November 10, 2021
In simple terms, calibration is the verification of an instrument against a standard. As test equipment becomes more complicated, successfully identifying and applying best practices can reduce expenses and improve organizational capabilities. Calibration has always been critical in test equipment but has become more essential with emerging wireless designs extending into higher frequencies using more complex schemes.
Regular calibration is necessary because it maintains measurement accuracy and increases confidence in product performance. Waiting too long between calibrations or not having complete and proper calibrations performed will lead to out-of-tolerance (OOT) instruments that produce poor results, and, potentially, dire consequences.
Why Calibrate Test Equipment?
Instrument performance is calibrated by the manufacturer when it is shipped. In any test system, however, instruments are exposed to a certain level of degradation based on how they are used and the aging of the instrument. Proper calibration needs to be conducted to assure the instrument is performing according to published specification.
For this reason, it is recommended that calibrations be done per manufacturer specifications, which is typically 12 months. Doing so prevents measurement drift and degraded performance, unless another calibration cycle is recommended by the manufacturer. While an annual service is a good rule of thumb, the type of instrument also should be considered. For example, handheld instruments, especially those that support lower frequencies, can be extended a few months past a year. Conversely, vector network analyzers (VNAs) and other high-frequency benchtop instruments should strictly adhere to the annual calibration.
It is recommended that instrumentation, such as VNAs, that have a calibration process still undergo annual calibration of the test setup. High-frequency VNAs and similar analyzers that are often used in the lab usually come with calibration kits from the manufacturer. These kits typically provide the passive components to perform a short-open-load-through (SOLT) calibration prior to measurements being conducted. Such calibrations do not equal calibrations performed by qualified test labs.
It should be noted that no measurement is ever 100% correct. There will always be an unknown, finite non-zero difference between a measured value and the corresponding reference value. This means that the user needs to make sure that the instruments are operating within the specified tolerance limits initiated by the factory.
Stable temperature and humidity in the test laboratory help create an accurate test environment but they are not enough. A regular traceable correction and calibration by comparing the instrument’s performance to a standard of known accuracy helps increase confidence in measurement results. Common standards used in a calibration are those published by the National Institute of Standards and Technology (NIST), Department of Defense (Mil-STD), or other government agency.
Failure to establish and implement a regular calibration schedule for test instruments can result in costly problems that can have long-term detrimental effects at various levels of the product life cycle. Identical issues can occur if a non-OEM calibration is conducted, as it is limited and increases the probability for corrections.
- Production – Conducting tests with uncalibrated instruments may create false passes or failures of devices under test (DUTs). There are two potential problems with this scenario. Inferior products can be shipped to customers who may soon become ex-customers due to the product performing out of specification. It can also result in a product recall or financial liability. Either way, your reputation will be tarnished.
False failures mean good products are subject to unnecessary rework or used as scrap. The result is poor yields and increased cost of manufacturing.
- R&D – In the engineering lab, inaccurate measurements can lead to incorrect conclusions about how an emerging design behaves. Unnecessary design turns can result, adding considerable expense to the project and delaying a product release in an ever-competitive market.
- Compliance – Many contractual or regulatory requirements stipulate regular calibration. If a product is non-compliant, repercussions can include fines incurred, business lost, and possible factory shutdown.
Types of Calibration Processes
It is always preferred to have the instrument manufacturer conduct the calibration. They are the most familiar with the equipment and are specially trained to conduct the calibration process on the specific instruments.
Third-party organizations perform calibrations, but they are not as thorough as those done by a manufacturer. In fact, in many instances, they will conduct only basic – aka standard – calibrations. It’s important for engineers to recognize the types of calibration and what they entail.
- Standard – The most common level of calibration provides verification that the model meets or exceeds all its published specifications. It includes execution of necessary adjustments as outlined in the operation and maintenance manual for that model, test data for out-of-tolerance parameters recorded before any necessary adjustments, and a certificate of calibration which attests to Z540 compliance, as well as a cal sticker.
- Premium – Similar to the standard calibration, this process verifies that the model meets or exceeds all of its published specifications and executes necessary adjustment procedures. Additionally, test data is taken before and after any necessary adjustments and the certificate of calibration attests to compliance with ANSI/NCSL Z540-1 and NIST traceability. Premium calibrations may include uncertainty calculations, depending on the generation of the equipment model.
Other steps taken in a premium calibration include providing a list of test equipment used in the verification process and the environmental conditions at the time of verification. A calibration sticker indicating date of calibration and recommended recalibration date is provided, as well.
- Accredited – Anritsu offers accredited calibrations that meet the requirements of the highest standards, such as ISO/IEC 17025 and ANSI/NCSL Z540-1/Z540-3 General requirements for the competence of testing and calibration laboratories. This process satisfies the specific calibration listed on the agreed scope of accreditation.
ISO/IEC 17025 accredited calibrations are required in many heavily regulated industries, such as conformance and certification labs, and military and government applications. An ISO/IEC 17025 calibration is a premium option that provides additional information about the quality of each measurement made during the calibration process by individually stating the uncertainty calculation of each test point.
Calibration is time well spent compared to countless hours lost from unnecessary rework on perfectly good products or shipping poorly performing products to a customer with the resulting loss of company credibility. Consider calibration time an investment in product confidence.
To learn more or schedule an instrument calibrations visit our Repair and Calibration page.