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Optimum Performance Gigabit Ethernet Technology with Purchased Components

Research, examine, test, and connect the purchased components to duplicate Gigabit Ethernet functionality. Explore VCSELs, PIN photodiodes, multimode fiber, photodetectors, and more.

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Optimum Performance Gigabit Ethernet Technology with Purchased Components

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  1. Final Presentation ECE 4006 C G3: Karen Cano, Scott Henderson, Di Qian April, 23 2002

  2. I: Project Tasks and Theory • 1. Research on the transmitting and receiving modules. • 2. Examine the testing board • 3. Search for the components • 4. Testing the evaluation board with purchased components • 5. Connecting the purchased components with parts from other groups.

  3. Project Goal • Duplicate the data transmitting and receiving module functionality of the Gigabit Ethernet technology with purchased components that provide optimum performance at a minimum price.

  4. Possible Solutions • Transmitting module (laser source) • VCSEL • Receiving module (Photo-detector) • PIN photodiode • Other Specs: - SC connectorized (optical) - SMA connectorized (electrical) - 850nm - Multimode (fiber) - relatively low cost

  5. Laser Basics • What is a Laser? • Light Amplification by Stimulated Emission of Radiation • How? 1) Electrons in low-energy levels bumped into high levels by injection of energy 2) When an electron drops to a lower energy level, excess energy is given off as light.

  6. VCSELs • Vertical Cavity Surface Emitting Lasers • Physical makeup • Bragg mirrors • Active region • Fabrication techniques • Molecular beam epitaxy • Vapor phase epitaxy

  7. VCSELs • In EELs no pre-cleaving tests can be performed, testing VCSELs is much cheaper • Less current required for VCSELs • Output beam easier couple into fiber and much less divergent than EELs • Smaller and faster than EELs

  8. VCSELs vs. EELs • Edge Emitting Lasers - give out their light from the sides or edges, therefore no pre-cleaving tests can be performed • Since VCSELs emit light from the top and bottom, they do not have this problem. Testing them is much cheaper

  9. Multimode • Multimode- light is injected into the core and can travel many paths through the cable (i.e. rattling in a tube). • Each path is slightly different in length, so the time variance this causes, spreads pulses of data out and limits the bandwidth.

  10. Singlemode • Fiber has such a narrow core that light takes one path only through the glass. • Not limited to modal-bandwidth. • Very small amount of pulse-spreading is consequential only in Gigabit speed applications.

  11. Photodetectors • Necessary for light pulse detection • Wide variety of of types • Photoconductors • Avalanche photodiodes • PIN photodiodes • MSM photodiodes

  12. Avalanche Photodiodes • Exemplify the “gain-bandwidth” tradeoff • Use the p-n junction model to operate • Take advantage of the avalanche effect • Carrier multiplication • Associated gain • Time constant associated with avalanche • Bandwidth penalty

  13. PIN Photodiode • PIN • Reason for name • Doped region, undoped region, doped region • Unity gain • Functions under reverse bias • Most important parameter for operation • Transit time

  14. Bandwidth vs. Depletion Width • Transit time • Time for subatomic particle to get from one electrode to the other • Based on quickest, typically electron • e- mobility > h+ mobility • Capacitance limited

  15. Transit Time (continued) • Dependence on intrinsic region length • Minimizing this region • High bandwidth applications

  16. MSM Photodiode • Metal-Semiconductor-Metal • Associated work functions • Atomic level metal-semiconductor marriage • High speed (up to 100GHz) • Majority carrier devices • Not developed for Gigabit Ethernet on scale as large as PIN

  17. II: Design OverviewKeeping track of Amps & Watts

  18. Signal Specification Overview

  19. Honeywell VCSELs • HFE4380-521 • Slope Efficiency 0.04 mW/mA • I threshold - 1.5 - 6 mA • HFE4384-522 • Slope Efficiency 0.15 mW/mA • I theshold - 1.5 - 6mA

  20. Link Budget • Analysis of current and power throughout the system, starting at the TX and ending at the RX or trans-impedance amplifier. • Losses • Not all of the light emitted by the VCSEL will reach the PD. • Losses are incurred from the fiber and the SC connectors.

  21. At the PD Side • Once the losses have been calculated into the output power range, this new range of power is to be converted back into current. • When the power or light hits the PD, it is multiplied by the responsivity of the PD, expressed in A/W. • This value is the current coming out of the PD and into the trans-impedance amp.

  22. At the RX Side • The current coming out of the PD has to be large enough to drive the trans-impedance amp, which takes at least 80 uA.

  23. Honeywell Option #1 • Ith =6 mA • DC bias of laser = 6 (1.2) = 7.2 mA • Slope efficiency 0.04 mW/mA • Power output at DC bias = 0.04 * 7.2 = 0.288 mW • Max TX modulation current. 300 mA • Power output at TX modulation current = 04 * 30 = 1.2 mW • Range of emission of light coming from the laser (lossless) =0.288 - 1.2 mW • Losses 3dB or 1/2 output power • Range of emission of light coming from the laser (with losses)=0.144 - 0.6 mW • Responsivity of lasermate's PD = 0.35 A/W • Min. current from PD = .000144 W * 0.35 A/W= 50.4 micro Amps • Max current from PD = .0006 W * 0.35 A/W = 210 micro Amps • Table 1. Link budget for low slope efficiency VCSEL (HFE4380-521).

