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)• 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

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

Sensitivity

Frequency

high low

high high/high, e.g., TIA input

high/low, e.g., VCSEL driver

outputs

Lowlow/high,

e.g., input side power

low/low, e.g., output side

power

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

– Transmission line issues– Signal return path issues (decoupling) – Shielding from larger high Frequency signals

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)

¼ wavelength or greater = transmission line = 5 cm

What is a Transmission line

• 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

1 wavelength =

rf

c

1/10 wavelength or less = wire = 2 cm

= 20 cm @ 500 MHz, 3r

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?• Depending on eye you want chose appropriate harmonic length to be less than a 1/10 th of a wavelength

First Harmonic = 1/10 * 20 cm = 2 cm

Second harmonic (present in real data) = 2 cm / 2 = 1 cm

Third Harmonic = 6.7 mm

FourthHarmonic= 5 mm

FifthHarmonic= 4 mm

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

• Terminate them in design impedance

• Ensure high frequency return path– Signal returns along the shield of Coax

50 ohms

“sees” 50 ohms immediately between core and shield - nothing else if terminated properly - “echo” after 2 x transmission delay otherwise

Signal arrives after transmission delay.

How to use Transmission Lines

• Special Case for Balanced Differential Signals– Connect shields together

Balanced = equal and oppositeThat is for AC components:

(+OUT) = -(-OUT)

+

-OUTGND 100 ohms

+OUT

+

“sees” 50 ohms immediately between core and shield

“sees” 50 ohms immediately between core and shield

How to use Transmission Lines

• Eliminate reflective features larger than 1/10th of a wavelength

• Avoid impendence changes

OK

BAD1/10th wavelength

1/10th wavelength

45 deg

45 deg

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)

• Every High Frequency input and output– All AC current out/in must return to both

“nearby” supplies

OUTVCC

VEE

Load

ground path – minimum length!“Decoupling Capacitor” – Must be a “short” at signal frequency

Decoupling Capacitors

• www.murata.com/cap/lineup– We are using 1.6 mm x 0.8 mm (0603) 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

High Frequency/Low Sensitivity

– Transmission line issues– 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

How to apply routing to PCB

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PCB design techniques

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