Problems emerge when electromagnetically “hot” optical transceivers are placed into systems that are bound by emissions limits. Because radiating transceivers are mounted externally or couple to inherent features of systems such as air vents, seams, and holes, they break the integrity of the enclosure shield and cause these systems to behave like antennas, amplifying energy generated inside systems and then transmitting to the outside world.
An in-depth understanding of the electromagnetic interactions inside optical transceivers and between transceivers and their environment is critical for effective design of high-performance opticalnetworking products. Virtual-prototyping of the transceivers themselves and their effect on a system could mean the difference between passing and failing FCC and CE emissions compliance tests.
All of the PCBs in the test system were modeled as solid copper sheets and the only details on the PCBs that were modeled were the chips located within the transceiver that drives the optics, as determined by previous research. The outputs for the single transceiver were monitored at 3.2 GHz and 7.4 GHz, which were known to be problem frequencies with small form factor transceivers.
Figure 1 shows cylindrical profiles of the emissions produced by the single transceiver at 3.2 GHz. The cylindrical profiles shown are at a distance of 3 m from the transceiver and simulate an antenna scan height of 3 m, which is in accordance with the test methods currently specified in EMC standards around the world where the device under test (DUT) is rotated in relation to the antenna and the antenna is scanned in height to determine the maximum emissions from the device under test (DUT). This type of output enables the user to determine the direction of the maximum emissions, which can be ultimately used to determine where design modifications need to be made to the DUT, and also help the test engineer to find the maximum emissions during precompliance testing.
From the cylindrical profile it can be seen that the worst position for the emissions is from the side at 3.2 GHz. These locations for the maximum emissions correspond well with measured data. Figure 2 shows the electric field pattern produced by the transceiver at the same frequency, monitored with a horizontal plane through the center of the transmit/ receive ports.
The field profiles enable a user of Flomerics FLO/ EMC to understand how the fields are escaping from a device and to focus any design modifications that need to be made in the correct area. Further investigation of these output files would enable a designer to fully identify the path the field was taking and enable corresponding design changes to be implemented. The design information generated for the optical transceiver in this example took only a matter of hours. Generating this amount of data without the analysis software would take days in the lab and the information gathered would be much less complete.
This article was written by David Johns, Vice President, Electromagnetic Engineering, at Flomerics, Inc. For more information, contact Mr. Johns at 508-357-2012 or visit http://info.ims.ca/5657-201.