  24. Honeywell Option #2 • Ith 6 mA • DC bias of laser = 6 (1.2) =7.2 mA • Slope efficiency 0.15mW/mA • Power output at DC bias = 7.2 * 0.15 =1.08 mW • Max TX modulation current. 300 mA • Power output at TX modulation current = .15 * 30 = 4.5 mW • Range of emission of light coming from the laser (lossless) =1.08 - 4.5 mW • Losses 3dB or 1/2 output power • Range of emission of light coming from the laser (with losses)=0.54 - 2.25 mW • Responsivity of lasermate's PD = 0.35 A/W • Min. current from PD = 0.00054 W * 0.35 A/W = 189 micro Amps • Max current from PD = 0.00225 W * 0.35 A/W= 787 micro Amps • Table 2. Link budget for high slope efficiency VCSEL (HFE4384-522).

  25. In Conclusion: Purchasing Choice • Prefer VCSEL with higher slope efficiency because it can drive the trans-impedance amplifier. More importantly, it can do this with the same amount of input current that is needed for the other VCSEL. • If the price difference is not too significant, this VCSEL is the most reliable option.

  26. III: Construction and Testing • Interface optical components with Maxim • Extensive testing to meet standards • Develop more optimal design utilizing newly ordered VCSEL and PD • Repeat testing procedures

  27. Circuit Layout for PD • Optical connection on top, (SC - Light In). Electrical connections on left and right (SMA) • R=1/(2*pi*f*CDET), where CDET = 1.5 pF (from spec. sheet). So, R = 53.1Ω.(impedance of PD). • Capacitors were chosen based on their frequency response at 1.25 GHz. From Murata’s site: C = .01uF.

  28. Circuit Layout for the VCSEL • Electrical connections on the left (SMA - In), optical connections on the right (SC - Light Out). • Constraint: Circuit’s RTOT has to be 50Ω, due to equipment requirements. • VCSEL’s RTYPICAL=25Ω (from spec.sheet) • So, a 25Ω resistor was placed in series with the VCSEL.

  29. Constructing the VCSEL Board • Radio Shack purchased “through” boards • SMA connectors, SMT components, and VCSELs • Impracticalities of initial design • Remedy for small holes and lack of a drill • Problem: GTS-1250’s outputs are AC-coupled • Bias-T applied and shown in new circuit

  30. VCSEL Testing Procedure • GTS-1250 • New VCSEL board • Old opto-board • Scope • Electrical connections and optical loop

  31. Testing Results • IEEE 802.3z standard mask application • Bit error rate settings in scope • New VCSEL vs. old VCSEL • Failure of complete Tx module thus far

  32. VCSEL PCB Design • Five components: - SMT Resistor, - LED, - Power Connector, - 2K ohms Resistor, - SMA Connector

  33. VCSEL PCB Design (Con’t) • SMT Resistor – 0.1 inch • LED – Two Leads • 2K Ohms Resistor – 0.5 inch • Power Connector – hole 0.2 x 0.1 inch

  34. The Complete PCB Design(Left: PD Circuit, Right: VCSEL Circuit)

  35. PCB Dimensions • SMA Leads – 0.045”X0.045”, Pad - .09”X0.09”. (given in the spec sheet) • VCSEL and PD holes – 0.02”X0.02” • Power Connectors – 0.125”X0.125”, Pad – 0.25”X0.25” • SMT – 0.01” • Resistor – Template in Library

  36. The Completed PCBsTop Layer and Bottom Layer

  37. Adjustment to the PCBThe SMA hole diameter was increase to 0.06-7” by using a razor

  38. Optimizing Circuit Design • Minimizing inductors and component quantities in general • Utilizing existing schematics and general EE knowledge • Soldering followed by repeated testing Intel/Agilent PC Interface Card

  39. IV: Construction: Photodetector(Connectorized) • Initial “through-component” board • Connectorized OSI PD • Standard SMT components • SMA connector and DC bias via 5V connection • PD maximum solder temperature of 500oF for total time of ten seconds

  40. V: Testing: Photodectector(Connectorized) • Lack of detectable eye • Simple signal analysis • 50% duty cycle, 1.25GHz square wave • Signal averaging functionality of scope • Comparison to input • Weak output (top)

  41. Testing (cont’d) • Fourier analysis via oscilloscope • Corresponding frequency concentrations, but apparent degradation • Averaging functionality in combination with Fourier • Input and output (bottom) still weakly associated with simple square pattern

  42. Testing (cont’d) • K28.5 (pseudorandom) bit test using Fourier • Averaging still turned on • Further degradation of signal with large amounts of stray artifacts • Corresponding frequencies still appear to be present

  43. Testing (cont’d) • Drastic measures to ensure PD board integrity • Fiber input to PD disconnected and new signal compared • Small amount of change in Fourier, zoomed in farther

  44. Testing (cont’d) • Scope (BNC) connection removed • New signal compared to previous noisy Fourier • Signal dissipated to nearly zero, proved that scope was not causing all noise

  45. Possible Problems at this Stage (1) • Ascertained that the noise was coming from PD board itself • Possibly slow or defective PD, inadequate board or construction • Process repeated, similar results • Circuit design errors unlikely

  46. VI: Construction: Photodetector(Unconnectorized) • Initial “through-component” board • Unconnectorized Hamamatsu PD • Standard SMT components • SMA connector and DC bias via 5V connection • PD maximum solder temperature of 500oF for total time of ten seconds

  47. VII: Testing: Photodetector(Unconnectorized) • “X-Y-Z” stage setup • Cleaved fiber (emission) • SC connector (other end) • Test setup identical to previous connectorized test sans the “stage” • Simple square wave input • Fine adjustment of fiber-vise • No apparent output

  48. Possible Problems at this Stage (2) • Previous semesters board tested • Satisfactory on simple pattern, not on K28.5 • Power connector on old board fickle • Possible broken solder joint on capacitor (VCSEL side) • Incorrect construction unlikely, but inadequate construction possible

  49. Concluding Remarks • Insight into opto-electronics gained • Work experience in team environment • Soldering, test equipment, and test methodology learned • Report writing and presentation experience for future job use

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