Originally posted Jun 23, 2016
How to avoid fooling yourself
Some years ago I bought an old building lot for a house, and hired a surveyor because the original lot markers were all gone. It was a tough measurement task because nearby reference monuments had also gone missing since the lot was originally platted. Working from the markers in several adjacent plats, the surveyor placed new ones on my lot—but when he delayed the official recording of the survey I asked him why. His reply: he didn’t want to “drag other plat errors into the new survey.” Ultimately, it took three attempts before he was satisfied with the placement.
Land surveys are different from RF measurements, but some important principles apply to both. Errors sometimes stack up in unfortunate ways, and an understanding of insidious error mechanisms is essential if you want to avoid fooling yourself. This is especially true when you’re gathering more information to better understand measurement uncertainty.
Keysight engineers have the advantage of working in an environment that is rich in measurement hardware and expertise. They have access to multiple measurement tools for comparing different approaches, along with calibration and metrology resources. I thought I’d take a minute to discuss a few things they’ve learned and approaches they’ve taken that may help you avoid sneaky errors.
Make multiple measurements and compare. I’m sure you’re already doing this in some ways—it’s an instinctive practice for test engineers, and can give you an intuitive sense of consistency and measurement variability. Here’s an example of three VSWR measurements.
VSWR of three different signal analyzers in harmonic bands 1-4. With no input attenuation, mismatch is larger than it would otherwise be. The 95% band for VSWR is about 1.6 dB.
It’s always a good idea to keep connections short and simple, but it’s worth trying different DUT connections to ensure that a cable or connector—or even a specific bit of contamination—isn’t impairing many measurements in a consistent way that’s otherwise hard to spot. The same thing applies to calibration standards and adapters.
The multiple-measurements approach also applies when using different types of analyzer. Signal analyzers can approach the accuracy of RF/microwave power meters, and each can provide a check on an error by the other.
Adjust with one set of equipment and verify with another. DUTs may be switched from one station to another, or elements such as power sensors may be exchanged periodically to spot problems. This can be done on a sample or audit basis to minimize cost impacts.
In estimating uncertainty, understand the difference between worst case and best estimate. As Joe Gorin noted in a comment on an earlier post “The GUM, in an appendix, explains that the measurement uncertainty should be the best possible estimate, not a conservative estimate. When we know the standard deviation, we can make better estimates of the uncertainty than we can when we have only warranted specifications.” A more thorough understanding of the performance of the tools you have may be an inexpensive way to make measurements better.
Make sure the uncertainties you estimate are applicable to the measurements you make. Room temperature specifications generally apply from 20 to 30 °C, but the “chimney effect” within system racks and equipment stacks can make instruments much warmer than the ambient temperature.
Take extra care as frequencies increase. Mismatch can be the largest source of uncertainty in RF/microwave measurements, and it generally gets worse as frequencies increase. Minimizing it can be worth an investment in better cables, attenuators, adapters, and torque wrenches.
This isn’t meant to suggest that you adopt an excessively paranoid outlook—but it’s safe to assume the subtle errors really are doing their best to hide from you while they subvert your efforts. Said another way, it’s always best to be alert and diverse in your approaches.