Calibrate VNAs over the Internet

A new service from NPL eliminates the need to transport your verification devices to a national measurement institute.

Nick Ridler, National Physical Laboratory, UK -- Test & Measurement World, 6/1/2002

Early in 2001, the UK's National Physical Laboratory (NPL) launched an Internet calibration service for microwave vector network analyzers (VNAs). Key to the success of this service is that traceability of the artifact standard you need for calibrating these analyzers depends upon physical rather than electrical parameters. As a result, the task of maintaining calibration artifacts at a remote site becomes much easier. Equally, because uncertainties in these artifacts depend only upon length measurements, your artifacts can have uncertainties comparable to the best standards held by national standards authorities themselves.

Having overcome the need for travelling electrical calibration artifacts, you are still left with the task of calibrating your VNAs. These instruments use electronic methods for calibration. In practice, a calibration involves measuring and recording readings for the artifact standard and instructing the analyzer to "correct" its readings to the actual values assigned to the artifact—processes that can be performed over the Internet. This has led to the establishment of NPL's Internet Primary IMpedance Measurement System (iPIMMS; www.internetcalibrations.com), which provides the UK primary national standard for electromagnetic impedance quantities at RF and microwave frequencies (Ref. 1).

Figure 1 shows the bare essentials of the system. At your remote site, you have the VNA that needs to be calibrated, the calibration artifacts, a PC, and a skilled calibration engineer. At NPL sits a server PC with the iPIMMS calibration firmware. During the process, the engineer receives instructions from the remote PC and executes a response to each one. While there may appear to be an opportunity to automate the whole exercise, the engineer actually needs a good level of skill, particularly as connecting and disconnecting cables and artifacts involves using torque spanners. Typically, you need around 30 min for the complete calibration of a VNA, including evaluation of the uncertainties in the measurements. During this time, iPIMMS automatically processes and archives results and sends them to the remote site for producing a calibration certificate. To date, iPIMMS has interfaced successfully with several models of microwave VNA from Agilent Technologies and Anritsu.

Figure 1. a) Internet calibration of VNAs shortens the conventional traceability chain by b) eliminating the need to transport electrical calibration data.

Without iPIMMS, VNA users that require traceability must transport their electrical verification devices—attenuators, matched and mismatched transmission lines—to a national measurement institute. These devices exist as verification kits, one for each type of connector that the analyzer requires. The institute evaluates the verification devices electrically, in terms of reflection and transmission coefficients, and issues a calibration certificate. Users then calibrate their own analyzers using in-house standards such as short-circuits, open-circuits, matched loads, and air lines, which they usually assume to be perfect. Users follow up with measurements of the certified verification devices and verify results by comparison with values on the calibration certificate. Agreement between the two data sets indicates the validity of the measurement uncertainty quoted by the laboratory—the laboratory having previously evaluated the uncertainty of measurement for their own system.

Because analyzers make measurements over a range of frequencies and nominal values, you need to verify the analyzer's performance under similar wide-ranging conditions. To do this, calibration certificates typically contain results at several hundred different frequencies for each of four electrical verification devices (for each connector type) with different characteristics—such as high and low reflects and different values of attenuation. The electrical behavior of these verification devices at RF and microwave frequencies is subject to drift with time, so recalibration of each device is recommended at intervals of 12 months.

iPIMMS removes the need for clients to transport all their electrical verification kits to a national standards authority for calibration. Instead, NPL now directly measures the standard—previously assumed perfect—instead of the verification kits that a client laboratory used to calibrate its analyzers. Under normal circumstances, iPIMMS needs only one or two standards, depending on the frequency range, to achieve a full calibration. For example, one standard covers 1 GHz to 18 GHz.

These standards are lengths of precision transmission lines, such as coaxial lines or rectangular waveguides. NPL makes purely dimensional measurements on these standards to directly assess the overall quality of the standard. This enables you to trace back the uncertainty of measurements directly to dimensional measurements (and therefore, the SI base unit, the meter), avoiding the need for the electrical calibrations of the verification devices. This arrangement reduces the traceability chain between the electrical measurements and the SI base units to the minimum (that is, a chain with just one link). This process prevents the usual broadening of uncertainty intervals at each transfer measurement along a traceability chain, from national standard to end user—the uncertainty level the client laboratory achieves being the same as at NPL.

In the case of standards in coaxial lines, NPL makes dimensional measurements of the diameters of the inner and outer conductors of the line using an air-gaging measurement system. You can use these values to establish the characteristic impedance, Z ₀, of the line using the following expression:

where:
a is the diameter of the inner conductor
b is the diameter of the outer conductor.

In the case of rectangular waveguide transmission lines, the dimensions of the waveguide apertures establish the quality of the line using other expressions (Ref. 2).

iPIMMS stores the data for each client's line and recalls it when that particular client logs on to the service. The system can then use that data as part of the calibration process and establish very tight uncertainty intervals based on the client's own primary reference standards. The iPIMMS calculation routines evaluate the uncertainty in the client's measurements using ISO recommendations (Ref. 3).

Figure 2. Typical iPIMMS screen shots (a) inform you of anomalies in the data and (b) show all results, with uncertainties, for all devices.

Establishing fast, two-way communication between the NPL server and a remote PC, maintaining data security, and overcoming firewalls all present significant challenges in setting up iPIMMS. The system uses Java, ActiveX, and Visual Basic scripts as well as the Secure Sockets protocol to maintain bidirectional data integrity. The system constrains data to standard Web browser communication ports to reduce a client's liaison with service providers and computer services. If you can currently browse the Internet and complete Web-forms, then you can easily access iPIMMS. Like any other data-acquisition system, iPIMMS allows you to catalog and view data files, recalculate uncertainties, or suspend or continue measurement runs. NPL's server stores and backs up data on a regular basis, but you can also download data for use in your own records or calibration certificates.

Figure 2 shows typical iPIMMS screen shots. Two post-processing screens called iDataScan and iRho allow you to assess the quality of the measurement data and the standard used during the calibration. Currently, the iPIMMS database supports measurements in type-N, APC-7, 3.5-mm coaxial line, and rectangular waveguides.

A principal goal for further development of iPIMMS is to establish full accreditation for the system. The key consideration is the distributed nature of iPIMMS, being split between NPL and remote sites. For a start, the UK's accreditation body, UKAS, is likely to conduct a full assessment of the software and data interfaces provided by the system's server at NPL. Later, UKAS will need to concentrate on the user's end of the system. Because iPIMMS is accessible internationally, more widespread involvement is likely with other national accreditation bodies. iPIMMS provides its subscribers with a common set of centrally controlled procedure documents, which should be useful in achieving accreditation for the system and the system's clients.

Note: A version of this article previously appeared in Test & Measurement Europe.