Amendra Koul

Improved technique for extracting parameters of low-loss dielectrics on printed circuit boards

The paper is devoted to a methodology and an improved technique of characterization of low-loss dielectrics on printed circuit boards. The technique is based on measuring S-parameters and recalculating them into complex propagation constant. Phase correction is proposed to assure that the phase constant passes through zero at zero frequency. An effect of dielectric loss upon a dielectric constant is considered in the analytical model for dielectric parameter extraction. Dielectric and conductor losses are separated using a model, which includes surface roughness of conductors. Network asymmetry is taken into account in the model. Extracted parameters for frequency-dispersive dielectrics satisfy Kramers-Krönig causality relations. The proposed model allows for extracting dielectric constant and dissipation factor with an increased accuracy.

Differential Extrapolation Method for Separating Dielectric and Rough Conductor Losses in Printed Circuit Boards

Copper foil in printed circuit board (PCB) transmission lines/interconnects is roughened to promote adhesion to dielectric substrates. It is important to characterize PCB substrate dielectrics and correctly separate dielectric and conductor losses, especially as data rates in high-speed digital designs increase. Herein, a differential method is proposed for separating conductor and dielectric losses in PCBs with rough conductors. This approach requires at least three transmission lines with identical, or at least as close as technologically possible, basic geometry parameters of signal trace, distance-to-ground planes, and dielectric properties, while the average peak-to-valley amplitude of surface roughness of the conductor would be different. The peak-to-valley amplitude of conductor roughness is determined from scanning electron microscopy images.

Differential and extrapolation techniques for extracting dielectric loss of printed circuit board laminates

The experiment-based differential and extrapolation techniques to extract frequency-dependent dielectric loss of printed circuit board laminates are proposed. Separation of dielectric loss from conductor loss on substantially rough copper foils is based on the analysis of frequency (ω) components in dielectric and conductor losses. Smooth conductor loss behaves as √ω, while dielectric loss behaves as ω and ω². However, conductor roughness behaves as √ω, ω, and ω², and these contributions may be lumped into the dielectric loss. A few examples of extracting the unique dielectric loss parameters for PCB test striplines with the same dielectric, but with either different types of foils, or with different widths of the signal traces, are presented.

Material parameter extraction using Time-Domain TRL (t-TRL) measurements

Characterizing materials used in Printed Circuit Board (PCB) manufacturing is becoming increasingly important in link path analysis as the data rates are increasing. The material properties governing the performance of the signal passing through a transmission line are frequency-dependent. Using frequency-domain vector network analyzer (VNA) measurements and Through-Reflect-Line (TRL) calibration, these parameters can be determined accurately. But a Time-Domain Reflectometer (TDR) provides a relatively inexpensive and simple way of characterizing transmission lines, and it is easily accessible to Signal Integrity engineers. With the time-domain TRL (t-TRL) calibration technique [1], it is now possible to de-embed such discontinuities as connectors, cables, etc., in the path of the transmission line using time-domain measurements. From the calibrated results, material properties can be extracted in the same way as it is done in the frequency domain. This paper describes a t-TRL technique to obtain accurate frequency domain S-parameters from time domain measurements. The calibrated results are converted into the ABCD parameters. The propagation constant is obtained through the ABCD parameters, from which attenuation loss and phase constant are extracted. Dielectric constant is extracted from the phase constant and the total attenuation constant. Curve-fitting technique is used to split the losses into conductor and dielectric loss. Once dielectric loss is determined, loss tangent can be calculated. The results are compared for three test vehicles, and are also compared with frequency domain VNA measurements. The results from the t-TRL calibration technique are also compared with another known extraction procedure.

Surface impedance approach to calculate loss in rough conductor coated with dielectric layer

The analysis presented herein contains closed-form analytical expressions to calculate attenuation in a layered structure “rough metal-dielectric-dielectric”, which is a practically important problem in separating dielectric loss from rough conductor loss in actual PCB stripline geometries, when measuring dielectric constant (Dk) and dissipation factor (Df) using travelling wave S-parameter methods. This approach is based on the surface impedance concept. It is shown that the presence of an epoxy layer on the conductor may affect extracted dielectric parameters, of a PCB substrate, especially the Df data.

Measurement and correlation-based methodology for estimating worst-case skew due to glass weave effect

Skew is unintentionally introduced within a differential pair, through misalignment of conductors and glass fiber bundles in Printed Circuit Board (PCB) dielectric layers. Manufacturers do not control interposition of specific glass bundles to supplied board design (artwork). Therefore, an unknown and random factor is added to each produced PCB. Current paper describes a method that utilizes a set of measurements and numerical models to estimate worst-case skew for the aforementioned effect. First, test vehicles are built, and then cross-sections are analyzed with Scanning Electron Microscope (SEM) for precise measurements. Numerical models are constructed to correlate with real DUT; after correlation is achieved, relative location of conductors to glass bundles is swept to obtain best and worst case skew.

Effective channel budget technique for high-speed channels due to differential P/N skew

It is understood very well that at 20Gbps and beyond, P/N skew from fiber weave effect in printed circuit board (PCB) has to be taken into account. Several studies have been done to measure and quantify the effect of glass weave but very few have offered techniques to budget for P/N skew. System companies need an effective way to capture glass weave skew and budget for it in channel designs. Using transmission line theory, we analytically calculate differential loss due to P/N skew, study the effect of P/N skew on eye performance for SerDes IP and explore dependency of P/N skew on PCB materials. This paper studies effect on S-parameters, SerDes eye performance due to skew that can help SI engineer to effectively budget for P/N skew as part of channel link budget.

Evaluation of propagation characteristics for PCB traces with periodic roughness using ASM-FDTD method

This paper presents an efficient technique to evaluate the propagation characteristics of PCB traces with periodic roughness. This technique is based on the application of the array-scanning-method finite-different time-domain technique in which only a single periodic element needs to be modeled. Numerical examples are used to demonstrate the effectiveness and efficiency of this approach.

Developing an SI tool set for engineering design discovery, physical insight, and education

This paper reports on the process of developing a signal integrity tool set for engineers and educators. These tools will complement well-known enterprise numerical tools by providing the designer, student, or educator a reliable means of finding accurate results for specific questions in signal integrity design and troubleshooting. They are intended to find use as accurate, traceable, and easy-to-use aids in design discovery and in education. Their computational engines will use recognized methods from the literature and these will be made available so that interested users will be in complete control of their enquiries; there will be no calculations in which the user is kept in the dark about the methods used. The SI tool set will be made freely available on the internet.