High-speed signal routing basics in PCB Design - Real Business Scenario

Scenario Mode

👤 Your Role: You are a PCB design engineer working on a new high-speed communication device.

📋 Request: Your manager wants a report showing how signal integrity can be maintained by proper routing techniques in high-speed PCB design.

📊 Data: You have data on signal trace lengths, impedance values, and crosstalk measurements from recent PCB prototypes.

🎯 Deliverable: Create a dashboard that visualizes signal trace lengths, impedance control, and crosstalk levels to identify routing improvements.

Sample Data

Trace ID	Length (mm)	Impedance (Ohms)	Crosstalk (dB)	Layer
T1	45	50	-40	Top
T2	60	48	-35	Inner1
T3	30	52	-42	Top
T4	55	49	-38	Inner2
T5	70	50	-33	Top
T6	40	51	-41	Inner1
T7	65	47	-36	Top
T8	50	50	-39	Inner2

1

Step 1: Create a table visual showing Trace ID, Length, Impedance, Crosstalk, and Layer.

Add all columns from sample data as table columns in the BI tool.

Expected Result

A table listing all 8 traces with their length, impedance, crosstalk, and layer.

2

Step 2: Calculate average impedance per layer to check impedance control.

Use DAX measure: Average Impedance = AVERAGE('Table'[Impedance]) grouped by Layer.

Expected Result

Average impedance values: Top=49.75 Ohms, Inner1=49.5 Ohms, Inner2=49.5 Ohms.

3

Step 3: Create a bar chart showing average impedance by layer.

X-axis: Layer, Y-axis: Average Impedance measure.

Expected Result

Bar chart with three bars showing average impedance close to 50 Ohms for each layer.

4

Step 4: Calculate maximum crosstalk per layer to identify worst interference.

Use DAX measure: Max Crosstalk = MIN('Table'[Crosstalk]) grouped by Layer.

Expected Result

Max crosstalk values: Top=-33 dB, Inner1=-35 dB, Inner2=-38 dB.

5

Step 5: Create a line chart showing max crosstalk by layer.

X-axis: Layer, Y-axis: Max Crosstalk measure.

Expected Result

Line chart showing max crosstalk levels per layer, highlighting Top and Inner1 layers have highest crosstalk.

6

Step 6: Create a scatter plot of Trace Length vs Crosstalk to see if longer traces have more interference.

X-axis: Length (mm), Y-axis: Crosstalk (dB), Points labeled by Trace ID.

Expected Result

Scatter plot showing trend that longer traces tend to have higher (less negative) crosstalk values.

7

Step 7: Add slicer/filter for Layer to allow filtering visuals by PCB layer.

Add Layer column as slicer in the dashboard.

Expected Result

User can filter all visuals to show data for Top, Inner1, or Inner2 layers.

8

Step 8: Summarize findings in a text box on the dashboard.

Add text: 'Impedance is well controlled near 50 Ohms across layers. Crosstalk is highest on Top and Inner1 layers, especially on longer traces. Consider routing improvements on these layers to reduce interference.'

Expected Result

Dashboard includes clear summary of signal integrity insights.

Final Result

High-Speed Signal Routing Dashboard

+----------------+----------------+----------------+
| Layer Slicer   |                |                |
+----------------+----------------+----------------+
| Table of Traces | Bar Chart: Avg | Line Chart: Max|
| Trace ID, Length| Impedance by   | Crosstalk by   |
| Impedance,      | Layer          | Layer          |
| Crosstalk, Layer|                |                |
+----------------+----------------+----------------+

Scatter Plot: Trace Length vs Crosstalk

Summary Text Box:
"Impedance is well controlled near 50 Ohms across layers.
Crosstalk is highest on Top and Inner1 layers, especially on longer traces.
Consider routing improvements on these layers to reduce interference."

✓Impedance is consistently controlled around 50 Ohms on all PCB layers.

✓Top and Inner1 layers show higher crosstalk, indicating more signal interference.

✓Longer traces tend to have higher crosstalk, suggesting routing length impacts signal quality.

✓Filtering by layer helps focus on specific routing issues per PCB layer.

Bonus Challenge

Add a calculated measure to estimate signal delay based on trace length and visualize it.

Show Hint

Use formula: Signal Delay (ns) = Length (mm) / 150 (assuming signal speed 150 mm/ns). Create a new measure and add a bar chart by layer.

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