James P. K. Gilb

Pulse distortion on multilayer coupled microstrip lines

The distortion of pulses due to dispersion and coupling on generalized planar microstrip is investigated. A simple, general form for the Greens function is obtained by solving the boundary value problem separately for the TE and TM modes. The mechanism of even/odd-mode distortion is discussed and numerical results are presented illustrating its effect. The design of structures with low even/odd-mode distortion is considered. Results for pulse distortion on this type of microstrip are also included. >

Asymmetric, multi-conductor low-coupling structures for high-speed, high-density digital interconnects

Due to the increasing speeds of digital circuits and the small interline spacings, a full-wave analysis of multilayer, multiconductor structures is required to accurately predict the coupling and crosstalk on these structures. Using the spectral domain approach, it is shown that substrate compensation can be used for asymmetric coupled lines and symmetric multiconductor lines. Some of the characteristics of substrate compensated low-coupling structures were also investigated, showing that one substrate configuration can be used to reduce coupling and crosstalk with a variety of conductor configurations. It is concluded that, by designing high-speed interconnects with substrate compensation, it will be possible to achieve an extremely high density of signal conductors, using interline spacings of less than one center conductor width, while keeping crosstalk and coupling distortion to a minimum. >

MIS slow-wave structures over a wide range of parameters

The authors study lossy multilayer, multiconductor microstrip structures using the spectral domain approach. The modal attenuation and propagation constants of these structures are given over an extremely wide range of substrate parameters and frequencies, covering all three regions: low loss, slow wave, and skin effect. The modal attenuation and propagation constants are presented for two and four conductor structures as a function of the substrate loss tangent. Single conductor structures are characterized with contour plots showing the complex effective dielectric constant as a function of both frequency and conductivity. >

Enhanced dominant mode operation of a shielded multilayer coplanar waveguide via substrate compensation

The cutoff frequency of the first higher-order even mode in a shielded multilayer coplanar waveguide (CPW) is studied using the spectral domain approach (SDA). The effective dielectric constant for the dominant odd and first higher-order even mode in a shielded multilayer CPW is computed and compared to other published numerical results. Dielectric constant and substrate height are varied with respect to even mode cutoff frequency and plotted for several CPW structures. Different combinations of internal substrates are shown to produce even mode cutoff frequency maximization for increased odd mode operation bandwidth. >

Transient analysis of distortion and coupling in lossy coupled microstrips

The transient response of lossy coupled microstrips is studied using the spectral domain approach (SDA) to compute rigorously the dielectric losses. Transient coupling is formulated in the frequency domain using an even/odd mode approach. Results for pulse distortion on a semiconducting substrate are presented showing how losses reduce the signal amplitude without significantly distorting the shape. Distortion due to even/odd mode coupling was the dominant distortion mechanism, while coupling due to differences in the modal attenuation constants had a secondary effect.<<ETX>>

Improved performance of microstrip couplers through multi-layer substrates

Edge-coupled microstrip directional couplers are widely used in microwave circuits, however practical limits on center conductor spacings limit the level of coupling that can be attained. In addition, differences in the even and odd mode phase velocities degrade the performance of broad-side coupled microstrip directional couplers. A new technique for equalizing the phase velocities is presented which enhances the coupling and directivity, but uses wider center conductor spacing than traditional designs. This new technique uses two substrate layers where the upper substrate dielectric constant is much larger than the dielectric constant of the lower substrate. This allows the design of directional couplers with tighter couplings and better performance.

Enhanced dominant mode operation of shielded multilayer coplanar waveguide

The effects of substrate height and dielectric constant on the propagation characteristics of a multilayer shielded symmetrical coplanar waveguide (CPW) are examined using a full-wave spectral-domain analysis (SDA). It was found that the effective dielectric constant of the shielded CPWs displayed a cutoff frequency for the even mode which can be controlled by varying the height and dielectric constant of the substrate, especially those directly below the coplanar slots. The highest cutoff frequencies found in three-layer CPW configurations utilized air as the lower substrate dielectric. In certain multilayer CPW combinations, when the lower substrate was not air, a bandwidth maximization effect was demonstrated using internal substrates.<<ETX>>

Distortion of transient signals on multilayer coupled symmetric microstrip transmission lines

Using a variation of the spectral domain technique, a recursion relation for general planar microstrip structures is derived that is simple to formulate and computationally efficient. Using this formulation and the Fourier transform, signal distortion for both simple and complex multilayer, multiconductor lines is investigated as a function of distance and strip spacing. A method for eliminating distortion due to coupling between lines is discussed, and results are presented that confirm the removal of coupling despite the close spacing of the lines.<<ETX>>

Closed-form expressions for the design of microstrip lines with two substrate layers

A simple closed-form expression for the effective dielectric constant of a microstrip on a two-layer substrate is presented. Results for single-layer cases are incorporated in the formula to simplify it and increase the accuracy. The formula is curve-fitted from results computed with the spectral domain approach and is accurate to within 3% relative error over a wide range of parameters.<<ETX>>

Wide-Band Reduction of Coupling and Pulse Distortion in Multi-Conductor Integrated Circuit Lines

Ultrafast clock speeds and short rise times of digital signals give them a very large bandwidth. With clock speeds in the Gigabit /second range and rise times on the order of 10 picoseconds, these signals will have significant frequency components in the tens to hundreds of GHz. This wide frequency range requires a full-wave analysis, rather than a a quasi-static one, to accurately predict the distortion and coupling of highspeed digital signals. High speed circuits also have small inter-line spacing to increase packing density, which in turn increases coupling distortion and crosstalk. Successful design of these high-speed circuits requires accurate prediction of pulse distortion and coupling, as well as a technique to reduce coupling over a very wide band of frequencies.