Chamber verifies quiet designs

A product line of network telemetry and alarm-monitoring equipment that includes alarm-monitoring master units, remote telemetry units, protocol mediators, communications interface adapters, building access units, and other facilities-management equipment

Shorten time to market by performing precompliance radiated and conducted EMI scans in-house. Build and equip an in-house chamber that is large enough to accommodate 3-m emissions tests and that lets engineers perform conducted emissions tests. Increase confidence that a product will pass compliance tests in a certified EMI lab and meet emissions requirements for Network Equipment Building System (NEBS) certification.

• CKC Labs: EMI testing lab. www.ckclabs.com.
• Com-Power: spectrum analyzer. www.com-power.com
• ETS-Lindgren: EMI receiving antenna, EMI test chamber. www.ets-lindgren.com.
• National Technical Systems: EMI testing lab. www.ntscorp.com

DPS Telecom (Fresno, CA) designs and manufactures telemetry equipment used to monitor telecommunication networks and remote site environments. When customers started asking DPS to provide NEBS-compliant products, the company encountered unacceptable delays from using outside testing labs to perform precompliance electromagnetic interference (EMI) testing.

An EMI chamber lets DPS engineers perform precompliance tests in house. Courtesy of DPS Telecom.

With the experimental chamber, engineers learned how to reduce emissions. "At first," said Stover, "we had to meet standards by using filtering and grounding in our chassis." Eventually, by reading books and attending seminars, the staff learned to design for low emissions. With a successful proof of concept, the DPS engineers were ready to pursue their goal of eliminating the need for an outside test lab for precompliance tests. DPS then purchased its own 4x8-ft EMI chamber that the engineers later expanded to 4x16 ft so they could perform 3-m tests.

DPS engineers redesigned existing products at the board level to reduce emissions, which eliminated the need to add ferrites and other components to their chassis. "Today," noted Stover, "there's very little difference in emissions, whether we have the chassis cover on or off. Once in a while, we put an old product into the chamber to verify that the test equipment works because new boards are so quiet."

After setting up the purchased chamber, the DPS engineers brought their measurement equipment—an EMI antenna and a spectrum analyzer—to a test lab and tested a product. Then, they went back to their chamber and retested the product. Doing so let them prove that tests done in their chamber would be consistent with those performed at an outside lab.

During a precompliance EMI scan, DPS engineers simulate the signals that a product sees in actual use. They exercise the Ethernet and RS-232 ports by connecting them to a computer and running diagnostic software. A manual switch box lets them simulate signals from external alarms.

DPS engineers develop eight to 20 new products each year, and they get those products to market four to eight weeks earlier with the in-house chamber. They use an outside lab for compliance certification with a high level of confidence.

Through building a preliminary chamber and studying the fundamentals of designing for compliance, DPS engineers learned that designing for NEBS compliance at the start makes their products less expensive to build and gets them to market faster than trying to retrofit NEBS compliance into existing designs. They've learned how to design quiet boards while using a minimum of ferrites and other components—which can add to a product's cost—to suppress emissions.