Design Con 2015

Selecting a handheld RF analyzer for tough conditions

Wilkie Yu, Agilent Technologies -November 12, 2012

Today, accurate RF and microwave measurements are becoming increasingly necessary in harsh and hard-to-reach environments. These scenarios span a wide range of conditions: day or night, rain or shine, hot or cold; aboard a ship, in an aircraft, or in a vehicle.

In these situations, a handheld instrument must be capable of making the required measurements with sufficient levels of performance and accuracy. Those attributes are important; however, given the expected operating conditions, physical and environmental specifications are equally important to the instrument and the user (see Figure 1). This article presents general suggestions and specific examples regarding the essential attributes of handheld analyzers that will be used in harsh conditions.


Figure 1: The ideal RF or microwave handheld analyzer must be capable of making accurate measurements in tough working conditions.

Establishing a reference for “ruggedness”
United States military specification US MIL-PRF-28800F provides a set of benchmarks for test equipment that will be used in the testing and calibrating of electrical and electronic equipment. The spec includes four classes that range from the “extremes of world climatic variation” (Class 1) to controlled, protected operational environments (Class 4).

The 88-page specification is quite detailed, and section 3.8 uses 13 major categories to define the environmental requirements. Six of these are especially important in handheld analyzers:

•    Environmental conditions (3.8.1)
•    Temperature and humidity (3.8.2)
•    Altitude (3.8.3)
•    Vibration (3.8.4)
•    Mechanical shock (3.8.5)
•    Water resistance (3.8.6)

Within the bounds of these specifications, an instrument that satisfies Class 2 requirements will be capable of operating in rugged operational environments that include unprotected, uncontrolled climatic conditions. These are high hurdles for handheld RF and microwave instruments.

Another important consideration for handheld analyzers is operation in potentially explosive conditions. One of the key benchmarks is MIL-STD-810G, which covers the environmental considerations a device may experience in its service life and provides relevant laboratory tests. Within the standard, Test Method 511.5 deals with operation in an explosive atmosphere. Testing typically includes operation of the unit-under-test (UUT) in a chamber filled with explosive gases that may ignite if the UUT produces a spark. Passing this test is especially important for handheld instruments that may be used near the flammable fuels or gases present around aircraft, vehicles, mining operations, and so on.

Drawing a line in the sand
As demonstrated by currently available RF and microwave instruments, there are two ways to create a handheld unit: one is to repackage a conventional benchtop analyzer, and the other is to create an all-new instrument designed with field use firmly in mind.

Using MIL-PRF-28800F as a yardstick, many of today’s “repackaged” instruments fall short in several key areas: temperature and humidity, vibration, mechanical shock, water resistance and dust exposure. Most stumble due to poor choices in areas such as component selection and package design. For example, components designed to work in an AC-powered device tend to be power-hungry, which has two undesirable consequences. One is shorter battery life, which is a significant shortcoming in the absence of AC outlets or spare batteries. The other is heat: these components often need fan-based cooling, and this requires vents to provide airflow through the instrument enclosure.

With these attributes, a typical repackaged design will have a hard time operating in tough—but typical—conditions: rain, dust, humidity, fluctuating temperatures, and so on. As a worst-case example, imagine a day in the desert, working on satellite ground stations with a fan-cooled handheld. Sandstorms are common and the instrument is likely to ingest significant amounts of foreign matter, which can lead to overheating more quickly than when operating in ideal conditions.

Designing from the ground up
The alternative to a repurposed instrument is a purpose-built device. In the ideal case, industrial and electronic designers would have the freedom to start with a clean slate. Design choices would be shaped by the need to provide high quality RF and microwave measurements in difficult conditions. To reduce the amount of equipment to be carried into the field, the overall design would be flexible enough to provide a wealth of capabilities in a compact package: cable and antenna analysis, vector network analysis, spectrum analysis, power measurement, interference analysis, and vector voltage measurement (see Figure 2).

Figure 2: This handheld analyzer was purpose-built to meet the needs of personnel who need to make a variety of precise, on-the-go measurements.

Creating a field-worthy industrial design
To get a firsthand perspective, an Agilent design team left the office behind and tagged along with technicians and engineers in the field as the field personnel performed routine maintenance, in-depth troubleshooting, and everything in between.

The team traveled to a variety of worksites in vans, pickups and trucks. The “good” instruments usually rode up front with the crew; the other gear was often tossed—sometimes literally—into the cargo area of a van or the bed of a pickup. In all cases, the people, vehicles and instruments had to be equipped for a wide range of conditions, day or night, rain or shine.

Those experiences translated into attributes that make a handheld analyzer ready for the toughest conditions. For example, a completely sealed enclosure that is compliant with US MIL-PRF-28800F Class 2 requirements will ensure durability in harsh environments. Consistent with the spec, a water-resistant chassis, keypad and case let the instrument withstand salty, humid environments. Gasket-sealed doors will protect instrument interfaces from moisture, and a dust-free design—with no vents or fans in the case—will help extend instrument availability and reliability.

The package should also withstand shock and vibration, and the connector bay should be designed to protect the RF connectors from damage due to drops or other external impacts. Two additional attributes will help a unit survive drops onto all six faces. One is a case with a curved bottom and rounded corners: These disperse impact and increase structural resistance to shock and impact from all angles. Another useful attribute is the polymer blend used in the case: In addition to its innate durability, the polymer can be formulated to resist shattering at the lowest temperature of the desired operating range.

>>Defining 'field ready'
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