Connectors: Not just schematic symbols

-October 24, 2017

When Maxim Integrated introduced the now classic MAX232 in 1988, circuit designers likely didn't give a thought to how the connector affected signal quality or radiated EMI. After all, the data rate for that bus is in the kilobits per second. While signal-integrity engineers have long known that connectors degrade signal quality at high data rates, now EMI engineers must get involved in dealing with radiated emissions from connectors. That's the lesson Colin Brench delivered on October 18 to engineers at the Boston Chapter of the IEEE EMC Society at Bose Corp. in Framingham, Mass.

Brench is Principal Engineer with the High Speed Inter-connect division of Amphenol in Nashua, NH and is well known in EMC circles for his many papers, presentations, and a book on computational modeling. His presentation covered why today's connectors in high-speed links radiate and how to measure their emissions.

According to Brench, EMI is low on the priority list when it comes to high-speed system design. He listed typical design priorities:

  1. Signal integrity
  2. Manufacturing
  3. Cost
  4. EMI
Brench started by explaining that when data rates exceed 20 Gbps (10 GHz clocks), as they do at 28 Gbps NRZ and 56 Gbps PAM4 (14 GHz clocks), EMI became a problem because of the short wavelengths relative to connector size. Today's backplane connectors designed to handle data rates of 56 Gbps have shields around each pair to minimize crosstalk. They are also made of lossy materials that absorb some radiated energy. "Plastics are not well behaved at high frequencies," Brench noted. Thus, you can’t rely on them not to emit energy.

Connectors such as those in the SFP family have their own set of problems. They have active components such as transmitters and receivers. Their size can cover the wavelength of multiple bits. At frequencies over 5 GHz, a backplane connector can be one wavelength in size. Under those conditions, connectors become radiators (Figure 1). Remember that EMI engineers want to keep things like enclosure opening to less than 1/20 wavelength, so a connector that's a full wavelength or more in size can easily cause EMI if not properly designed.

Figure 1. This slide makes a point of how connectors can affect EMI at high data rates.

Even though the SFP family of connectors mount into shielded cages, they need EMI testing. "We have no control over what passes through the connectors," said Brench. Unfortunately, frequencies in the range of 20 GHz to 30 GHz are in a transition area where enclosure shielding is no longer practical because of thermal issues. "When data rates exceed 30 Gbps," noted Brench in a slide, "EMI must be considered in backplane connectors and measurement now go to 70 GHz. Connectors are no longer points on a schematic."

Front-panel connectors such as SFP and its derivatives contain active components that are inherently radiators of RF energy. In addition, they generate heat that must be managed. To cool those components, connector designers add fins (Figure 2). Unfortunately, this kind of design also drives RF current that can result in radiated emissions.

Figure 2. Colin Brench points our cooling fins on an optical SFP connector.

How do you test the emissions from a connector? Brench explained how a connector under test is mounted on a card that then mounts to the wall of a reverberation chamber. A stirrer in the chamber causes emissions to radiate throughout, making it easier for an EMI antenna to receive the connector's radiated emissions. Not only must connectors be tested, but cable assemblies using those connectors must also be tested. Figure 3 shows a test-setup diagram for cable assemblies.

Figure 3. A test setup for measuring emissions in cable assemblies uses a stirrer (left), a reference antenna, and an antenna for capturing the radiated emissions.

Even when connectors and cable have been tested and emit acceptable amounts of RF energy, thermal management often requires network equipment such as server and switch blades to have openings for ventilation. Such opening can let RF energy out or in. According to Brench, cages used to house SFP-type connectors will shield EMI when the connectors are installed, provided the cases are attached to the equipment's face plate through EMI gaskets.

The next meeting of the IEEE EMC Society Central New England Chapter will take place November 16, 2017, 6 PM, at National Technical Systems in Boxborough, Mass. The meeting will include a presentation on EMC chamber design and a demonstration of the time-domain measurement technique described in ANSI C63.25 for test-site validation and diagnostics. Dr. Zubiao Xiong, RF Engineer, ETS-Lindgren will speak. Click here for more information and registration.

Martin Rowe covers test and measurement for EDN and EE Times. Contact him at Circle me on Google+ Follow me on TwitterVisit my LinkedIn page

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