Gigabit Ethernet PMD

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Gigabit Ethernet PMD

• Opto-Link, Inc. Status Report 9
• Vinh Nguyen
• April 14, 2005

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Gigabit EthernetProgress Summary

• Completed Tasks
• Finished troubleshooting
• Updated Bill of Materials
• Updated budget to reflect latest purchases

• Next Weeks Tasks
• Complete second board layout
• Design review for next fabrication
• Place order for board fabrication
• Assemble second board

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Gantt Chart
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Current Budget
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Projected Budget
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Estimated Bill of Materials
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Transmitter Testing

• Last weeks 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
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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

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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

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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

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

• Tried using PRBS7 test signal as input

PRBS7 input obtained using the probe plotted
beside the clock signal
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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

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

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Testing Hypothesis

• Replaced the bias potentiometer with a larger 50
  kO 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

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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

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Board Layout Review