Pcb design for 1 gbps
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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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Pcb design for 1 gbps

PCB Design for 1 Gbps

ECE 4006

Dr Brooke


Overview

Overview

  • What signals are being routed?

  • How can you route those signals?

  • How to apply routing to PCB?

  • PCB design techniques


Signals being routed

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)


Signal type matrix

Signal Type Matrix

  • Red = Challenging, Yellow =Care needed, Green = Easy


Different types need different treatment

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


High frequency high sensitivity

High Frequency/High Sensitivity

  • Transmission line issues

  • Signal return path issues (decoupling)

  • Shielding from larger high Frequency signals


Transmission line issues

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)


What is a transmission line

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


What is a transmission line1

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%


How to avoid transmission lines

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


How to avoid transmission lines1

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


How to use transmission lines

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


How to use transmission lines1

+

+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)


How to use transmission lines2

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


Non traditional transmission lines curves tapers

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


Signal return path issues decoupling

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!


Decoupling capacitors

Decoupling Capacitors

  • www.murata.com/cap/lineup

    • We are using 1.6 mm x 0.8 mm (0603) caps


Decoupling caps

Decoupling caps

  • 10000 pF = 0.01 uF

    • S11 = reflected/incident power ratio when grounded

    • S21 = ratio of power passed to 50 ohm load


Shielding from high frequency

Shielding from high Frequency


High frequency low sensitivity

High Frequency/Low Sensitivity

  • Transmission line issues

  • prevent coupling to sensitive


Low frequency high sensitivity

Low Frequency/High Sensitivity

  • Shielding from high frequency

  • Return path (ground loops)


Low frequency low sensitivity

Low Frequency/Low Sensitivity

  • Low Frequency decoupling

  • Resistive Loss


How to apply routing to pcb

How to apply routing to PCB

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Pcb design techniques

PCB design techniques

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