7.8: Summary

A microwave system or subsystem is generally constructed as a cascade of two-port modules. Typically such modules are interconnected on a circuit board using microstrip structures with the system designer providing matching networks, transmission line networks, system architecture, and frequency plan. One or two decades ago crafting such a system using vendor-supplied modules would have required a significant performance compromise. As a result microwave companies necessarily developed large portions of a design in-house that resulted in long design cycles. With programmability and user-defined adjustments, perhaps by setting bias levels, the gain, bandwidth, and distortion performance of off-the-shelf modules can be adjusted. Active modules necessarily introduce distortion and noise, and so managing dynamic range while managing DC power consumption is a considerable concern.

Design and analysis of RF and microwave systems is complicated by the complex signals, i.e., modulated signals and not discrete sinewaves, used in RF and microwave systems and by the excessively long simulation times required to analyze microwave circuits with these signals. Design of linear RF and microwave subsystems such as an amplifier or matching network can proceed very well by simulating the structure one frequency at a time and then stepping the signal frequency over the range of interest.

The analysis of nonlinear microwave subsystems such as amplifiers is more complicated but often it is sufficient to consider two-tones, signals with two sinusoidal components and then there are efficient simulation techniques such as harmonic balance analysis that enables subsystem performance to be efficiently but approximately evaluated. However, neither of these strategies is sufficient when trying to determine the performance of RF and microwave systems. In this case complex signals such as digitally modulated signals must be considered, and there are usually signals at widely different frequencies to be incorporated in any analysis. There is however one special property of microwave signals that is exploited, and this is that nearly every modulated microwave signal has a relatively small bandwidth compared to the center frequency of the signal. Thus system simulation strategies have been developed that require only the smaller bandwidth signal to be fully considered in analysis and bookkeeping used to track the center frequency of the signal.

In this chapter metrics and design techniques were introduced that enable competitive designs to be realized quickly. Often a company may decide that it is to their competitive advantage to design one or more of their own modules rather than using those from module vendors. Even then, the large number of modules available enables design concepts to be tested for feasibility early in the design cycle. Module vendors are challenged with designing modules with close to ultimate performance, but with enough adaptability that the module is suited to a wide variety of system applications. The greater customer base for modules justifies the higher module design costs leading to acceptable unit costs and high performance.