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PCB design for 1Gbps

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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 Sensitivity Frequency high low high high/high, e.g., TIA input high/low, e.g., VCSEL driver outputs Low low/high, e.g., input side power low/low, e.g., output side power

Different Types Need Different Treatment :

Different Types Need Different 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:

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 = 1/10 wavelength or less = wire = 2 cm = 20 cm @ 500 MHz,

What is a Transmission line:

What is a Transmission line What frequency to use? Gbps data ~ 500 MHz sq wave (10101010…) Square Wave = 1 st + 3 rd + 5 th … Harmonics Using up to 5 th harmonic has eye closure ~15% Using up to 3 rd harmonic has eye closure ~30% Using only 1 st 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 1/10 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 Fourth Harmonic = 5 mm Fifth Harmonic = 4 mm

How to avoid Transmission lines?:

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 “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:

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

How to use Transmission Lines:

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

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 1/10 th 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 OUT VCC VEE Load ground path – minimum length! “Decoupling Capacitor” – Must be a “short” at signal frequency

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 fff

PCB design techniques:

PCB design techniques fff

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