Vector Network Analyzer Calibration Options For Ensuring Measurement Accuracy In The Field
By Afsi Moaveni, Agilent Technologies
Installation, troubleshooting, and maintenance of RF and microwave systems and components requires measurement of their reflection and transmission characteristics within a variety of indoor and outdoor environments and conditions, some of which can be quite extreme. One day the measurements may need to be taken in an area with freezing temperatures, the next, at very high elevation, such as on top of an outdoor tower and mast installation. Even measurements taken in shipboard, aircraft, or vehicle applications can be problematic, as they may require the engineer or technician to operate in small, confined spaces. Typically, these measurements are made using a handheld vector network analyzer (VNA) having a user calibration that is performed in the field.
Calibration is necessary to remove the effects of cables and adapters placed between the instrument and the device under test (DUT) when measuring the S-parameters of components and systems. Selection of the appropriate calibration type is often a trade-off between measurement accuracy, speed, and complexity of the calibration process. Making the right decision is critical, since the VNA’s measurement accuracy is directly influenced by the type of user calibration selected. Luckily, recent advances in VNA calibration now present operators (engineers and technicians) with a number of compelling options.
Due to the extreme conditions under which engineers and technicians must often make their measurements in the field, measurement accuracy, calibration convenience, sweep speed, and analyzer portability must all be considered when selecting an infield measurement solution. When using a VNA, the measurement accuracy will be dependent on the user calibration selected, as well as the associated test configuration. The VNA hardware determines how the DUT is measured, either in the forward direction only using a transmission/reflection (T/R) hardware configuration, or in both the forward and reverse directions using a full two-port configuration.
In practice, these different test configurations may result in a variety of test cables and adapters being connected to the VNA. It is preferable to not have the effects of these cables and adapters included in the measurement. For that reason, a user calibration is performed to establish the calibration plane beyond the test cables and adapters. User calibration often requires a calibration kit (cal kit), which includes a set of high-quality coaxial or waveguide standards that are measured by the VNA during the calibration process.
Most VNAs offer a choice of user calibrations that can remove the effects of test cables and adapters in the test configuration, and correct for systematic errors. Systematic errors stem from a number of sources. They are created by the frequency response of the test cables, adapters, and analyzer components, and by the analyzer’s internal leakage paths. They can also be caused by multiple reflections between the DUT and the analyzer. These errors are characterized and mathematically removed from the measurements during the user calibration. Some types of user calibrations correct only a subset of the errors, while others offer a more extensive correction.
User calibrations are performed by the operator on a regular basis and are independent of the general instrument calibration that may be required on a yearly cycle. This yearly calibration is a traceable process, performed by a certified test lab, to confirm an instrument is operating according to its stated specifications. The calibration should be traceable to International System (SI) units through a national metrology institute such as NIST, NPL, or BIPM.
Currently, there are several types of user calibrations available. Regardless of which one is used, any test cables and adapters utilized must be phase-stable, high-quality items and maintained in very good condition. Doing so will improve the accuracy and repeatability in the measured data and also ensure the calibration is accurate and/or stable.
Mechanical calibration is a traditional VNA calibration requiring the use of a high-quality cal kit. This method is recommended when the highest level of measurement accuracy is desired. Mechanical calibration is performed using discrete calibration standards with well-defined characteristics, contained in a high-quality cal kit. Measurements of these standards enable the VNA to mathematically determine systematic errors in the test system. Some of the mechanical calibration types available are summarized in Figure 1.
Figure 1: This table lists the types of mechanical calibrations available on Agilent Technologies’ FieldFox VNA, as well as the relative accuracy and calibration speed for each type. Measurement speed relates to calibration speed as some measurements require only forward or reverse sweeps (e.g., enhanced response and normalization), while others require both forward and reverse sweeps (e.g., Full 2-Port and QSOLT).
Full two-port calibration is the most comprehensive mechanical calibration, removing most of the test system’s systematic errors. However, the process is complicated, requiring seven connections to various high-quality calibration standards (e.g., open, short, and load). Various techniques can be used to improve the accuracy and speed of this process. One such technique, QSOLT, speeds the calibration by reducing the number of steps when measuring cal kit standards, while still maintaining the high level of accuracy.
When lower measurement accuracy is acceptable and faster measurement time is desired, enhanced response mechanical calibration is a good choice. It places the calibration plane at the interface to the DUT, which results in higher measurement accuracy. Since the calibration and measurements only occur in one direction (either forward or reverse), the measurement times are faster. The downside is that by not having measurements in two directions, uncorrected systematic errors result that reduce the overall measurement accuracy.
