Signal Integrity & High-Speed Design Flashcards

What is the primary cause of inter-symbol interference (ISI) in high-speed serial links?

Answer: Frequency-dependent channel losses that cause pulse spreading

ISI occurs when frequency-dependent dielectric and skin-effect losses attenuate high-frequency components of a pulse, causing it to spread and interfere with adjacent bits.

Which equalization technique is implemented in the transmitter to pre-compensate for channel loss in a serial link?

Answer: Feed-forward equalization (FFE) / de-emphasis

FFE/de-emphasis reduces the amplitude of repeated bits at the transmitter, effectively boosting the relative energy of transitions to compensate for high-frequency channel loss.

A stripline trace has a propagation delay of 180 ps/inch. What is the approximate dielectric constant (Dk) of the surrounding material?

Answer: 4.1

Propagation delay TD = 85 × √Dk ps/inch; solving for Dk gives (180/85)² ≈ 4.48, closest to 4.1 for standard FR4 in stripline.

What is the effect of a poorly placed via anti-pad on differential pair routing?

Answer: It increases the differential impedance locally, causing mode conversion

An asymmetric or overly large anti-pad disrupts the reference plane locally and can create a differential-to-common-mode conversion point, degrading signal integrity.

In a PCB stackup, which configuration provides the best EMI shielding for high-speed signals?

Answer: Signal layers sandwiched between adjacent reference planes on both sides

Embedding signal layers between tightly coupled reference planes minimizes radiated emissions by containing the electromagnetic field within the stackup.

What does the term 'skin depth' refer to in the context of conductor loss at high frequencies?

Answer: The depth below a conductor surface at which current density falls to 1/e of the surface value

Skin depth is the depth at which current density decreases to approximately 37% (1/e) of the surface value, and it decreases with increasing frequency, concentrating current at the surface.

Which parameter in an S-parameter matrix describes the signal reflected back to port 1 when port 2 is properly terminated?

Answer: S11

S11 is the input reflection coefficient, representing the ratio of reflected wave amplitude to incident wave amplitude at port 1 with port 2 matched.