Signal Integrity & High-Speed Design Flashcards

What is the primary advantage of using a differential signaling scheme over single-ended signaling in high-speed PCB design?

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.

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?

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.

What is the purpose of a guard trace placed between two high-speed signal traces?

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.

What PCB design rule directly limits the maximum length of an unterminated stub on a high-speed transmission line?

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.

In a high-speed design, what is 'fiberglass weave effect' and how does it impact signal integrity?

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.

Which of the following is a correct statement about backward crosstalk (NEXT) in a microstrip configuration?

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.

When performing time-domain reflectometry (TDR) on a PCB trace, a downward step in the reflected waveform indicates what type of impedance discontinuity?

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.