Gigabit Ethernet PMD

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Gigabit Ethernet PMD

• Opto-Link, Inc. Progress Summary
• Vinh Nguyen, Clifton Kerr, Andrew Meyerson, Bryan
  Justice
• April 21, 2005

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Project objective

• Design, assemble, and test the Physical Medium
  Dependent (PMD) layer of a Gbps Ethernet
  optoelectronic link

3
Defining Success

• IEEE compliance is necessary at a minimum
• Staying within our allowed budget
• Assuming the specs are met, the most successful
  board will feature the least costly BOM.

4
Project Planning
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Project Planning

• To ensure that the project was completed on time,
  a Gantt chart was developed
• The Gantt chart shows the scheduled tasks and the
  progress made on each task
• The Gantt chart also shows whether work is
  proceeding on-schedule

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Initial Gantt Chart
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Gantt Chart Revisions

• An initial Gantt chart developed based on project
  objectives and deadlines
• Actual progress rapidly deviated from initial
  Gantt chart
• The initial Gantt chart revised based on rate of
  progress
• The Gantt chart finalized after 4 weeks

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Final Gantt Chart
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Final Gantt Chart Cont.
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Hindsight

• Time crunch towards end of semester
• Should have allocated more time to testing
• Should have worked harder/allocated less time to
  early project phases

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Ideal Gantt Chart
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Ideal Gantt Chart Cont.
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Project Specifications
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Project Specifications

• PMD should conform to the IEEE 802.3
  specifications for type 1000BASE-SX (Short
  Wavelength Laser)
• Key specs Bit-Error rate lt 1 x E9
• Proper operation with 7dB attached attenuation
• Open and defined eye diagram (low noise)
• Extinction ratio gt 9dB
• Eyesafe laser output (lt 1mW)

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• Transmit characteristics (from 802.3z standard)

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• Receive characteristics (from 802.3 Standard)

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Part Selection
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Part Selection Process

• Factors in part selection were
• Product specs (chosen parts must result in an
  IEEE compliant optical link budget)
• Ability to contact and get responses from
  companies and vendors
• Stocking and a sufficiently fast lead time for us
  to obtain the parts in time to build our prototype

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VCSEL Selection

• AOC HFE419x-541
• Suited to our specifications
• Available within two weeks
• Best pricing
• Suitable Emcore sample VCSELs were also secured

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AOC HFE419x-541 4-Corner Analysis
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PD Selection

• AOC HFE3180-108 ROSA
• Suited to specifications
• Delivery within two weeks
• Relatively inexpensive in all quantities
• We would eventually find that incorporating the
  PD and TIA into one can completely eliminated
  crosstalk issues.

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HFE3180-108 ROSA 4-Corner Analysis
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Optical Link Budget
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Optical Link Budget Description

• An optical link budget was computed to ensure
  that all active components would function
  together
• Data from the 4-Corners analysis of the VCSEL and
  the ROSA was used

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Optical Link Budget
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Design and Assembly
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The Design Process

• Schematics based largely off of past designs,
  with some modification.
• Filtering and decoupling a major focus, to make
  sure everything worked as planned.
• PDs no longer widely available ROSA replacing
  both the PD and the trans-impedance amp and
  simplifying circuit
• Schematic design translated to PCB layout
• In translation, emphasis on correctness first and
  spacing second
• Transmission line considerations important

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Transmitter Design Schematic
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Receiver Design Schematic
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Board Layout
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Board Construction

• First design assembled with no problems.
• 0603 components very small and hard to solder, in
  part due to smaller pads
• Dont underestimate how long it takes to put
  together a board

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Board Construction (continued)

• Second design construction was rushed after first
  failed to work
• Communication mishap (and the depths of Hudson)
  left one person to assemble board
• Soldering alone is no fun. Bring a solder buddy,
  as one person only has two hands.

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Board Construction (continued)

• Aggressive design construction last ditch
  attempt to get a working board
• Primarily done because debugging the then-broken
  common-cathode design was not efficient.
  Something had failed, but we couldnt isolate it.
• While soldering, the cause of our previous
  failures became clear. Corrected on this
  assembly.
• Returned later to add a receiver to this design
• Needed to do our most aggressive loop-back test.
• The solder job was rushed once, and the receiver
  wasnt perfect the first time around. Limiting
  amp had to be replaced.

