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 Sensitivity high low high high high low Frequency high Low e g TIA input e g VCSEL driver outputs low high low low e g input side power e g output side power Red Challenging Yellow Care needed Green Easy 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 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 What is a Transmission line 1 wavelength c f r 20 cm 500 MHz r 3 EM wave wavelength or greater transmission line 5 cm 1 10 wavelength or less wire 2 cm Less that 1 10 of a wavelength use arbitrary geometry connections More that wave length use wideband RF design techniques for geometry stripline coplanar In between use special angles tapers curves 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 Fourth Harmonic 5 mm Fifth Harmonic 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 Signal arrives after sees 50 ohms immediately transmission delay between core and shield nothing else if terminated properly echo after 2 x transmission delay otherwise How to use Transmission Lines Special Case for Balanced Differential Signals Connect shields together sees 50 ohms immediately between core and shield OUT GND OUT sees 50 ohms immediately between core and shield Balanced equal and opposite That is for AC components OUT OUT 100 ohms How to use Transmission Lines Eliminate reflective features larger than 1 10th of a wavelength Avoid impendence changes 45 deg OK 45 deg 1 10th wavelength BAD 1 10th wavelength Non traditional transmission lines curves tapers If you want to use these features either Do it in the transition region between 1 10th and 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 VCC OUT Load VEE Decoupling ground path minimum length 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 fff PCB design techniques fff