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PCB Design for 1 Gbps

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PCB Design for 1 Gbps

ECE 4006

Dr Brooke

- What signals are being routed?
- How can you route those signals?
- How to apply routing to PCB?
- PCB design techniques

- 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)

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

- 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

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

- What is a Transmission line? What is not?
- How to avoid (short lines)
- How to use (50 ohms)
- Non traditional transmission lines (turns, tapers)

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 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%

- 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

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

- 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

+

+OUT

100 ohms

GND

-OUT

+

“sees” 50 ohms immediately

between core and shield

- 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)

- Eliminate reflective features larger than 1/10th of a wavelength
- Avoid impendence changes

45 deg

45 deg

1/10th wavelength

1/10th wavelength

- 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

- 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!

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

- 10000 pF = 0.01 uF
- S11 = reflected/incident power ratio when grounded
- S21 = ratio of power passed to 50 ohm load

- Transmission line issues
- prevent coupling to sensitive

- Shielding from high frequency
- Return path (ground loops)

- Low Frequency decoupling
- Resistive Loss

- fff

- fff