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PCB Design for 1 Gbps

PCB Design for 1 Gbps. ECE 4006 Dr Brooke. Overview. What signals are being routed? How can you route those signals? How to apply routing to PCB? PCB design techniques. Signals being routed. High Frequency Sensitive Analog (e.g., IN from PD)

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PCB Design for 1 Gbps

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  1. PCB Design for 1 Gbps ECE 4006 Dr Brooke

  2. Overview • What signals are being routed? • How can you route those signals? • How to apply routing to PCB? • PCB design techniques

  3. Signals being routed • High Frequency Sensitive Analog (e.g., IN from PD) • High Frequency: Data, and Noisy Analog (e.g., +OUT from Limiting Amp, +OUT from VCSEL driver) • Low Frequency sensitive : Bias, Analog (e.g., DC Power on input side of most chips esp. TIA) • Low Frequency insensitive: Bias, Analog (e.g., DC Power on output side of most chips, low frequency data)

  4. Signal Type Matrix • Red = Challenging, Yellow =Care needed, Green = Easy

  5. Different Types NeedDifferent Treatment • High Frequency/High Sensitivity • Transmission lines, return path (decoupling), Shielding from high frequency • High Frequency/Low Sensitivity • Transmission lines, prevent coupling to sensitive • Low Frequency/High Sensitivity • Shielding from high frequency, return path (ground loops), • Low Frequency/Low Sensitivity • Low Frequency decoupling, Resistive Loss

  6. High Frequency/High Sensitivity • Transmission line issues • Signal return path issues (decoupling) • Shielding from larger high Frequency signals

  7. Transmission line issues • What is a Transmission line? What is not? • How to avoid (short lines) • How to use (50 ohms) • Non traditional transmission lines (turns, tapers)

  8. What is a Transmission line 1 wavelength = = 20 cm @ 500 MHz, • Less that 1/10 of a wavelength use arbitrary geometry connections • More that ¼ wave length use wideband RF design techniques for geometry (stripline, coplanar) • In between use special angles, tapers, curves EM wave ¼ wavelength or greater = transmission line = 5 cm 1/10 wavelength or less = wire = 2 cm

  9. What is a Transmission line • What frequency to use? • Gbps data ~ 500 MHz sq wave (10101010…) Square Wave = 1st + 3rd + 5th … Harmonics Using up to 5th harmonic has eye closure ~15% Using up to 3rd harmonic has eye closure ~30% Using only 1st harmonic has eye closure ~50%

  10. How to avoid Transmission lines? • Depending on eye you want chose appropriate harmonic length to be less than a 1/10th of a wavelength First Harmonic = 1/10 * 20 cm = 2 cm Second harmonic (present in real data) = 2 cm / 2 = 1 cm Fifth Harmonic = 4 mm Third Harmonic = 6.7 mm Fourth Harmonic = 5 mm

  11. How to avoid Transmission lines? For Gigabit Ethernet • Nice eye for lines less than 4 mm not a transmission line • OK eye for lines less than 7 mm not a transmission line • Poor eye for lines less than 2 cm not a transmission line

  12. How to use Transmission Lines • Terminate them in design impedance • Ensure high frequency return path • Signal returns along the shield of Coax 50 ohms Signal arrives after transmission delay. “sees” 50 ohms immediately between core and shield - nothing else if terminated properly - “echo” after 2 x transmission delay otherwise

  13. + +OUT 100 ohms GND -OUT + “sees” 50 ohms immediately between core and shield How to use Transmission Lines • Special Case for Balanced Differential Signals • Connect shields together “sees” 50 ohms immediately between core and shield Balanced = equal and opposite That is for AC components: (+OUT) = -(-OUT)

  14. How to use Transmission Lines • Eliminate reflective features larger than 1/10th of a wavelength • Avoid impendence changes 45 deg 45 deg 1/10th wavelength 1/10th wavelength

  15. Non traditional transmission lines (curves, tapers) • If you want to use these features either: • Do it in the transition region between 1/10th and ¼ wavelength • Or use an RF design tool (e.g., ADS) to verify operation with finite element analysis

  16. Signal return path issues (decoupling) • Every High Frequency input and output • All AC current out/in must return to both “nearby” supplies VCC OUT Load VEE “Decoupling Capacitor” – Must be a “short” at signal frequency ground path – minimum length!

  17. Decoupling Capacitors • www.murata.com/cap/lineup • We are using 1.6 mm x 0.8 mm (0603) caps

  18. Decoupling caps • 10000 pF = 0.01 uF • S11 = reflected/incident power ratio when grounded • S21 = ratio of power passed to 50 ohm load

  19. Shielding from high Frequency

  20. High Frequency/Low Sensitivity • Transmission line issues • prevent coupling to sensitive

  21. Low Frequency/High Sensitivity • Shielding from high frequency • Return path (ground loops)

  22. Low Frequency/Low Sensitivity • Low Frequency decoupling • Resistive Loss

  23. How to apply routing to PCB • fff

  24. PCB design techniques • fff

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