PCB Technical Publications

This report provides detailed information on a study of the effects of internal series termination between positive and negative I/O pads in LVDS buffers. Comparisons between IBIS and SPICE simulations are used to emphasize these effects. Different loading conditions and modeling approaches are considered. IBIS modeling approaches for LVDS buffers are presented. Model validation shows the power of these approaches over other conventional techniques.

New software technology has been developed that enables effective parallel design of a circuit board. This technology enables multiple designers, processes and heterogeneous tools to work on the same design database simultaneously and achieve significant gains in design productivity. But, unlike classical divide - and conquer methods that break down a design into pieces and operate on them independently, this new technology enables concurrent progress on a common database, automatically synchronizing changes and resolving conflicts - a first in the EDA industry. This paper focuses on the methods and applications of new parallel design technology that offers a novel paradigm for circuit board design. Topics include:

• Review of design problems and concurrent methods
• Parallel design architecture

Applying the parallel design technology to:

• Layout
• Autorouting
• Circuit and Board Design
• System Design

There is no shortage of design rules available when people talk about differential traces on circuit boards. At various times you can hear people argue that there is a need for, or there is no need for, a variety of special rules regarding continuity of ground planes underneath the traces, equal length traces, equal separation between traces, differential impedance control, etc. So let's set the record straight. NONE of these rules are inherently required by the fact that we are using differential signals! But some of them might be required if we are worried about signal integrity issues in our designs. This article looks at these individual types of rules from the standpoint of various signal integrity issues to see when, if ever, they need be applied.

Termination strategies are effective in eliminating, or at least controlling, transmission line reflections. There are five types of termination strategies commonly used with transmission lines (parallel, AC, Thevenin, Series, and Diode.) This paper looks at each strategy and summarizes its strengths and weaknesses. In addition, simulations are illustrated for two of tem (AC and Series), helping to illustrate their unique characteristics.

Most of us have been using incorrect values for the propagation speed of our microstrip traces! The correction factor for Er we have been using all this time is based on an incorrect premise. In particular, it results in a value for propagation speed that is independent of variations in trace width and height above the reference plane. But the propagation speed for microstrip traces depends significantly on such variations. This article explains why and develops a superior model for estimating propagation speeds and propagation delays for microstrip configurations.

When we use transmission line techniques to control reflections on circuit board traces, we must terminate the lines. Typically we do so with resistors placed at the beginning (series termination) or at the end (parallel or Thevenin termination) of the trace. An interesting question is, "Where do we place these terminating resistors?" The more obvious assumption that we place them" as close as possible" to the end of the trace may not be the best answer. This article looks closely, with the aid of some simulations, at precisely where terminating resistors should be placed, and why.