Messtechnik Figure 3: Actual SELDS test setup tion of small leakages within a larger contiguous surface, not for detecting intended openings (e.g. open chamber door, waveguide feedthrough). At any place in the enclosure with a small leak (e.g., a crack or seam gap), there will be a small magnetic field formed. The Receiver is designed to detect these small magnetic fields. As the Receiver is moved along a seam and encounters this magnetic field, the display will indicate a relative decibel value when detecting a sufficient field level, dependent upon the Receiver sensitivity settings. An audible tone will increase in volume at higher value indications. The audible tone is not calibrated but is an indicator that simplifies chamber probing without requiring continuous observation of the display. If a poor seam or opening exists in the enclosure walls, a signal will be detected when scanning with the Receiver. Detected signals will cause the Receiver display to indicate larger values, indicating less shielding effectiveness. The unit of measure is Shielding Units (SU), which is a relative measurement from the calibration level set at AR’s factory. The measured values obtained by the system allow identifying areas of potential RF leakage and decreased shielding effectiveness but cannot be directly converted to Shielding Effectiveness measurements obtained using other methods. The indication of shielding effectiveness is directly related to the quality of workmanship of each seam, door seal, and filter connection. When the system is used to probe around these places the maximum displayed value indication pinpoints the location of the fault, which may be, among other things, a loose bolt, bent panel, or corroded door facing. The data acquired by this system may be used as a guide for: (1) immediate corrective actions, (2) correlation for proof of performance test as a basis for future acceptance tests, or (3) enclosure profiling. Due to the high energy electromagnetic fields which surround each feed wire, a strong local effect is created which may be detected by the Receiver along the path of the feed wire, and if this effect is not taken into account, the resulting readings near the feed wires could be misconstrued as indicating a faulty seam. Due to this effect, a second feed wire configuration is used in which the feed wires are placed in such a way as to be an approximate mirror image of the first configuration. Chamber probing is then repeated along the path of the original feed wire to confirm leakage points. The user who is familiar with attenuation scales on conventional RF measuring equipment should notice that the system is calibrated to indicate direct shielding effectiveness rather than field strength. That is since an increase in received signal occurs when the shielding decreases, an increased signal causes the displayed value to decrease. Integration Careful attention must be given to the connection and dress of the Transmitter feed wires. For all Transmitter feed wire configurations used, the Transmitter is connected at diagonally opposite corners of the structure being tested. Connecting across diagonal corners will provide the most evenly distributed signal coverage across the structure. By convention, the Transmitter is physically located at and connected to the front wall, which is defined as the most accessible, and is typically where the door is located. The second Transmitter feed wire should be dressed along the vertical and horizontal edges of the enclosure to a connection point at the opposite diagonal corner (see Figure 3 for an actual setup). Use approximately 12 inches of No. 12AWG insulated, stranded wire for the short cable and as much as necessary for the long cable. If a bolt or screw head is not available for attaching the ends of the feed wires, drill and tap the nearest frame member. Do not tap through the panel. These feed wires can be installed permanently if desired to facilitate future testing. The polarity of the connections is inconsequential. Enclosure probing To perform enclosure probing, first examine all six surfaces inside the enclosure with the door closed. Initially adjust the Receiver audio volume knob and gain setting to its maximum (highest sensitivity), then adjust both down as necessary to accommodate the environment. Holding the Receiver, so that the probe tip is as close as possible to and perpendicular to the wall (floor, ceiling, or one of the four sides), move the unit slowly (about six inches a second) down each seam. Stop when a sharp increase in the audible signal level is heard, or when the displayed value fluctuates noticeably, record the seam position, the distance of the apparent leak from the nearest active feed wire, and the Receiver reading. For optimum sensitivity, the probe should be perpendicular to the surface being tested. The Receiver units are on a logarithmic scale. When taking measurements, the value indicated on the Receiver display is in relative SUs. Conclusion No matter which standards must be met, selecting the right Shielding Effectiveness Leakage Detector is the key to ensuring commercial electronics and military EMI rooms are adequately shielded and are in compliance with standards. So, AR has all the technical expertise and Shielding Effectiveness Leakage Detector System that you need to meet the MIL-STD-188/125 requirements. To learn more about AR SELDS system, feel free to contact one of our applications engineers at 800-933-8181 or visit our website at www. arworld.us. ◄ 10 hf-praxis 6/<strong>2019</strong>
BOOSTING your overall test & measurement quality by patented TDEMI® TECHNOLOGY 685 MHz REAL-TIME BANDWIDTH TDEMI® TECHNOLOGY 40 GHz ULTRA-FAST RECEIVER SCANNING TDEMI® TECHNOLOGY The TDEMI® ULTRA is the only Solution providing all the Features of the "FFT-based measuring Instrument" according to the new Standards with 685 MHz Real-time Bandwidth and CISPR Detectors. by the inventors of the full compliance real-time FFT based measuring instrument. gauss-instruments.com
Change language