The fastest way to a verified VNA measurement is with an airline, but not the kind that flies.
"I use airlines every week," said Al Neves, the self-proclaimed SI (signal integrity) Practitioner, at DesignCon 2013. He's also the Chief Technologist with Wild River Technology. Of course Al is referring not to taking a plane every week, but to using the specially constructed precision coaxial transmission line structure called an airline.
An airline is a cylindrical coax transmission line with precision machined center conductor and inner diameter cavity. It's designed to act as a low loss, single mode, uniform transmission line, which can be used as a reference. Its impedance is accurate to better than 1 percent, and is often traceable to NIST (Figure 1).
Figure 1. Example of the measured return and insertion loss of an airline that is designed with a stepped impedance. Courtesy of Wild River Technology.
Al suggests every SI practitioner doing routine VNA or TDR measurements should have at least one 50Ω airline handy. "I use it whenever I want a quick way of verifying a VNA calibration or to compare a VNA measurement to a TDR measurement," he said. "Airlines are really about risk reduction. Measuring an airline is the start to a benchmarking strategy. You can pay a little bit to buy insurance and reduce your risk with an airline."
It's easy to get a measurement from a VNA. The hard part is getting one that is free of artifact. "A VNA requires a lot of discipline to calibrate. Every now and then, it’s an important sanity check to look at the measurements on a known standard like an airline." Adding an airline in series to general TDR measurements gives an immediate measure of confidence.
Neves will tell you that one of the advantages of an airline is that the measurements are clean to high bandwidth. There are rarely transverse modes, so the measurements should be artifact free.
An SI Practitioner's goal is to get a successful product out the door on schedule. When signal integrity problems pay a role in the design, we need to leverage all the tools available to us.
Al suggests these tools are in four important areas: metrology and measurement, 3D modeling, general SI issues, and the business and economics side of things. Figure 2 illustrates the relationship among these issues.
Figure 2. A signal-integrity practitioner leverages four categories of tools.
A version of this article appeared on DesignCon Community.