Calibration standards: Introduction

If you are into amateur electronics, you will have experienced a requirement for reliable calibration sources to calibrate your instruments against.

In a professional lab environment, instruments are being shipped to a calibration laboratory in defined intervals where their accuracy is validated to be within planned limits and deviations are documented. The reference source for such a calibration is in turn compared against a central standards institutions source so that any calibration can be traced back to this central source.

Unfortunately, conducting such a calibration is often far beyond the reach of amateur means and often exceeds the instrument value (which is probably why the same is then written off).

In the past I was fairly surprised to find out that my (simple) instruments are deviating significantly from each other and I had no reliable calibration sources to validate them against so I just arbitrarily defined one of my meters as the "master" against which all other instruments are being calibrated.

Over time I had the desire to achieve a better understanding and quantification of accuracy and precision of my instruments so I took the time to build some hardware to help me with that task.

The intention of this small series of articles is to present simple means for calibration and help with judging whether or not an instrument needs to be adjusted. If you, like me, own mostly old instruments pulled from work environments, you will be surprised about your findings during the calibration process.

Accuracy, Resolution and Precision

It is critical to understand the difference between Accuracy, Resolution and Precision in measurement instruments. As usual, an excellent article can be found in Wikipedia.

In short, Precision quantifies the instruments ability to produce exactly the same measurement within over and over again for a given input condition. Resolution defines the "fineness" of numerical representation of the result.  Accuracy on the other hand quantifies a deviation of the measurement from "reality" or, for better definition, a given reference value. All three metrics are independent on an instrument and may vary over time.

Both accuracy and precision can be different in their characteristics, based on the type of instrument being looked at. For a voltage measurement, an accuracy error for example can exist dependent (like a percentage) and independent (like an offset) of the measured input condition.

For a frequency measurement, accuracy is potentially unreliable, depending on the quality of the time base and it's stabilization but precision can be expected to be fairly good once the time base has settled. 

Understanding these instrument specifics can greatly help in getting a good feel for interpreting a measured value. 

DC Voltage

Probably the most basic calibration standard should be a reliable DC voltage source.

Continue here...

Resistance

The second most useful (and probably cheapest) standard to calibrate against is the resistance standard.

Continue here...

AC voltage

A simple peak detector allows for a fairly precise measurement of sine peak voltages which can be converted to rms (effective) voltages for calibration purposes.

Continue here.... 

Frequency

Different methods exist to find cheap and accurate sources for precision frequencies to calibrate instruments like generators, counters and scopes but also radio transceivers against.

Continue here...

RF level

This is one of the more complicated calibration standards and it would normally not be required in a regular hobby workshop. If you enjoy building radios though, it is an elegant way of providing a reference signal to calibrate a RF milliwatt meter or an oscilloscope against.

Continue here...