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Gigabit Ethernet PMD

Gigabit Ethernet PMD. Opto-Link, Inc. – Status Report 9 Vinh Nguyen April 14, 2005. Completed Tasks Finished troubleshooting Updated Bill of Materials Updated budget to reflect latest purchases. Next Week’s Tasks Complete second board layout Design review for next fabrication

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Gigabit Ethernet PMD

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  1. Gigabit Ethernet PMD Opto-Link, Inc. – Status Report 9 Vinh Nguyen April 14, 2005

  2. Completed Tasks Finished troubleshooting Updated Bill of Materials Updated budget to reflect latest purchases Next Week’s Tasks Complete second board layout Design review for next fabrication Place order for board fabrication Assemble second board Gigabit Ethernet:Progress Summary

  3. Gantt Chart

  4. Current Budget

  5. Projected Budget

  6. Estimated Bill of Materials

  7. Transmitter Testing • Last week’s transmitter problems persisted • Testing resulted in closed “eyes” for both the AOC and the Emcore board designs • The VCSEL is lasing but no signal is getting though to the receiver • Probing the board resulted in a blown inductor Transmitter PRBS7 Signal with no attenuation

  8. Troubleshooting • Hypothesis: Limiting resistor values may be incorrect • Would prevent the VCSEL from receiving enough current to bias it above threshold • Adjusting the potentiometers would have little effect on the output signal beyond a certain level

  9. Testing the Hypothesis • Set bias potentiometer to high resistance • Shorted the limiting resistor • Reduced the potentiometer resistance • Resulted in closed eye • Continued reduction of potentiometer resistance resulted in a blown Emcore VCSEL • Probing the AOC board design resulted in another blown VCSEL

  10. Repairs • Replaced all damaged components on the Emcore design • Testing the repaired board resulted in closed eyes again • Tried to systematically tune potentiometers • Careful tuning did not result in open eyes • Carefully probed inputs and outputs

  11. Test Signals Output signal for D21.5 input obtained using the optical test equipment D21.5 input signal obtained using the probe

  12. Test Signals Cont. • Tried using PRBS7 test signal as input PRBS7 input obtained using the probe plotted beside the clock signal

  13. The Magic Probe • Probing between the laser driver and the VCSEL resulted in the “Magic Probe Effect” • The optical output signal formed excellent eye diagrams when the probe leads were placed between the laser driver and the VCSEL

  14. Magic Probe Results Eye diagram for optical output for PRBS7 input with probe

  15. Possible Explanations • The use of smaller potentiometers may have resulted in the overdriving of the laser driver chip • Board layout issues • Unbalanced circuit • Weird layout effects

  16. Testing Hypothesis • Replaced the bias potentiometer with a larger 50 kΩ potentiometer from the practice board • Unable to obtain an open eye diagram by tuning the potentiometer • Can still obtain good eye diagrams using the “Magic Probe” • Effectively rules out overdriving hypothesis • Only leaves potential layout issues

  17. Next Week • Complete second board layout • Second board layout will use passive components to try to replicate the effect of the probe on the board • Design review for next board fabrication? • Place order for board fabrication • Start Final Report • Assemble second board • Get board blessed in chapel

  18. Board Layout Review

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