Signal Integrity Interview Questions

Every board with a modern processor is a high-speed board. Interviewers want the physics in one sentence, then a number, then what you do about it on the board.

Before you start

What you should already be comfortable with:

• Rise time and bandwidth (BW ≈ 0.35/tr)
• Characteristic impedance and propagation delay in FR4 (about 150–180 ps per inch)
• The reflection coefficient Γ = (ZL − Z0)/(ZL + Z0)
• What an S-parameter plot shows (helpful, not required)

What interviewers are really testing

• Deciding when a trace is a transmission line from the edge, not the clock
• Reading reflections on a waveform and fixing them with the right termination
• Return paths, crosstalk and loss — each with a number
• Via stubs, differential pairs and length matching to a timing budget
• Jitter, eye diagrams and TDR as measurement tools

Start with these two — free full answers

Study order

Work through the stages in order. ● screening · ●● onsite technical · ●●● design / senior. Pages marked "Going deeper" add explanation that is not in the book; the two marked "Free answer" show the full answer; every page tells you where the full answer is in the book.

1. Transmission-line basics
   • SI-01 — When is a trace a transmission line? ● Free answer
   • SI-02 — Reflections and the reflection coefficient ● Free answer
   • SI-03 — Overdriven vs underdriven lines ●● Going deeper
   • SI-04 — Termination choices ●● Going deeper
2. Routing
   • SI-05 — Microstrip vs stripline ● Going deeper
   • SI-06 — Changing layers with a controlled-impedance line ●●
   • SI-07 — Crossing a plane split ●●
   • SI-08 — Crosstalk: NEXT vs FEXT ●●
   • SI-16 — Length matching ●●
3. Multi-gigabit channels
   • SI-09 — Loss above 1 GHz ●●
   • SI-10 — Via stubs and back-drilling ●●
   • SI-11 — Differential impedance, odd and even mode ●●
   • SI-12 — Glass-weave effect ●●●
4. Measurement
   • SI-13 — Jitter taxonomy ●●
   • SI-14 — Eye diagrams ●●
   • SI-15 — TDR ●●

Related questions in other chapters

• HS-10 — 10 Gb/s channel design ●●
• HS-06 — Equalization: CTLE, FFE, DFE ●●
• PCB-04 — Top-layer vs inner-layer routing ● Going deeper
• MD-01 — How a scope probe works ●

The book

The book has all 16 questions in this chapter with the full answer: how to explain it on a whiteboard, the numbers worth quoting, and the follow-ups that come next.

More topics

PCB Design Interview Questions · Power Electronics Interview Questions · Analog & Op-Amp Interview Questions · Embedded Hardware Interview Questions · Power Integrity Interview Questions