two MTF functions are the same, the total area between two MTF curves are calculated and compared by the software. The total areas between two curves are the same. Therefore without changing the duty cycle of grid pattern, only to change line spacing and line width of EMI grid pattern has no effect on image quality regarding system overall MTF function. Hence, as mentioned above while deciding line space and line width parameter of EMI grid, the best way is to keep line spacing as small as possible to get the best SE performance. The minimum line space value will be limited by mask and manufacturing process technology which will be applied. 2.5. Modifying EMI grid pattern After Electro optic system prototype completed the EMI tests will be performed. If selected and designed EMI grid pattern does not satisfy enough shielding in system level, the SE can be increased by increasing duty cycle of EMI grids. However, this decision will reduce the system optical transmission; therefore at this moment trade off must be done very carefully.

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3. CONCLUSION In this work we tried to find simple optimization method to decide EMI mesh geometries for our particular system. Then we made some simple simulation and comparison to see the effect of our decision on the system overall MTF performance or other meaning on image quality performance. Our future work is to realize the result of designed EMI grid pattern on our Electro Optic system experimentally and to observe effect of the grid pattern on system MTF performance. Later step is to compare theoretical results with the measured value.

4. REFERENCES [1] Robert J. Noll, “Some trade issues for EMI windows”, Proc. SPIE Vol.2286 / 403 [2]Yongmeng Liu, Jiubin Tan, “FQUENCY DEPENDENT MODEL OF SHEET RESISTANCE AND EFFECT ANALYSIS ON SHIELDING EFFECTIVENESS OF TRANSPARENT CONDUCTIVE COATINGS”, Progress In Electromagnetic Research, Vol. 140, 353-368, 2013 [3]Jennifer I. Halman, Keith A. Ramsey, Michael Thomas, Andrew Griffin, “Predicted and measured transmission and diffraction by a metallic mesh coating”, Proc. Of SPIE Vol. 7302 73020Y-1 [4]Keith T. Jacoby, Matthew W. Prieratt, Jennifer I. Hallman, Keith A. Ramsey,”Predicted and measured EMI shielding effectiveness of metallic mesh coating on a sapphire window over a broad frequency range”, Proc. Of SPIE Vol. 7302 73020X-1 [5]Ian B. Murray, Victor Densmore, Vaibhav Bora, “Numerical comparison of grid patterndiffraction effects through measurement and modeling with Optiscan software”, Proc. Of SPIE Vol. 8016 80160U-1 [6] Kurt A. Osmer, Mike I. Jones, “ Optical characterization of photolithographic metal grids”, SPIE Vol. 1498 Tactical Infrared Systems (1991) [7] Jennifer Halman, Keith Ramsey, Vashti Sawtelle, “Effects of mesh voids on insertion loss of metallic mesh coatings”, Proc. of SPIE Vol. 6545 65450X-1 [8] Hua Xu, Steven M. Anlage, Liangbing Hu, George Gruner, “Microwave shielding of transparent and conducting single walled carbon nanotube films”, APPLIED PHYSICS LETTERS 90, 183119 (2007) [9] Lin Biao Wang, Jun Wu Zhang, Kye Yak See, Tengiz Svimonishvili, “Ultra Thin and Flexible Multi Band Rejection EMI Shield”, Journal of Electromagnetic Analysis and Applications, 2014, 6, 163-173

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