Test equipment represents that last barrier between a design concept and a shipped product. Reliable test gear can keep production lines running and ensure that a customer is satisfied with the performance of those products. In the research and development (R & D) laboratory, the test instruments provide invaluable insight into the performance characteristics of a design, allowing engineers to refine and optimize. The essential nature of test equipment to microwave engineering has changed little over the last decade, but the form and function of the equipment is ever evolving.

Traditionally, microwave and RF test instruments have been built into 19-in.-wide enclosures that either sat on an engineer's bench or were mounted into a rack with other gear. R & D equipment has generally been connected in different configurations as needed, while production-line test equipment has usually remained in a fixed installation with a set number of functions under computer/software control.

These racks of equipment were usually filled with single-function boxes, such as power meters, frequency counters, signal generators, precision power supplies, spectrum analyzer, and voltmeters, along with a few microwave-specific instruments, such as scalar network analyzers and vector network analyzers. Over the last decade, and largely due to the specialized requirements of many wireless applications, the extent of this functionality has changed and instruments have tended to become multifunction devices. Frequency counters and power meters are integrated into spectrum analyzers, and spectrum analyzers are rapidly becoming "real-time" analyzers capable of digitizing wide instantaneous bandwidths.

The sizes and forms of the "racks" are also changing, with the evolution of VXI/VME cards and Agilent/Hewlett-Packard's high-performance Modular Measurement System (MMS) architecture into current-day instrument-on-a-card formats such as CompactPCI and PXI.

The logical next step in test-equipment evolution, of course, is to realize as many of these measurement functions as possible in software. This is essentially the concept of a synthetic instrument (see p. 57), which is built largely around high-speed data converters, such as analog-to-digital converters (ADCs) and digital-to-analog converters (DACs). Once test signals are digitized, they can be processed and manipulated to extract modulation, phase, amplitude, and other information. Of course, capturing as much of an analog input signal as possible remains the challenge for synthetic instruments, since they are ultimately limited by the processing speed of the data converters.