CID Case Analysis & Practical Application 3 — Questions and Answers
Question 1: A case study presents a DDR5 memory bus with excessive jitter at the data strobe (DQS). Which parameter most directly contributes to this jitter?
- Reference plane thickness
- Power delivery network (PDN) noise coupling into the DQS signal (Correct answer)
- Number of memory modules installed
- DRAM die temperature
Correct answer: Power delivery network (PDN) noise coupling into the DQS signal
PDN noise modulates the switching threshold of the transmitter, converting supply ripple into deterministic jitter on time-critical strobes like DQS.
Question 2: An interconnect designer reviews a USB 3.2 layout and finds the differential impedance measures 87 ohms instead of 90 ohms. What is the most likely correction?
- Increase trace width slightly
- Decrease trace width or increase trace spacing (Correct answer)
- Add a ferrite bead to each trace
- Reduce via count on the pair
Correct answer: Decrease trace width or increase trace spacing
Differential impedance is increased by narrowing the trace width or increasing the edge-to-edge spacing between the pair traces.
Question 3: During eye diagram analysis, a design shows excessive jitter floor even after addressing reflections. Which factor is most consistent with a high jitter floor (random jitter)?
- Via stub resonance
- Thermal noise from lossy dielectric and resistive traces (Correct answer)
- Return path discontinuity
- Coupled stub resonance
Correct answer: Thermal noise from lossy dielectric and resistive traces
Random jitter (RJ) originates from thermal and shot noise sources such as resistive conductor loss and dielectric loss, creating an unbounded Gaussian jitter distribution.
Question 4: A case involves a multi-drop bus where one node shows significantly higher reflection than others. What practical investigation step identifies the offending node?
- Measure DC resistance of the bus
- Use time-domain reflectometry (TDR) to locate impedance discontinuities along the bus (Correct answer)
- Increase pull-up resistor values
- Replace all connectors on the bus
Correct answer: Use time-domain reflectometry (TDR) to locate impedance discontinuities along the bus
TDR sends a step signal and measures reflections versus time (distance), pinpointing the exact location and magnitude of impedance discontinuities.
Question 5: An engineer analyzing a 25 Gbps serial link case observes that insertion loss reaches the loss budget limit at 5 GHz. What is the impact on the overall channel at the Nyquist frequency of 12.5 GHz?
- The channel performs exactly at the limit
- Insertion loss at Nyquist is likely far beyond the budget, causing link failure (Correct answer)
- The channel automatically compensates at higher frequencies
- Insertion loss decreases above 5 GHz
Correct answer: Insertion loss at Nyquist is likely far beyond the budget, causing link failure
Insertion loss increases with frequency (approximately proportional to √f for skin effect), so reaching budget limits at 5 GHz means severe excess loss at 12.5 GHz.
Question 6: A practical PCB case requires routing a high-speed differential pair across a layer transition. What is the correct via strategy?
- Use a single via for both traces to save space
- Use a matched via pair with adjacent GND vias to maintain impedance continuity (Correct answer)
- Route across layers using capacitive coupling only
- Use through-hole vias only on the positive trace
Correct answer: Use a matched via pair with adjacent GND vias to maintain impedance continuity
A matched differential via pair with nearby return vias preserves the differential impedance and provides low-inductance return paths through the transition.
Question 7: A case study shows that a high-speed link passes simulation but fails physical testing. The most likely reason to investigate first is:
- The simulation model omitted the PCB surface finish
- The simulation used ideal transmission line models not accounting for fabrication tolerances and connector parasitics (Correct answer)
- The test equipment is faulty
- The power supply is unregulated
Correct answer: The simulation used ideal transmission line models not accounting for fabrication tolerances and connector parasitics
Physical channels include real-world effects like trace roughness, connector parasitics, and manufacturing tolerances that ideal simulation models typically neglect.
CID Case Analysis & Practical Application 3
A case study presents a DDR5 memory bus with excessive jitter at the data strobe (DQS).
Which parameter most directly contributes to this jitter?