CID Signal Integrity & High-Speed Design 3 — Questions and Answers
Question 1: What is the primary advantage of using a differential signaling scheme over single-ended signaling in high-speed PCB design?
- It doubles the data rate without increasing trace count
- It provides inherent common-mode noise rejection (Correct answer)
- It eliminates the need for impedance matching
- It reduces the number of required reference planes
Correct answer: It provides inherent common-mode noise rejection
Differential signaling rejects common-mode noise because both conductors pick up the same interference, which cancels at the differential receiver.
Question 2: A PCB trace exhibits a measured insertion loss of -18 dB at 5 GHz. Which loss mechanism typically dominates at this frequency on an FR4 board?
- Radiation loss
- Dielectric loss (Correct answer)
- Resistive (skin effect) loss
- Leakage loss through the substrate
Correct answer: Dielectric loss
On FR4 PCBs at frequencies above a few GHz, dielectric loss (due to the high loss tangent of FR4) typically dominates over conductor skin-effect loss.
Question 3: What is the purpose of a guard trace placed between two high-speed signal traces?
- To carry return current and reduce crosstalk by providing a low-impedance path between victims (Correct answer)
- To slow down one signal to match propagation delays
- To increase the characteristic impedance of the adjacent traces
- To act as a length-matching serpentine for timing closure
Correct answer: To carry return current and reduce crosstalk by providing a low-impedance path between victims
A guard trace tied to ground at multiple points provides a low-impedance path for induced currents, reducing the electric and magnetic coupling between adjacent signal traces.
Question 4: What PCB design rule directly limits the maximum length of an unterminated stub on a high-speed transmission line?
- The stub length must be less than λ/20 at the highest frequency of interest (Correct answer)
- The stub length must be greater than the skin depth of the conductor
- The stub length must equal the source termination resistor value
- The stub length must match the via barrel height
Correct answer: The stub length must be less than λ/20 at the highest frequency of interest
A stub shorter than λ/20 (one-twentieth of a wavelength) produces a resonant null beyond the useful frequency range, preventing significant degradation of the signal.
Question 5: In a high-speed design, what is 'fiberglass weave effect' and how does it impact signal integrity?
- Variation in glass bundle spacing causes local Dk variation along a trace, leading to phase skew (Correct answer)
- The glass cloth reduces copper adhesion, increasing insertion loss
- Weave patterns increase via drill wander, degrading impedance control
- Glass fibers reflect signals at high frequencies, causing multipath interference
Correct answer: Variation in glass bundle spacing causes local Dk variation along a trace, leading to phase skew
The periodic variation between glass bundles (higher Dk) and resin-rich areas (lower Dk) creates periodic Dk variation that introduces phase skew, especially for differential pairs.
Question 6: Which of the following is a correct statement about backward crosstalk (NEXT) in a microstrip configuration?
- NEXT is typically lower in microstrip than in stripline due to asymmetric field distribution (Correct answer)
- NEXT in microstrip is exactly zero because there is no return plane
- NEXT in microstrip equals FEXT when trace spacing equals trace width
- NEXT in microstrip is always higher than in stripline for the same geometry
Correct answer: NEXT is typically lower in microstrip than in stripline due to asymmetric field distribution
In microstrip, the electric and magnetic coupling coefficients are unequal due to the asymmetric dielectric environment, which partially cancels NEXT and makes it lower than in stripline.
Question 7: When performing time-domain reflectometry (TDR) on a PCB trace, a downward step in the reflected waveform indicates what type of impedance discontinuity?
- An inductive discontinuity such as a via barrel
- A capacitive discontinuity such as an unterminated stub or pad (Correct answer)
- An open-circuit termination
- A resistive termination greater than the characteristic impedance
Correct answer: A capacitive discontinuity such as an unterminated stub or pad
A capacitive discontinuity causes the TDR waveform to dip below the baseline level because the capacitance momentarily lowers the local impedance, increasing transmitted energy.
CID Signal Integrity & High-Speed Design 3
What is the primary advantage of using a differential signaling scheme over single-ended signaling in high-speed PCB design?