PCB Design for 1 Gbps
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Transcript of PCB Design for 1 Gbps
PCB Design for 1 Gbps
ECE 4006B
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 high Frequency
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 opposite
+
-OUTGND 100 ohms
+OUT
+
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 outp
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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