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You need to test, we're here to help.

17 August 2017

Back to Basics: Metrology

Some sciences are more obscure than others; these may be of interest and importance to relatively few. The term “metrology” may leave even some science geeks scratching their heads, but for those in the test and measurement arena, metrology is an extremely critical area of scientific endeavor.

Metrology is the science of studying the errors in measurements. Test equipment, such as the oscilloscopes we manufacture here at Teledyne LeCroy, are only as good as long as their measurements are consistently precise and accurate. So for us, metrology is a science that is central to our mission.

At the root of our practice of metrology are international standards of measurement for quantities such as currents, voltages, frequency, time, resistance, impedance, and power. These standards are defined by bodies such as the National Institute of Standards and Technology (NIST) in the United States, or the International System of Quantities as represented by the ISO/IEC 80000-1 standard.

An oscilloscope, like any other electronic system, is the sum of its parts. We design the instruments so as to maximize measurement accuracy, and specify the components built into them to deliver that accuracy. These components have themselves been characterized by their respective manufacturers, and their characterization process has some amount of error associated with it. Thus, the overall accuracy of our instruments incorporates that error as well.

Bear in mind that an instrument can be very precise and not accurate at all – or very accurate but not very precise. “Precise” means the measurement has many significant digits. “Accurate” means the digits are the correct ones.

It’s important to note that the objective of metrology is not to ensure that a given instrument is tweaked for perfect accuracy, but rather at determining how accurate the instrument actually is as measured against NIST or ISO/IEC standards. The metrologist documents that comparison so that the instrument’s end user knows exactly what he or she is getting. Unless specified otherwise, measurements are reported quoting an error of two “sigma”, which indicates a >95% chance that estimated errors are not exceeded. Another rule of good metrology is that the instrument is measured, when possible, against a standard that is 4X more accurate than the unit under test.

The Deeper Dive of Metrology

Of course, all Teledyne LeCroy oscilloscopes are subjected to quality control inspections. But metrology takes a deeper dive into how the parts come together to form the whole. It tells the customer, and our own staff, how close each instrument is to ever-elusive perfection.

All of our oscilloscopes come with a calibration certificate, and that calibration must be traceable to the definitions of quantities emanating from one or more international standard. The errors found through metrology reduce the margin of effort you can afford in your system. When the certificate states that an instrument is precise in measuring a given quantity to within ±1%, it typically means that we must measure the quantity to a fraction of 1% error. Thus, the instrument’s specifications are demonstrably met.

Calibration certificates spell out what equipment was used in the calibration of the instrument, and what the results of the testing showed. For example, a datasheet might show that a particular oscilloscope model is specified for DC measurement accuracy to within 1% ± some number of millivolts. The calibration certificate might show that a specific instrument is accurate to within 0.75% ± some smaller number of millivolts. It tells you the maximum amount of uncertainty in the instrument’s measurements.

To the oscilloscope’s owner, the calibration certificate represents a certificate of trust. It tells them how close the instrument is to the center of the range called for in the specification. It’s a matter of confidence in the owner’s measurements over time.

Metrology gives you a firm handle on your measurement margin for error. But with the advent of high-speed communications standards such as PCIe, that margin of error has been squeezed such that it becomes very difficult to ensure that everything works as it should. As a result, all oscilloscope makers have been pushed for greater measurement accuracy across ever-growing instrument bandwidths.

When instruments arrive at Teledyne LeCroy’s service department for calibration, some owners want the oscilloscope returned exactly as it was. They want only to learn how aging has affected the instrument. Did its measurement accuracy change, and if so, by how much? Other customers want their instrument restored to like-new condition, complete with software updates.

With the advent of high-speed communications standards such as PCIe, the margin for measurement error is minimal, making it extremely difficult to make sure everything is working correctly. Thus, the oscilloscope industry strives for more precision across the instruments’ wide frequency range. It could almost be said that Teledyne LeCroy’s LabMaster 10 Zi-A oscilloscope is akin to a 100-GHz voltmeter. There is an overriding need for high precision at every frequency. Thus, metrology will remain an integral part of the engineering effort for all instrument manufacturers.

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