Are Your Measurements Corrected? Calibrated? Aligned? Or What?

Blog Post created by benz on Sep 14, 2016

Originally posted Aug 15, 2014

Getting the accuracy you’ve paid for

You’ve probably had this experience while using one of our signal analyzers: The instrument pauses what it’s doing and, for some time—a few seconds, maybe a minute, maybe longer—it seems lost in its own world. Relays click while messages flash on the screen, telling you it’s aligning parts you didn’t know it had. What’s going on? Is it important? Can you somehow avoid this inconvenience?

There’s a short answer: The analyzer decided it was time to measure, adjust and check itself to ensure that you’re getting the promised accuracy.

That seems mostly reasonable. After all, you bought a piece of precision test equipment (thanks!) to get reliable answers, so you can do your real job: using RF/microwave technology to make things happen—important things. The last thing you want is a misleading measurement.

That’s not the whole story. Your time is valuable and it’s useful to understand the importance of these operations and whether you can stop them from interrupting your work.

The second short answer: the automatic operations are sometimes important but not crucial (usually). You can do several things to avoid the inconvenience, but it helps to first understand a few terms:

  • Calibrations are the tests, adjustments and verifications performed on an instrument every one to three years. The box is usually sent to a separate facility where calibrations are performed with the assistance of other test equipment.
  • Alignments are the periodic checks and adjustments that an in situ analyzer performs on itself without other equipment or user intervention. The combination of calibration and alignment ensures that the analyzer meets its warranted specifications.
  • Corrections are mathematical operations the analyzer performs internally on measurement results to compensate for known imperfections. These are quantified by calibration and alignment operations.

Alas, this terminology isn’t universal. For example, if you execute the query “*Cal?” the analyzer will tell you whether it is properly (and recently) aligned, but will say nothing about periodic calibration. Still, the terms are useful guides to getting reliable measurements while avoiding inconvenience.

As a starting point, you can use the default automatic mode. The designers have decided which circuits need alignment, how often and over what temperature ranges. Unfortunately, this may result in interruptions, and these can be a problem when you’re prevented from observing a signal or behavior you’re trying to understand. It’s especially frustrating when you’re ready to make a measurement and find that the analyzer has shifted into navel-gazing mode.

Switching off the automatic alignments will ensure that the instrument is always ready to measure—and it will notify you when it decides that alignments are needed. You can decide for yourself when it’s convenient to perform them, though this creates a risk that alignments won’t be current when you’re ready to make a critical measurement.

You can schedule alignments on your own, and tell the instrument to remind you once a day or once a week. This is a relatively low-risk approach if the instrument resides in a temperature-stable environment. However, with your best interests in mind, the analyzer will display this stern warning:

Switching off automatic alignment creates a small risk of compromised performance, and produces this popup.

Switching off automatic alignment creates a small risk of compromised performance, and produces this popup.

The default setting is governed by time and temperature, and in my experience it’s temperature that makes the biggest difference. I once retrieved an analyzer that had been left in a car overnight in freezing weather and, upon power up, found that for the first half hour it was slewing temperature so fast that alignments occurred almost constantly.

If you want to optimize alignments for your own situation, just check out the built-in help in the X-Series signal analyzers. You can even go online and download the spectrum analyzer mode help fileto your PC and run it from there.