Another mechanical calibration type — one-port OSL calibration — is used for highly accurate reflection-only measurements of one-port DUTs (e.g., an antenna, termination, or detector). Another option, response (normalization) calibration, is simple to perform but only corrects a minor subset of the systematic errors in the test system and therefore has the lowest accuracy. It is essentially a normalized measurement where a reference trace is stored in memory and subsequent measurement data is divided by this memory trace. This type of calibration is good for a quick check of the operation of a DUT and in cases where measurement accuracy is not a consideration.
Electronic calibration (ECal) is another calibration technique that provides excellent accuracy. Users can perform one- or two-port calibration using Agilent’s ECal modules. ECal calibrations are simple, fast, provide excellent accuracy, and are commonly used in benchtop applications. ECal modules are generally costlier than mechanical cal kits and are more susceptible to damage due to high power. As a consequence, they are generally used less in the field.
QuickCal is a user calibration technology that does not require a cal kit, making calibrating the instrument in the field a fairly simple and rapid process. Because it removes systematic errors in the test configuration, QuickCal enables easy S-parameter measurements.
QuickCal is a two-step process requiring a measurement sweep of the open-ended test cables or adapters and a measurement sweep of a THRU connection. The first step is to leave the end of any adapters and/or test cables open. The second step requires direct connection of the two test ports, which may or may not require an adapter for this THRU connection. This process is required for two-port DUT measurements (Figure 2a). One-port measurements require only one step, the open-ended sweep. Unlike mechanical calibrations, no calibration standards are required when performing a QuickCal.
Figure 2: Figure 2a shows the test configuration for the two-step process used to perform QuickCal on FieldFox, the only analyzer offering this technology. Figure 2b shows the DUT connection for measurements at the QuickCal calibration plane. If the DUT has an expected S11 or S22 less than –15 dB, an optional LOAD standard may be measured during QuickCal to improve DUT measurement accuracy. The LOAD standard should have the same connector type as the DUT and doesn’t need to be a cal kit load standard.
Once the QuickCal process is complete, the DUT can be measured without including the effects of any test cables and adapters. An example is shown in Figure 2b. Notice that as the calibration plane is moved to the DUT plane, the test cable and adapters are removed from the S-parameter measurements.
QuickCal can be performed using options for one- and two-port DUTs. A DUT with only one port requires a calibration and reflection measurement from just one port. The QuickCal process for one-port devices only requires the open measurement sweep, and possibly the optional LOAD for higher accuracy. QuickCal for two-port DUTs follows the same procedure in Figure 2a, but can be selected as a full two-port calibration or an enhanced response calibration. The full two-port option requires measurements in the forward and reverse direction and therefore provides the most accurate method of VNA calibration. However, when faster measurements are required, the enhanced response option is recommended. It only requires measurements in the forward or reverse direction and, as a result, has a much shorter measurement sweep time.
Comparing Mechanical Calibration And QuickCal
To better illustrate the tradeoffs operators make when selecting calibration type, consider the image in Figure 3, which compares the measured insertion loss (S21) of a short coaxial cable using two different calibration types available on a FieldFox VNA. The yellow trace represents the cable measurement using QuickCal calibration, while the blue trace is the same cable measurement using a traditional full two-port mechanical calibration. The full two-port calibration process requires the engineer to make seven connections to different calibration standards. In contrast, QuickCal was performed without calibration standards. Despite the fact that QuickCal is a much simpler calibration process, there is very little difference between the two measurement results.
Figure 3: Measurement comparison of a coaxial cable using a traditional full two-port calibration (blue) and the rapid QuickCal (yellow) available on a FieldFox VNA
Field measurement of the reflection and transmission characteristics of RF and microwave systems and components can be a tricky proposition, one that’s complicated by extreme conditions and the need to select the most appropriate calibration type for the VNA. With the VNA’s measurement accuracy directly related to the type of user calibration selected, it’s critical that the engineer or technician make the right choice. When the highest level of measurement accuracy is required, a traditional mechanical calibration or electronic calibration can be used. For a much faster and simpler calibration, QuickCal can be used. QuickCal offers reasonable measurement accuracy with significantly less complexity, making it a much more convenient calibration for today’s field operations.
For more information, download Agilent’s application note Techniques for Precise Measurement Calibration Techniques in the Field at www.agilent.com/find/fieldfoxapps.
About The Author
Afsi Moaveni is a senior applications engineer working in Agilent Technologies’ Component Test Division in Santa Rosa, California. Afsi has worked at Agilent for eighteen years, as a microelectronics manufacturing engineer, product marketing and applications engineer. She holds a BSEE from the University of Colorado and a master’s in MS&E from Stanford University.