Scenario: Supply Noise Coupling - Interview Scenario

Scenario

A mixed-signal macro shows intermittent spur bursts when neighboring digital clusters enter high-activity bursts. Local guards were added, but field behavior remains unstable.

OBSERVED METRIC
Supply-induced spur amplitude and in-band SNR degradation exceed product limits under worst-case activity.

45-MINUTE INTERVIEW FLOW
0-5: define context and KPI
5-15: classify first failing boundary
15-25: identify proving artifact packet
25-35: propose bounded fix with owner
35-45: define validation matrix and rollback

Common traps to avoid

• Using one low-frequency PSRR number as broadband evidence.

• Assuming guard rings alone close coupling without PDN/return-path analysis.

• Applying global decap increases without identifying source-path-victim dominance.

Scenario debrief

Score response quality on mechanism classification, proving artifact, owner assignment, and rollback-safe mitigation.

assumption -> mechanism -> evidence -> mitigation -> replay

risk reduction trend

Debrief prompts

1. Which boundary failed first and how is it proven?

2. Which analog mechanism best explains the symptom?

3. What bounded fix closes risk with the smallest collateral change?

Key takeaways

• Tie every analog claim to one measurable metric and one proving artifact.

• Prefer minimal reversible mitigations with explicit owner and rollback criteria.

Common pitfalls

• Treating all noise as one scalar instead of path and frequency dependent behavior.

• Changing multiple analog knobs at once and losing causality.

• Declaring closure from nominal behavior without stress replay evidence.