Sealing the EMI
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 5/1/2008
PROJECT DESCRIPTION
Quell (Albuquerque, NM, www.quell.us), a maker of silicone rubber EMI filters used in connectors, needed an automated tester that could adapt to fit any filter. A division of National Technical Systems (NTS; www.ntscorp.com) that was formerly B&B Technologies developed a PXI-based tester for the filters.
| More about the device under test, the challenge, and the tools. |
To provide flexibility, NTS system integrator Brian Shea designed an interconnect box that uses a 17x17 “bed of nails” array with 289 pogo pins spaced 0.1-in. apart in a grid. Quell engineers then design custom adapter boards with partial connectors that hold the filters under test. The number of pins in a connector determines the number of filters that a test operator can test at a time. The adapter board provides the tester with access to the required pins.
 A PXI-based system measures resistance and capacitance in EMI filters. |
The system performs a check on the adapter board by measuring the capacitance of open and short circuits with the 6½-digit digital multimeter (DMM) card. If the measurements are within tolerance, testing can begin. If the measurements are out of tolerance, the system prompts the operator to insert a calibration fixture with opens and shorts that lets engineers calibrate the signal path.
Next, the system measures capacitance and resistance for each of the 289 test points on the adapter card. The system stores the calibration data on each capacitance range on each pin. Once the system is operating within tolerance for each range, it begins the test. The system measures resistance with the 7½-digit DMM, then measures capacitance, compensating for errors produced by cables and switches. Switches connect the DMM cards to the appropriate pins in the array.
After measuring capacitance and resistance among all required pins and between each required pin and ground, the tester stores the results for each test point in a comma-separated variable (CSV) file. It then generates a pass report or fail report based on the test results and it generates a summary report. When a filter fails, the operator will print a failed-part report and send the part to the repair department. The system holds test data until an administrator downloads the data to read it with Excel. In the future, the system may store data in a database.
LESSONS LEARNEDShea encountered cabling issues when he first developed the tester. “We wanted to provide for disconnecting the interconnect box from the switch cards in case the test fixture needed repair,” he said. “We tried to use a 100-pin straight-through connector, but the 1-mm pitch on the switch card connectors prevented the cable’s connector from connecting to the switch card. So, we used the manufacturer’s recommended ribbon cable, which is permanently attached to the switch card.”
| DEVICE UNDER TEST
Electromagnetic interference (EMI) filters used on standard and custom connectors. Made of silicone rubber, the filters fit inside connector shells and holes let pins through the filter material. Standard connectors include commercial connectors such as DB-25 and military connectors such as MIL-C-38999, MIL-C-5015, and ARINC-404. THE CHALLENGEDevelop a tester that measures the capacitance and resistance of the rubber filters. The tester must be flexible to handle any standard or custom connector, and it must be able to automatically test several connectors in one setting. Tester must measure capacitance and resistance (1 Ω to 5 GΩ) between any two connector pins and from any pin to ground. Provide for self-calibration in each capacitance or resistance range. THE TOOLS
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