Synthetic Instruments

Concepts and Applications

Synopsis

The way electronic measurement instruments are built is is making an evolutionary leap to a new method of design called Synthetic Instruments. This promises to be the most significant advance in electronic test and instrumentation since the introduction of automated test equipment (ATE). The switch to Synthetic Instruments is beginning now, and it will profoundly affect all test and measurement equipment that will be developed in the future.

Synthetic Instruments are like ordinary instruments, in that they are specific to a particular measurement or test. They might be a voltmeter that measures voltage, or a spectrum analyzer that measures spectra. The difference is that Synthetic Instruments are implemented purely in software that runs on general purpose, non-specific measurement hardware with a high speed A/D or D/A converter at its core. In a Synthetic Instrument, the software is specific; the hardware is generic. Therefore, the personality of a Synthetic Instrument can be changed in an instant. A voltmeter may be a spectrum analyzer a few seconds later, and then become a power meter, or network analyzer, or oscilloscope. Totally different instruments are realized on the same hardware, switching back and forth in the blink of an eye, or even existing simultaneously.

The union of the hardware and software that implement a set of Synthetic Instruments is called a Synthetic Measurement System (SMS). This book studies both Synthetic Instruments, and systems from which they may be best created.

Powerful customer demands in the private and public sectors are driving this change to Synthetic Instruments. There are many bottom-line advantages in making one, generic and economical SMS hardware design do the work of an expensive rack of different, measurement-specific instruments. ATE customers all want to reap the savings this promises. ATE Vendors like Teradyne, as well as conventional instrumentation vendors like Agilent and Aeroflex, have announced or currently produce Synthetic Instruments. The US Military, one of the largest ATE customers in the world, wants new ATE systems to be implemented with Synthetic Instruments. Commercial electronics manufacturers such as Lucent, Boeing, and Loral are using Synthetic Instruments now in their factories.

Despite the fact that this change to Synthetic Instrumentation is inevitable and widely acknowledged throughout the ATE and T&M industries, there is a paucity of information available on the topic. A good deal of confusion exists about basic concepts, goals, and trade-offs related to Synthetic Instrumentation. Given that billions of dollars in product sales hang in the balance, it is important that clear, accurate information be readily available.

Plan of this Book

The basic goal of this book is to explain Synthetic Instrumentation at a high level, focusing on specific details when necessary to illustrate a crucial point of note. The first half of the book is generally of a hardware flavor, and the second half is generally oriented toward theory and software, but unifying themes of Synthetic Measurement System high-level design are presented throughout.

The foremost unifying concept in the book, tying together hardware, system theory, and software in a tidy package, is the Measurement Map. This unique and powerful concept serves as a bridge between the synthetic cascade hardware architecture, the abstract idea of a measurement, and the expression of a Synthetic Instrument as an XML schema for automated software implementation and processing.

The power of the XML schema based on the Measurement Map is that concise, structured descriptions of measurements can be given directly by the test engineering user (possibly with the help of a GUI tool). These descriptions of a measurement can be automatically processed, optimized, and performed. Most importantly, the test engineer can specify exactly the measurement wanted without writing procedural test scripts or doing any other programming.

Thus, the book is a collage of hardware, software, and system concepts all aimed at the single goal of explaining and extending this new approach to measurement system design.

Chapter Outline

The book begins by answering the question: “What is a Synthetic Instrument?” First, we review the history of measurement, automated and otherwise. The advantages of the synthetic approach are enumerated and discussed, along with motivation for the fundamental idea of using generic hardware to perform specific measurements. Also presented are the necessary distinctions between synthesis and analysis, and between test versus measurement.

Once this basic concept of a Synthetic Instrument is explained, the book moves on to a detailed discussion of hardware architecture. This discussion begins with the introduction of the Control, Codec, Conditioning (CCC) Cascade architecture, as either a stimulus or response asset. Architecture variations are described, including the basic Chinese Restaurant Menu (CRM) variation, compound stimulus, and multiplexing options.

The hardware discussion then moves sequentially through the stimulus and response system cascades, touching on critical issues and challenges these systems present. Among the issues considered for the stimulus side are direct digital synthesis (DDS) and controller characteristics, triggering and digital up-conversion, linearity, gain control, and adaptive fidelity improvement. In the response cascade, input conditioning, quantization, system fidelity trade-offs, adaptive interference cancellation, and matched filters are discussed.

Examples of commercial, state of the art subsystems are presented for stimulus and response. The book also contains a complete chapter containing a detailed description of a real world, production oriented Synthetic Measurement System.

The real world system example is roughly the midpoint of the book. At this point the focus shifts back to the theoretical as the central concept of a measurement map is introduced. The discussion links the measurement map to the test engineer's desired test, as well as to the hardware and calibration. This lays the groundwork for subsequent theory and software architecture discussions.

After the measurement map, there is a detailed discussion of signals and signal processing issues often encountered in Synthetic Measurement Systems. These include discussions of making measurements at different levels of the signal coding hierarchy, encoding and decoding strategies, bandwidth, and sampling strategies. Some practical issues with up and down converters are explored.

A chapter on calibration and accuracy attempts to clarify the way to think about measurement topics. There is also a general discussion of reference standards and uncertainty analysis. Along with general calibration topics, there is a major section on the topic of stimulus calibration which includes a detailed analysis of certain interpolation issues. The concept of de-embedding is also presented.

The next two chapters are an introduction to the XML method for encapsulating measurement descriptions, and an annotated example of a measurement description expressed in XML.

The book concludes with a listing of The Ten Mistakes in Synthetic Measurement System Design. This chapter draws from many of the concepts introduced throughout the previous material, drawing conclusions that apply to real world applications.

Errata in the first edition.

About C.T. Nadovich

Mr. Nadovich is a working Engineer with 25 years of experience in the design and development of advanced instrumentation for RF and Microwave test. His current work in Synthetic Measurement Systems is in collaboration with Pallas Systems where he is actively involved in SI related design and development efforts. He is at the forefront of the Synthetic Instrumentation revolution.