Differential pair routing basics in PCB Design - Real Business Scenario

Scenario Mode

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

📋 Request: Your manager wants you to analyze the differential pair routing quality and signal integrity for the USB data lines on the board.

📊 Data: You have the PCB layout data showing the lengths and spacing of differential pairs, and signal integrity test results including eye diagram measurements and crosstalk levels.

🎯 Deliverable: Create a report dashboard that shows the differential pair length matching, spacing compliance, and signal integrity metrics with visual indicators for any issues.

Sample Data

Pair Name	Length Positive (mm)	Length Negative (mm)	Spacing (mm)	Max Allowed Spacing (mm)	Eye Height (mV)	Crosstalk (dB)
USB_DP_DM	45.2	45.0	0.15	0.20	350	-40
HDMI_TMDS0	50.5	50.7	0.18	0.20	320	-38
HDMI_TMDS1	50.3	50.1	0.22	0.20	310	-35
HDMI_TMDS2	49.8	49.9	0.19	0.20	330	-37
PCIe_TX	60.0	60.1	0.16	0.18	400	-42
PCIe_RX	59.9	60.2	0.17	0.18	390	-41

1

Step 1: Calculate the length difference between positive and negative lines for each differential pair.

Length Difference = ABS(Length Positive - Length Negative)

Expected Result

USB_DP_DM: 0.2 mm, HDMI_TMDS0: 0.2 mm, HDMI_TMDS1: 0.2 mm, HDMI_TMDS2: 0.1 mm, PCIe_TX: 0.1 mm, PCIe_RX: 0.3 mm

2

Step 2: Check if the spacing between the differential pair lines is within the maximum allowed spacing.

Spacing Compliance = Spacing <= Max Allowed Spacing

Expected Result

USB_DP_DM: True, HDMI_TMDS0: True, HDMI_TMDS1: False, HDMI_TMDS2: True, PCIe_TX: True, PCIe_RX: True

3

Step 3: Create a dashboard table showing Pair Name, Length Difference, Spacing Compliance, Eye Height, and Crosstalk.

Display calculated Length Difference and Spacing Compliance alongside Eye Height and Crosstalk values.

Expected Result

Table with columns: Pair Name, Length Difference (mm), Spacing Compliance (Yes/No), Eye Height (mV), Crosstalk (dB)

4

Step 4: Add conditional formatting to highlight length differences greater than 0.2 mm and spacing compliance failures in red.

If Length Difference > 0.2 mm or Spacing Compliance = False, highlight row in red.

Expected Result

PCIe_RX row highlighted for length difference 0.3 mm; HDMI_TMDS1 row highlighted for spacing compliance failure.

5

Step 5: Plot a bar chart of Eye Height for each differential pair to visualize signal quality.

X-axis: Pair Name, Y-axis: Eye Height (mV)

Expected Result

Bar chart showing Eye Height values with PCIe_TX highest and HDMI_TMDS1 lowest.

6

Step 6: Plot a line chart of Crosstalk levels for each differential pair to identify potential interference issues.

X-axis: Pair Name, Y-axis: Crosstalk (dB)

Expected Result

Line chart showing Crosstalk values with PCIe_TX lowest interference (-42 dB) and HDMI_TMDS1 highest (-35 dB).

7

Step 7: Summarize key findings in the dashboard with recommendations for pairs needing review.

Highlight pairs with length difference > 0.2 mm or spacing compliance failure as needing routing review.

Expected Result

PCIe_RX and HDMI_TMDS1 flagged for routing review due to length mismatch and spacing issues.

Final Result

PCIe_TX

↓

PCIe_RX

↓

USB_DP_DM

↓

HDMI_TMDS2

↓

HDMI_TMDS0

↓

HDMI_TMDS1

✓Most differential pairs have length differences within 0.2 mm, which is acceptable.

✓HDMI_TMDS1 spacing exceeds the maximum allowed, which may cause signal integrity issues.

✓PCIe_RX has a length difference of 0.3 mm, which is above the recommended tolerance and should be reviewed.

✓Eye height is highest for PCIe_TX, indicating good signal quality.

✓Crosstalk is worst for HDMI_TMDS1, correlating with its spacing issue.

Bonus Challenge

Create a DAX measure or SQL query to automatically flag differential pairs that fail either length matching or spacing compliance for use in a BI report.

Show Hint

Use a conditional expression to check if ABS(Length Positive - Length Negative) > 0.2 OR Spacing > Max Allowed Spacing, then return 'Fail' else 'Pass'.