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Testing and Troubleshooting
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Receiver Board Testing
Receiver eye with no attenuation
Receiver eye with 7dB optical attenuation

• First receiver circuit constructed worked from
  the start
• No appreciable signal loss with 7dB optical
  attenuation
• No errors detected with the BER tester in 5
  minutes of operation

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Receiver Board Testing (continued)
Bad receiver eye with 7dB optical attenuation

• Second receiver circuit wasnt so easy
• Eye not clean on regular test (but loopback was
  no worse)
• Error rate of about 10, so signal was good
  enough for the equipment to get a lock but not
  much better.

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Receiver Board Testing (continued)
Fixed receiver eye with 7dB optical attenuation

• But was easily fixed
• Limiting amp poorly attached and multiple pins
  bridged/
• BER of at worst 1e-10 once repaired

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Transmitter Testing

• First two transmitters didnt work so well.
• First, no optical output as the laser was in
  upside down
• Fixed orientation, and got a very messy noise
  band with the traces of an eye inside.
  Insufficient signal?

Transmitter PRBS7 Signal with no attenuation
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Transmitter Testing (continued)

• Troubleshooting accidentally led to part
  failures.
• We blew two VCSELs and a handful of ferrite bead
  inductors.
• Replaced parts, and then got the magic probe
  effect
• Probing the output pins of the laser driver
  cleaned up the eye

Signal output when using the probe across the
output pins
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Transmitter Testing (continued)

• Third time was the charm
• Aggressive design transmitter just worked.
• Same eye as with the magic probe on the other
  design
• At minimum currents, 1e-10 BER with 7dB optical
  attenuation
• Tracked down the source of the Magic Probe
  while testing the good transmitter
• Only happened when probe touched laser driver
  output pins
• Pushing down on the chip with excessive force
  produced the same result
• Bad solder joint!

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The Loop-Back Test

• Once we got a working transmitter and receiver on
  one board, it just worked.
• Lots of jitter on the eye, but lots on the clock
  too
• Connection seems to be getting less reliable with
  time at the splitter
• Did not effect bit error rate measurements
• After 15 minutes of continuous testing, still no
  errors and a BER of 0
• Eye totally disappears when optical cable is
  removed, so entirely a product of transmitted
  light and not electrical cross-talk.

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The Loop-Back Test

• A good, clean eye with a tiny bit of
  clock-induced trigger jitter

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Budget and Ordering
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Preliminary Budgeting (estimation)

• AOC VCSEL 14.50 (2)
• AOC ROSA 10.00 (2)
• Two board fabs 70.00
• Maxim driver and limiting amp, Digikey passives,
  and Murata inductors, plus allowances for
  shipping costs 80.00
• Total projected budget
• Approximately 210.00

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Estimated Budget

• Realized that our preliminary budget was very off
  (i.e. didnt even add up right)
• More itemized for actual parts we intended to use
  as well as quantities of parts
• Based on previous shipping costs estimated total
  costs for entire project
• Still under budget, although not by much (327.85
  for the project)

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Final Budget

• Determined that a second board fabrication was
  unnecessary since first design was adequate
• Includes total amounts paid for parts, shipping
• Total of 268.85 for the project, which is almost
  100 below budget

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Final Budget
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Bill of Materials

• Total cost of mass producing the board was found
  to be 23.93

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Bill of Materials
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Ordering

• Vendors included
• Digi-Key
• Jameco
• PCB Express
• All parts were received in timely fashion and
  progress was never delayed due to waiting for
  parts

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Ordering

• It was learned that Digi-Key has a 5 handling
  charge for any orders under 25
• Therefore it is a good idea to know and get all
  of your parts at once in order to save some money
• Back-ordered parts are not good
• Sometimes you pay for next day shipping when
  ground is what you wanted

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In Retrospect

• When determining the number of parts to order, it
  should be assumed that you will need both
  multiple board fabrications as well as extra
  parts. This means you should initially order
  more than 10 ferrite beads.
• Although individual parts may be more expensive
  from a certain vendor, it is still best to order
  as many parts as possible from the same vendor
• It is possible to hide embezzlement under the
  line Shipping and Handling in the budget