Good models require great imaginations. Whether at the device, circuit, or system level, computer-aided-engineering (CAE) software simulators will only take you so far when trying to model a design. The rest must come from good old-fashioned engineering imagination. it is being able to foresee parasitic circuit elements, or time delays in certain types of transmission lines that can result in phase shifts in the frequency domain. And it used to be simpler, with the analog designers dealing with one portion of the project and the digital folks working with a different part. But more and more, as this month's front cover shows, the analog and the digital parts of a high-frequency/high-speed design may be in the hands of the same designer.

This collision between analog and digital worlds occurred long before signals were being broken into in-phase (I) and quadrature (Q) components for digital modulation in high-data-rate communications systems. The conversion from intermediate frequency (IF) to digital signals in RF/microwave systems ranging from radars to spectrum analyzers has required at least a working knowledge of high-speed analog-to-digital converters (ADCs) and how they interface with other logic components.

Simulating the behavior of analog circuits requires reasonable models of complex components. Analog designers have had choices in different circuit simulation tools as well as numerous electromagnetic (EM) simulation programs based on the use of Maxwell's equations for predicting EM field generation and behavior. in recent years, the term "co-simulation" in terms of high-frequency analog modeling has come to mean modeling efforts with a combination of such tools as circuit and EM simulators.

With the growing use of digital components (including ADCs and DACs) in high-frequency circuits and systems, however, effective modeling becomes an even greater challenge. The term "co-simulation" must now take on a different meaningthat of modeling both analog and digital component and circuit functions so that their interactions can be understood. For example, many papers and even books have been written about modeling the static and dynamic behavior of ADCs in attempts to better understand the error sources associated with data converters. Although most studies cite linearization errors as the greatest cause of concern when trying to construct a software model for an ADC or DAC, many also mention some of those analog sources of error.

As digital circuitry becomes a larger part of RF/microwave designs, it is clear that new models and imaginative modeling tools will be needed to predict the interactions of different types of components at high analog frequencies and high digital clock rates. Fueled by great imaginations, those RF/microwave/digital models will help future designs achieve higher levels of integration at lower levels of power consumption.