CID Interconnect Modeling & Simulation 2 — Questions and Answers
Question 1: In a transmission line SPICE model, what does the 'TD' parameter represent?
- Time domain step size
- Propagation delay of the line (Correct answer)
- Terminal damping coefficient
- Transient decay constant
Correct answer: Propagation delay of the line
TD (time delay) specifies the one-way propagation delay of the transmission line, derived from length divided by the phase velocity.
Question 2: Which S-parameter is used to characterize the return loss of a two-port interconnect network?
- S21
- S12
- S11 (Correct answer)
- S22
Correct answer: S11
S11 represents the input reflection coefficient, and return loss is defined as -20·log10|S11|, measuring how much signal is reflected at port 1.
Question 3: What is the primary advantage of using W-element models over RLGC lumped models for high-speed interconnects?
- Lower simulation memory requirements
- Frequency-dependent loss and dispersion are accurately captured (Correct answer)
- Simplified netlist syntax
- Compatibility with older SPICE simulators
Correct answer: Frequency-dependent loss and dispersion are accurately captured
W-element (lossy transmission line) models support frequency-dependent RLGC parameters, accurately capturing skin effect and dielectric loss at high frequencies.
Question 4: In electromagnetic simulation, what does 'de-embedding' accomplish?
- Removes the effect of fixture or launch structures from measured data (Correct answer)
- Converts time-domain data to frequency domain
- Adds port boundary conditions to the model
- Scales the simulation mesh to a finer resolution
Correct answer: Removes the effect of fixture or launch structures from measured data
De-embedding mathematically removes the influence of test fixtures, probes, or launch structures so only the DUT characteristics remain.
Question 5: A via stub resonance in a PCB causes signal integrity problems primarily because it creates:
- Increased DC resistance
- A notch in the insertion loss at the stub resonant frequency (Correct answer)
- Higher differential impedance
- Increased crosstalk between adjacent vias
Correct answer: A notch in the insertion loss at the stub resonant frequency
The unused via stub acts as a shunt stub resonator, producing a transmission null (notch) at the frequency where the stub length equals a quarter wavelength.
Question 6: When building a behavioral model of an interconnect using vector fitting, what is the key output used in SPICE simulation?
- A table of S-parameters
- A rational function approximation implemented as an RLC network (Correct answer)
- A 2D field solution
- A time-domain impulse response waveform
Correct answer: A rational function approximation implemented as an RLC network
Vector fitting produces a rational function (poles and residues) that can be synthesized into a passive RLC equivalent circuit for SPICE transient simulation.
Question 7: Which simulation technique is most appropriate for capturing radiation and resonance effects inside a PCB power delivery network cavity?
- SPICE lumped element simulation
- 2D transmission matrix method (TMM)
- Full-wave 3D electromagnetic simulation (FEM or FDTD) (Correct answer)
- Quasi-static RC extraction
Correct answer: Full-wave 3D electromagnetic simulation (FEM or FDTD)
Cavity resonances and radiation are full-wave phenomena requiring 3D EM solvers (FEM or FDTD) to accurately model field distributions inside the power plane cavity.
CID Interconnect Modeling & Simulation 2
In a transmission line SPICE model, what does the 'TD' parameter represent?