CID Case Analysis & Practical Application 3

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?

Accepted Answer

Power delivery network (PDN) noise coupling into the DQS signal

Answer

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?

Accepted Answer

Decrease trace width or increase trace spacing

Answer

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)?

Accepted Answer

Thermal noise from lossy dielectric and resistive traces

Answer

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?

Accepted Answer

Use time-domain reflectometry (TDR) to locate impedance discontinuities along the bus

Answer

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?

Accepted Answer

Insertion loss at Nyquist is likely far beyond the budget, causing link failure

Answer

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.

(CID) Certified Interconnect Designer Practice Test

(CID) Certified Interconnect Designer Practice Test

CID Case Analysis & Practical Application 3