Performing component verification in the field using a rugged vector network analyzer
To address this need, Agilent’s FieldFox family of RF and microwave analyzers makes it possible to make precise measurements under non-ideal conditions: in a hangar during a snowstorm, aboard a ship sailing rough seas, or at a satellite trailer in a sandstorm (Figure 1).
Figure 1. The handheld FieldFox microwave vector network analyzer enables favorable tradeoffs between speed, accuracy and calibration in the field.
When working in the field, reality often dictates the need for a balance between precision and practicality. In the case of vector network analysis, the key tradeoffs are between speed, accuracy and the complexity of the calibration process. This article will focus on analyzer configurations and calibration methods that support high quality vector network measurements in a short amount of time.
Accounting for the range of potential problems
Any scenario that involves accurate device characterization usually conjures up images of a network analyzer and one or more calibration kits on a lab bench. Today, more and more engineers and technicians need to make accurate measurements in remote locations and harsh environments—and they want to do so with less equipment and in less time.
Extreme conditions make it difficult to perform essential measurements and get accurate results. Indoor tests may be in limited space, perhaps near other operating equipment. Examples include vehicles, aircraft, ships and submarines. Hazards to the equipment-under-test include heat, stress, vibration and mishandling. Also, oils and other contaminants may leak into the components.
Outdoor measurements may be performed in extreme temperatures, heavy precipitation and strong winds, and perhaps with hostile combatants nearby. These conditions have the potential to damage cables, antennas, filters and other system elements.
Severe conditions make it difficult to produce accurate, repeatable measurements of characteristics such as voltage standing wave ratio (VSWR), return loss, insertion loss, isolation, distance-to-fault (DTF), S-parameters and cable loss. What’s more, the possible physical configurations and distances will dictate the practicalities of making one- or two-port measurements.
The need to carry a minimum amount of equipment into the field has implications for calibration. More external accessories mean more a larger, heavier field kit. Complex calibration setups mean spending more time in nasty conditions, and can lead to user errors in the cal process.
Getting accurate, repeatable results in the field
A FieldFox analyzer can be configured for cable-and-antenna test (CAT), vector network analysis and spectrum analysis—individually or in combination—with maximum frequency coverage that ranges from 4 GHz to 26.5 GHz. To survive field use, the analyzers are durable enough to comply with MIL-PRF-28800F, which includes water resistance and a dust-free design, and type tested under IP53 (dust and rain). FieldFox has also been type-tested for operation in explosive environments according to MIL-STD-810G, Method 511.5 Procedure 1. As an added element of user safety and convenience, the analyzers can be controlled and monitored remotely through an iOS app.
Configuring for the required results
The minimalist configuration for field use is the CAT analyzer. With these capabilities, the instrument measures magnitude-only S-parameters and reports values such as return loss, VSWR and insertion loss. These are useful when characterizing antenna components and transmission line systems including cables and waveguides.
As a VNA, FieldFox can be configured for either transmission-and-reflection measurements (forward only) or full two-port measurements (forward and reverse). In either case, the analyzer can measure the full set of complex-valued (magnitude and phase) S-parameters.
In a transmission/reflection (T/R) configuration, the DUT must be disconnected and reconnected to make measurements in the reverse direction. Because the full two-port configuration includes an internal switch matrix, the device-under-test (DUT) need be connected only once. A full two-port unit provides the highest level of measurement accuracy because it allows the characterization and removal of all systematic measurement errors.
Utilizing built-in calibration capabilities
FieldFox offers several types of calibration, ranging from simple to advanced. The simplest types are CalReady and QuickCal. Neither requires an external cal kit, and this reduces the amount of equipment to be carried into the field. It also means fewer steps and therefore shorter total measurement times.
With CalReady, the instrument is already calibrated at power-on (or after a preset) and is ready to go without any additional steps. It applies full two-port error correction at the test ports.
QuickCal enables the user to perform calibration without additional accessories. Because the cal plane is extended out to the actual connection to the DUT, this method can compensate for test cables and adapters placed between the instrument and the DUT.
In field applications, QuickCal coupled with a full two-port configuration provides the best balance between precision and practically. It also offers the optimum blend of speed, accuracy and cal simplicity.
Figure 2 compares two measurements of forward transmission, S21, made on a short coaxial cable using QuickCal and a traditional full two-port mechanical calibration. As shown, there is very little difference between the two traces, and this suggests the value of QuickCal as an alternative to mechanical calibration when making measurements in the field.
Figure 2. By extending the measurement plane to the DUT, QuickCal can produce results that are comparable to those from a full two-port calibration.
If even greater precision is needed, FieldFox also supports advanced calibration methods: one-port open-short-load (OSL), full two-port short-open-load-through (SOLT), quick SOLT (QSOLT) and enhanced response. Although these offer the highest level of accuracy, they do require the user to carry high-quality cal kits into the field. In terms of tradeoffs, greater accuracy comes at the expense of cal complexity and set-up time. Also, the cal kits include coaxial and waveguide standards that must be kept clean and protected from damage.
Carrying precision into the field
With the configurations and capabilities outlined above, it is possible to carry precision into the field and perform accurate, repeatable vector network analysis. Making this a reality requires a handheld instrument that is on the leading edge of today’s technologies—electrical and mechanical.
Making it economical comes from configurability: the ability to carry multiple instruments in a compact, durable package provides a meaningful cost advantage. For example, the initial purchase can be for a unit that provides many useful capabilities while fitting into existing budget constraints. Additional capabilities can be added later as budgets allow.
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