Differential Vias Transition Modeling in a Multilayer Printed Circuit Board

A 26-layer printed circuit board including several test sites has been analyzed. All the sites have a transition from coupled microstrips to coupled striplines through signal vias. Differential measurements have been performed on some of these test sites to estimate the effect on S-parameters and eye diagrams due to via and antipad radius variation, and different lengths of via stub. The focus of this paper is on a test site with a transition from top to the sixth layer. At the same time, a physics based circuit model has been assembled in a spice-based simulation tool and a full-wave model has been generated as well. The paper will show that the process of modeling can require a series of adjustments to get reasonable results. A brief discussion about possible issues associated with fabrication tolerances is presented in the last chapter

Influence of an Extended Stub at Connector Ports on Signal Launches and TRL De-embedding

Characterization of PCBs (Printed Circuit Boards) is usually associated with measurement using a VNA (Vector Network Analyzer) in the frequency-domain or a TDR (Time Domain Reflectometer) in the time-domain. The often used signal launch techniques on PCBs based on the VNA or TDR measurement in the microwave frequency range use SMA or 3.5 mm connectors, in edge-launch or vertical-launch fashions. The signal transition between the launch port and the DUT (Device Under Test) introduces errors in the measurement, which is dominant when compared with a transmission line itself on the PCB as the technologies of PCB manufacturing well developed today. Discontinuities at connector ports depend on the port structures and the dielectric properties of the substrate materials. However, an extended stub at a connector port may significantly influence signal launches, or even corrupt a TRL calibration in a measurement

A Hybrid Approach to Decrease Port Influence in Transmission Line Characterization

Characterization and models for multi-gigabit signaling is an important issue in modern digital system. A good physical based model relies on a precise characterization of the test board. Typically, the characterization of the test board is associated with scattering matrix parameter measurement, which can be done with a VNA (Vector Network Analyzer) in the frequency-domain or a TDR (Time Domain Reflectometer) in the time-domain. The commonly used launch techniques on PCBs (Printed Circuit Boards) associated with the VNA or TDR measurement in the microwave frequency range use SMA or 3.5 mm connectors, in edge-launch or vertical-launch fashions. The transition between the launch port and the DUT (Device Under Test) introduces errors in the measurement. Embedding/deembedding techniques are used to remove the port influences in the measurement generally. For example, TRL (Through, Reflect, and Line) calibration is the typical method used in measurement to eliminate port influences. However, extra test kits are needed for TRL calibration, and furthermore the TRL calibration is sometimes difficult to implement, such as in coupled differential lines. In this paper, an effective hybrid approach for transmission line characterization is proposed, which includes choosing a suitable port launch technique for the test board, port parasitic parameters estimation, and building up a proper circuit model for evaluation with genetic algorithms (GA)

Signal Link-Path Characterization Up to 20 GHz Based on a Stripline Structure

Dielectric properties and losses are two critical issues in signal link-path characterization. To obtain the substrate dielectric properties for a planar transmission line, an analytical solution is derived and validated based on a stripline structure and measured scattering parameters with TRL de-embedding. The characterized dielectric property is used to evaluate dielectric loss and conductor loss. The total loss is thereby found from their summation. The calculated total loss is compared to the measured total loss, and the conductor loss and dielectric loss are then quantifiable. Since the conventional description using the loss tangent and dielectric constant to represent material properties is usually insufficient as the frequency reaches 20 GHz, a Debye model is proposed. The second order Debye parameters are subsequently extracted using a genetic algorithm. A full wave simulation is implemented to verify the determination of two-term Debye model parameters

Causal RLGC( Ƒ ) Models for Transmission Lines from Measured S-Parameters

Frequency-dependent causal RLGC(f) models are proposed for single-ended and coupled transmission lines. Dielectric loss, dielectric dispersion, and skin-effect loss are taken into account. The dielectric substrate is described by the two-term Debye frequency dependence, and the transmission line conductors are of finite conductivity. In this paper, three frequency-dependent RLGC models are studied. One is the known frequency-dependent analytical RLGC model ( RLGC-I), the second is the RLGC(f) model (RLGC-II) proposed in this paper, and the third (RLGC-III) is same as the RLGC -II, but with causality enforced by the Hilbert transform in frequency domain. The causalities of the three RLGC models are corroborated in the time domain by examining the propagation of a well-defined pulse through three different transmission lines: a single-ended stripline, a single-ended microstrip line, and an edge-coupled differential stripline pair. A clear time-domain start point is shown on each received pulse for the RLGC-II model and the RLGC-III model, where their corresponding start points overlap. This indicates that the proposed RLGC(f) model (RLGC-II) is causal. Good agreement of simulated and measured S-parameters has also been achieved in the frequency domain for the three transmission lines by using the proposed frequency-dependent RLGC (f) model

Reconstruction of Dispersive Dielectric Properties for PCB Substrates using a Genetic Algorithm

An effective method for extracting parameters of a Debye or a Lorentzian dispersive medium over a wideband frequency range using a genetic algorithm (GA) and a transmission-line model is presented. Scattering parameters (S-parameters) of the transmission-line sections, including a parallel plate, microstrip, and stripline, are measured. Wave equations for TEM/quasi-TEM mode with a complex propagation constant and a frequency-dependent wave impedance are used to evaluate the corresponding S-parameters in an analytical model. The discrepancy between the modeled and measured S-parameters is defined as the objective function in the GA. The GA is used for search of the dispersive-medium parameters by means of minimizing the objective function over the entire frequency range of interest. The reconstructed Debye or Lorentzian dispersive material parameters are corroborated by comparing the original measurements with the FDTD modeling results. The self-consistency of the proposed method is demonstrated by constructing different test structures with an identical material, i.e., material parameters of a substrate extracted from different transmission-line configurations. The port effects on the material parameter extraction are examined by using through-reflection-line calibration

Investigation of Mixed-Mode Input Impedance of Multi-Layer Differential Vias for Impedance Matching with Traces

In multilayer printed circuit boards (PCBs), vias are commonly used to connect traces on different signal layers. This paper derives the mixed-mode input impedance of differential vias in typical multilayer structures, and proposes to use the input impedance concept to achieve impedance matching at the via and trace connections. Effects of several geometrical parameters on the input impedance of differential vias have also been studied in this paper. This method can be used to optimize via structures in PCB design processes for smooth via-trace transitions

Methodology of Physics-Based Model Development for Differential Vias

Signal transition through vias is among the components of most concern in high-speed signal link-path analyses. Full-wave is widely used to predict the electric performance of vias. However, for better understanding the characteristic of via transition, physics-based via model is desired. This paper will discuss the development of physics-based via model from the perspective of methodology and behind physics. A simple differential via is used as an example to introduce the development of physics-based via model where the via structure is mapped to circuit components by tracking current-path. A via with ten-layer stack-up is then analyzed using peeling and partitioning method by cutting the via model into via blocks at its reference planes. The observation of electric field distribution along the interface circle of via anti-pad wall and the cutting plane demonstrates that the peeling and partitioning method is physics-based. The determination of TEM/quasi-TEM reference break-points on signal traces is essential for port assignment. Where and how to find the break-point is depicted in the paper as well. Finally, a simple case is studied to verify the methodology