Gigabit Ethernet PMD

1. Gigabit Ethernet PMD Opto-Link, Inc. – Progress Summary Vinh Nguyen, Clifton Kerr, Andrew Meyerson, Bryan Justice April 21, 2005

2. 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

5. 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

6. Initial Gantt Chart

7. 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

8. Final Gantt Chart

9. Final Gantt Chart Cont.

10. 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

11. Ideal Gantt Chart

12. Ideal Gantt Chart Cont.

13. Project Specifications

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

15. Transmit characteristics (from 802.3z standard)

16. Receive characteristics (from 802.3 Standard)

17. Part Selection

18. 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

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

20. AOC HFE419x-541 4-Corner Analysis

21. 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.

22. HFE3180-108 ROSA 4-Corner Analysis

23. Optical Link Budget

24. 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

25. Optical Link Budget

26. Design and Assembly

27. 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

28. Transmitter Design Schematic

29. Receiver Design Schematic

30. Board Layout

31. Board Construction • First design assembled with no problems. • 0603 components very small and hard to solder, in part due to smaller pads • Don’t underestimate how long it takes to put together a board

32. 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.

33. 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 couldn’t 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 wasn’t perfect the first time around. Limiting amp had to be replaced.

34. Testing and Troubleshooting

35. Receiver Board Testing • 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 Receiver eye with no attenuation Receiver eye with 7dB optical attenuation

36. Receiver Board Testing (continued) • Second receiver circuit wasn’t 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. Bad receiver eye with 7dB optical attenuation

37. Receiver Board Testing (continued) • But was easily fixed • Limiting amp poorly attached and multiple pins bridged/ • BER of at worst 1e-10 once repaired Fixed receiver eye with 7dB optical attenuation

38. Transmitter Testing • First two transmitters didn’t 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

39. 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

40. 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!

41. 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.

42. The Loop-Back Test A good, clean eye with a tiny bit of clock-induced trigger jitter

43. Budget and Ordering

44. 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

45. Estimated Budget • Realized that our preliminary budget was very off (i.e. didn’t 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)

47. 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

48. Final Budget

49. Bill of Materials • Total cost of mass producing the board was found to be $23.93

50. Bill of Materials