In Quest Of The Highest Accuracy

Vector network analyzers (VNAs) are instruments unique to the microwave industry. Other areas of electronics have protocol analyzers, or power meters, or spectrum analyzers, but only RF/microwave engineers have VNAs. In many ways, the microwave industry of the last few decades has been built upon these instruments, which provide measurement results accurate enough to model a device under test (DUT).

The "ancient" among us can remember groundbreaking VNA products such as the HP 8409A and HP 8510A from Hewlett-Packard Co. Agilent Technologies>, and the model 360 from Wiltron Co. Anritsu Co.>. While the bandwidth, resolution, and accuracy of those instruments was impressive by 1980's standards, few engineers from that decade could envision the kind of VNA performance embodied in Anritsu's VectorStar VNAs (see pp. 89 and 28).

Development of those earlier VNAs represented multiyear projects, calling upon superb machine-shop skills from gifted engineers like Julius Botka at Hewlett-Packard Co. and Bill Oldfield at Wiltron. Precision transmission lines, reference standards, and even coaxial connectors were needed to realize those VNAs. Fabricating these components required not only expertise in mechanical engineering, but in how those dimensions applied to high-frequency signals.

Which makes the accomplishment of the VectorStar analyzers even more impressive. High directivity is essential in a VNA, even with error correction. In many ways, a VNA is as much about mechanical engineering as it is about electrical engineering; the VectorStar VNAs show what can be done in terms of fabricating precisely controlled transmission paths in a high-performance RF/ microwave instrument.

In the era of the HP 8510A and 360, few would have projected that microelectromechanical systems (MEMS) structures could be used for switches. The advances in integrated circuits (ICs) over the last few decades have grabbed headlines and captured the imaginations of designers seeking to combine analog, RF, digital, and even optical functions on a single chip.

Advances in micromachining are not nearly as exciting as those leaps in IC and active device technologies, but they are as important, especially for the development of instruments such as the VNA. With the performance levels already possible with modern VNAs such as the VectorStars, dramatic improvements in micromachining capabilities will be needed to significantly improve upon current VNA performance and accuracy levels. But we have been surprised before.

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