High-speed signal routing basics in PCB Design - Cell-by-Cell Formula Trace

Sample Data

This data shows trace lengths in millimeters and signal speed in meters per nanosecond for high-speed PCB signal routing. The goal is to calculate the propagation delay in nanoseconds.

Cell	Value
A1	Trace Length (mm)
B1	Signal Speed (m/ns)
C1	Propagation Delay (ns)
A2	100
B2	0.15
A3	150
B3	0.15
A4	200
B4	0.15

Formula Trace

=A2/1000/B2

Step 1: A2 = 100

Step 2: Convert 100 mm to meters: 100 / 1000

Step 3: Signal speed B2 = 0.15 m/ns

Step 4: Calculate propagation delay: 0.1 / 0.15

Cell Reference Map

    A       B       C
1 |Trace Length|Signal Speed|Propagation Delay
2 |   100     |   0.15    |  =A2/1000/B2
3 |   150     |   0.15    |
4 |   200     |   0.15    |

The formula in C2 uses A2 for trace length and B2 for signal speed to calculate propagation delay.

Result

    A       B       C
1 |Trace Length|Signal Speed|Propagation Delay
2 |   100     |   0.15    |  0.6667
3 |   150     |   0.15    |  1.0000
4 |   200     |   0.15    |  1.3333

Propagation delay is shown in column C, calculated by dividing trace length (converted to meters) by signal speed.

Sheet Trace Quiz - 3 Questions

What does the formula =A2/1000/B2 calculate?

A Trace length in meters

B Signal speed in meters per nanosecond

C Propagation delay in nanoseconds

D Trace length in millimeters

Key Result

Propagation delay = (Trace length in meters) / (Signal speed in meters per nanosecond)

Practice

1. What is the main reason to match trace lengths in high-speed signal routing on a PCB?

easy

A. To ensure signals arrive at the same time and avoid timing errors

B. To reduce the cost of manufacturing the PCB

C. To make the PCB look more organized

D. To increase the thickness of the PCB

Solution

1. Step 1: Understand signal timing in high-speed routing

   Signals traveling on different length traces can arrive at different times, causing errors.

2. Step 2: Importance of matching trace lengths

   Matching trace lengths ensures signals arrive simultaneously, preventing timing mismatches.

3. Final Answer:

   To ensure signals arrive at the same time and avoid timing errors -> Option A

4. Quick Check:

   Signal timing = matched trace lengths [OK]

Hint: Signals must arrive together; match trace lengths [OK]

Common Mistakes:

• Thinking trace length affects cost only
• Believing trace length is for PCB appearance
• Confusing trace thickness with length

2. Which of the following is the correct way to describe a controlled impedance trace in PCB design?

easy

A. A trace with random width and spacing

B. A trace designed with specific width and spacing to maintain constant impedance

C. A trace that is thicker than all others

D. A trace that is routed only on the top layer

Solution

1. Step 1: Define controlled impedance

   Controlled impedance means the trace width and spacing are designed to keep impedance steady.

2. Step 2: Identify correct description

   The correct description is of a trace designed with specific width and spacing to maintain constant impedance.

3. Final Answer:

   A trace designed with specific width and spacing to maintain constant impedance -> Option B

4. Quick Check:

   Controlled impedance = specific width and spacing [OK]

Hint: Controlled impedance means fixed width and spacing [OK]

Common Mistakes:

• Assuming random width controls impedance
• Thinking thickness alone controls impedance
• Believing layer choice controls impedance

3. Consider a high-speed PCB trace routed with two 90-degree corners versus one with smooth 45-degree bends. What is the expected effect on signal integrity?

medium

A. The 90-degree corners cause signal reflections and degrade integrity more than 45-degree bends

B. The 90-degree corners improve signal speed compared to 45-degree bends

C. There is no difference in signal integrity between the two

D. 45-degree bends cause more noise than 90-degree corners

Solution

1. Step 1: Understand effect of sharp corners on signals

   Sharp 90-degree corners cause impedance discontinuities leading to reflections and signal degradation.

2. Step 2: Compare with 45-degree bends

   Smoother 45-degree bends reduce reflections and maintain better signal integrity.

3. Final Answer:

   The 90-degree corners cause signal reflections and degrade integrity more than 45-degree bends -> Option A

4. Quick Check:

   Sharp corners = more reflections [OK]

Hint: Avoid sharp corners to reduce signal reflections [OK]

Common Mistakes:

• Thinking sharp corners speed up signals
• Believing no difference between corner types
• Assuming 45-degree bends cause more noise

4. A PCB designer routed a high-speed signal trace near a noisy power supply area causing interference. What is the best fix to reduce noise coupling?

medium

A. Add more 90-degree corners to the trace

B. Increase the trace width without changing location

C. Move the trace away from the noisy area and add a ground plane

D. Route the trace on the same layer as the power supply

Solution

1. Step 1: Identify noise source and coupling

   Routing near noisy power supplies causes interference through electromagnetic coupling.

2. Step 2: Best practice to reduce noise

   Moving the trace away and adding a ground plane shields and reduces noise coupling effectively.

3. Final Answer:

   Move the trace away from the noisy area and add a ground plane -> Option C

4. Quick Check:

   Distance + ground plane = less noise [OK]

Hint: Keep traces away from noise and use ground planes [OK]

Common Mistakes:

• Thinking wider trace reduces noise alone
• Adding corners increases noise, not reduces
• Routing near power supply increases noise

5. You need to route a differential pair for a high-speed USB signal on a PCB. Which combination of routing practices ensures best signal quality?

hard

A. Use random trace lengths, increase spacing between traces, and add sharp corners

B. Make one trace wider than the other to balance impedance

C. Route traces on different layers far apart to reduce crosstalk

D. Match trace lengths, maintain controlled impedance, and keep traces close with consistent spacing

Solution

1. Step 1: Understand differential pair requirements

   Differential pairs require matched lengths, controlled impedance, and close spacing for noise immunity and signal integrity.

2. Step 2: Evaluate options

   Only Match trace lengths, maintain controlled impedance, and keep traces close with consistent spacing combines all correct practices: matched lengths, controlled impedance, and consistent close spacing.

3. Final Answer:

   Match trace lengths, maintain controlled impedance, and keep traces close with consistent spacing -> Option D

4. Quick Check:

   Differential pair = matched + controlled + close [OK]

Hint: Match, control impedance, keep pair close [OK]

Common Mistakes:

• Using random lengths breaks timing
• Increasing spacing reduces differential benefits
• Unequal trace widths cause imbalance