Specifying Accredited Calibration Services: A Review of Electrical Test Equipment

It is not unusual for electromagnetic compatibility (EMC) testing laboratories to experience complications in obtaining appropriate calibration certificates due to issues related to such things as purchasing procedures, specifications, and review of calibration services upon completion. This article highlights topics for consideration when obtaining an accredited calibration service. Also included is a step-by-step tutorial on how to obtain an accredited calibration for a log-periodic dipole array (LPDA) antenna.

Obtaining an Accredited Calibration Service

Before purchasing a calibration service, there are key details that need to be taken into account to ensure that an acceptable calibration service is received and that the test laboratory is also meeting the requirements of ISO/IEC 17025. The first step is to identify an appropriate calibration service provider. In accordance with ISO/IEC 17025 Section 4.6.1, a testing laboratory shall have a policy and procedure for the selection of calibration service providers. When searching for a calibration laboratory, the following factors should be considered:

1. First, the calibration laboratory must be accredited to ISO/IEC 17025 by an accreditation body that is a signatory to the International Laboratory Accreditation Cooperation (ILAC) Mutual Recognition Arrangement (MRA) in order to ensure appropriate traceability of the measurements made.
2. Additionally, the calibration laboratory’s scope of accreditation must include the specific parameters that need to be calibrated, and the ranges within which it needs to be calibrated. When physically large equipment is being calibrated, the location of the calibration laboratory can be an important factor to keep in mind as well. When equipment is too expensive to ship, calibration laboratories will typically send field technicians to the testing laboratory in order to provide a more cost-effective calibration service. Such on-site calibration services, performed by an accredited calibration laboratory, must also be covered within the terms of their scope of accreditation.
3. Finally, ensure that the scope of accreditation is current and not expired, which can usually be verified on the relevant accreditation body’s website.

Once a capable calibration laboratory has been selected, the testing laboratory must complete several additional steps before submitting a purchase order. Per ISO/IEC 17025 Section 4.6.4, all calibration service suppliers must be evaluated by the testing laboratory prior to procurement of such services. The rigor of the evaluation is left to the testing laboratory’s discretion, but could include a combination of the following:

• Review of the calibration laboratory’s accreditation status and scope of accreditation;
• Review of any previous services supplied by the calibration laboratory; and
• Proximity to the testing laboratory.

Once a calibration laboratory is found to be acceptable, it can be added to the testing laboratory’s approved supplier list as required by Section 4.6.4 of ISO/IEC 17025.

After selection of a suitable calibration laboratory, a purchase order is written for the equipment to be calibrated. The following should be considered:

• A purchase order must include information describing the requested services, and this information must be reviewed and approved for technical content per Section 4.6.3 of ISO/IEC 17025.
• When writing the purchase order, it is important to be specific to ensure that the proper calibration is performed. Items to consider for inclusion are specifications, drawings, range of calibration, decision on whether to include the calibration interval on the calibration certificate, any special inspection instructions, an endorsement of
  ISO/IEC 17025 accreditation by the calibration lab’s specific accreditation body, and any specific calibration methods that are required by the testing standard or regulatory bodies.

Once the calibration has been completed and the calibration certificate has been received, the testing laboratory should look for specific details to ensure that the requested calibration service and necessary documentation have been received.

First, there should be verification that the calibration certificate includes all of the information that was requested in the purchase order. The most important items to look for are an endorsement from the accreditation body, the calibration method and/or standards used, and measurement uncertainty. It is also important to check the equipment’s functionality, calibration status and calibration label (if affixed by the calibration service provider) prior to its return to service as required by section 5.5.9 of ISO/IEC 17025.

Obtaining Calibration of a Log-Periodic Dipole Array (LPDA) Antenna

As an example to illustrate the points made above, the following outlines the steps toward obtaining an accredited calibration for a LPDA antenna, used to perform radiated emissions measurements in the frequency range 30 MHz to 1 GHz and above, in accordance with ANSI C63.4-2014. The usage of antennas in general is described in Table 1 of ANSI C63.4-2014 for exploratory measurements and in Table 2 for compliance (final) measurements. If antennas are to be used for test site validation measurements (e.g., normalized site attenuation measurements) Table 3 of ANSI C63.4-2014 is to be consulted and the specific calibration requirements are to be observed.

Step 1

Since the testing standard ANSI C63.4-2014 requires antennas to be calibrated to ANSI C63.5 (the current revision is ANSI C63.5-2006) a calibration service provider is to be selected that is accredited to perform antenna calibrations in accordance with ANSI C63.5-2006 in the operating frequency range of the antenna. Typically, accreditation bodies will provide a list of accredited calibration service providers for the public’s reference. Be sure to check the calibration laboratory’s scope of accreditation to ensure it is accredited to calibrate LPDA antennas in the range of 30-1000 MHz (or higher, as required by the operating frequency range of the antenna) in accordance with ANSI C63.5-2006 and to ensure that the scope is current.

Step 2

Once an appropriate, accredited calibration service provider has been identified, it should be added to the test laboratory’s approved vendor listing, if not already included. Record of the test laboratory’s evaluation of the accredited calibration supplier must be kept for an appropriate time period.

Step 3

A purchase order should be prepared, documenting the following technical aspects:

• The frequency range for calibration of the antenna (e.g., 30 MHz to 1 GHz);
• The calibration standard with the proper revision (i.e., ANSI C63.5-2006);
• The actual antenna calibration method
  (e.g., Reference Antenna method or
  Standard Site method);
• Frequency points for antenna calibration
  (if deviating from standard minimum requirement); and
• Any additional parameters like antenna VSWR or cross-polarization performance.

Step 4

When the LPDA antenna is received back from the calibration service provider, an appropriate inspection must be performed. First, the calibration certificate should be reviewed to ensure that the following information has been included:

• Statement of calibration standard (including calibration method) and measurement method;
• Measurement uncertainty;
• Antenna factors versus frequency in tabular format (or in electronic format);
• Inclusion of any special parameter requested (e.g., VSWR or cross polarization performance); and
• Endorsement of certificate with accreditation body symbol.

It is good practice to compare antenna factors against the factors from previous calibrations by recording them in a spreadsheet. That way, trends in antenna factors or significant changes can be easily determined. It is important to determine the root cause for any significant changes observed in the antenna factors in order to avoid erroneous future measurements.

This article was written for the American Association for Laboratory Accreditation (A2LA) by Jordan Acton and Adam Philp, A2LA Accreditation Officers,
with technical input from Werner Schaefer. They may be reached at jacton@A2LA.org, aphilp@A2LA.org and wernerschaefer@comcast.net.