Electronics Protection - September/October 2013 - (Page 8)
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.
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.
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.
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.
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
Calendar of Events
Five Ways to Realize Server Room Profitability
Electronics Protection - September/October 2013