Electronics Protection - September/October 2013 - (Page 8)

Feature Predicting Gasket Performance: SE Measurements with a TEM Cell to Study Gasket Reliability Dr. James Drewniak, Professor, EMC Lab, Missouri University of Science and Technology Parisa Faraji, EMI Scientist, Laird Dr. Douglas McBain, Director of Technology, EMI, Laird Dr. David Pommerenke, Associate Professor, EMC Lab, Missouri University of Science and Technology Shielding Effectiveness (SE) is a means to measure EMI gasket performance in order to predict how well a structure is protected from EMI that may enter from or escape to the outside environment. EMI gaskets are used to maintain proper shielding in a variety of different commercial and military applications, which may be subjected to environmental stresses. EMI gasket exposure to harsh environments can result in physical or electrical degradation of the gasket performance, which can lead to a loss of SE. Environmental reliability testing is fundamental in reducing potential problems that the system can experience when it is operating in its natural environment. Proper SE data after a variety of different reliability tests such as accelerated aging, salt fog, or galvanic corrosion is essential for long term gasket performance prediction. Measurement System The SE measurement system includes a gasket testing enclosure (GTE), a Vector Network Analyzer (VNA), and a laptop computer. A low noise 20 dB amplifier is utilized to increase the dynamic range. The GTE provides a means for testing how well a test gasket can limit the amount of RF energy transmitted from one side of the enclosure to the other. Further, the GTE permits variation in the amount of compression on the gasket holder during testing. The GTE is a rectangular double-sided Transverse Electro-Magnetic (TEM) cell made with 0.375 inch thick aluminium plates and external dimensions of 22.5 by 12.75 by 4.75 inches. It is a closed cavity with 50Ω transmission line and the Device under Test (DUT) is placed in a gasket holder underneath the separating plate, an internal view of the fixture and the gasket holder are shown in Figure 1. Variety of gasket profiles and different gasket-holder mating materials can be tested with this structure. Figure 1. (A) TEM Cell Interior, (B) Gasket Holder with Gasket Under Test The field at all cross-sections along the axis of the TEM horn has a uniform amplitude and phase, similar to a 1D uniform plane wave. The TEM horns directly inject the current and need to maintain a constant width-to-height ratio, ensuring constant characteristic 50Ω impedance across the TEM Structure. The gasket holder fixture maintains a constant and uniform compression on the DUT during measurement and aging. The fixture along with the compressed gasket can be removed for accelerated aging without altering the compression or disturbing the gasket to holder interfaces. 8 Measurements There are several measurements that should be considered when predicting gasket performance, including open slider reference measurement, virtual noise floor, dynamic range and gasket SE. Open Slider Reference Measurement The insertion loss measurement of the TEM structures without a gasket provides a frequency-based means for the usable bandwidth of the GTE. The insertion loss should be as small as possible to ensure that there is not significant energy loss between the ports as a result of mismatch losses. Ideally, the insertion loss is zero dB (without amplifier), because all of the transverse fields developed in the driven structure should be captured by the receiving structure. However, the insertion loss of each structure drops as frequency increases, but remains less than 5 dB over the entire usable range of operation. Beyond 2 GHz the insertion loss is affected by overmoding of the enclosure resonant cavity. By adding an amplifier, the virtual reference level or 100% transmission shifts from 0 dB to 20 dB which is the amplifier gain. Virtual Noise Floor The virtual noise floor is the best possible measurement of the GTE in terms of shielding effectiveness between the TEM structures. The term virtual noise floor corresponds to the noise floor of the GTE as measured by the network analyzer. It is a measure of how well a structure placed between cavities can keep a signal on one TEM structure from appearing on the other TEM structure. Since the GTE is utilized for testing gaskets placed between the slider plate and the GTE floor, it must be guaranteed that there are no other points along the slider where the signal can leak. The virtual noise floor is quantified by measuring the magnitude of S21, when the two sides of the TEM cell are isolated entirely by placing some copper wool under the separating plate. Then the TEM horns are slid into the cell and contact the slider. A good highfrequency contact is ensured with a gasket between the slider face and the thin edge of the TEM horn along its entire length. This effectively provides a short between these two structures. The goal is to achieve isolation as close as possible to the VNA noise floor. Some modification to the VNA settings and fixture gasketing between the two TEM cells is required to maintain proper isolation. The virtual noise floor is nearly -105 dB over the optimal range of operation (<3 GHz), and is almost consistent across the entire measurement range. Dynamic Range The Dynamic Range (DR) is the range of amplitudes over which the system operates linearly. It is numerically equal to the difference between the maximum and minimum signal amplitudes in decibels that can be measured. For shielding effectiveness measurements, DR is determined from the reference level to the virtual noise floor. For DR validation of SE procedure this value needs to be verified, which represents the maximum SE measurement at the specific frequency with particular equipment and settings. The TEM cell operational DR is verified by two tests, which are the open slider reference measurements, with the two TEM structures electrically connected and the closed slider noise floor measurements. These measurements are presented in Figure 2 and the difference between these values determines the fixture DR. The TEM has a DR of approximately 120 dB across operational bandwidth. September/October 2013 www.ElectronicsProtectionMagazine.com http://www.ElectronicsProtectionMagazine.com

Table of Contents for the Digital Edition of Electronics Protection - September/October 2013

Geist Unveils Rapid Deployment Data Center Environmental Monitoring System
Enabling Effective Thermal Management with DCIM
Predicting Gasket Performance: SE Measurements with a TEM Cell to Study Gasket Reliability
Rack Containment 101
The Nine Core Elements of DCIM
Using Electronic Locking Solutions to Secure Enclosures and Meet Storage Compliance Needs
Hammond’s HJ Series Ticks All the Boxes
Rogers Introduces Poron SlimGrip Foam
Ferrite Suppressors Clear Interference
Fujipoly Thermal Sheets are a Cooling Influence on LED Lighting
GE Introduces TLE Series UPS Platform
Gore PolyVent XL Improves Reliability of Large Outdoor Enclosures
IMI Sensors Launches Linear Adjust Mechanical Vibration Switch
Industry News
Calendar of Events
Five Ways to Realize Server Room Profitability

Electronics Protection - September/October 2013