Supplier eBooks

| 56 require sending test communication signals between a remote test unit or connecting directly to the wireless communication system and energizing RF circuits and interconnects to perform the measurements. Precision RF measurements, which help reduce uncertainty within a measurement to capture meaningful physical insights about a test target, often include a calibration process to further minimize uncertainties associated with the test apparatus and ambient environmental factors. To help improve test system repeatability, regulatory and standards agencies release specifications for calibration standards, among other standards for test apparatuses. For example, such a standard may include specifications for the exact EMC environment within which a test must be conducted. The two most relevant types of RF measurements are free- space testing, which refers to either radiated or near-field RF emissions, and conducted RF signal testing, which refers to RF emissions along a cable or power line (sometimes called "conducted" measurements). In free-space testing, antennas act as transducers that convert electrical signals into electromagnetic waves. Conducted testing uses components like calibrated loads, which are designed to absorb all signal energy and act as a precise reference for measurements. Basics of RF Free Space When conducted RF signals are transduced to radiated or coupled fields, the passing, reflected, absorbed, or coupled fields can be used to discern behaviors of RF materials and systems. Free-space measurements involve capturing the electromagnetic fields emitted by an RF system or exposing a DUT to externally generated fields. These tests are considered free space when there is no direct conductive contact and when the medium between the DUT and the test equipment is either air or a vacuum, rather than a physical transmission line or a non-air dielectric. Some free-space testing, such as spectrum monitoring, uses equipment to capture ambient signal energy generated by natural and human-made signal sources. However, this free-space energy could also be intentionally focused to reflect or pass through a material (Figure 2), providing insights into the material's electromagnetic behavior, such as with microwave or millimeter-wave (mmWave) free- space dielectric characterization. Some non-contact types of RF measurements, such as waveguide or open-ended transmission line probes, use the coupled fields generated by proximity to the exposed probe end and the DUT, material, or test system to determine the behavior of the test target. In free-space testing, it's important to know which region of the field—near-field or far-field—the test apparatus is operating in and whether it's capturing or inducing signals. The way test apparatuses, such as test antennas and probes, interact with another system depends on the field dynamics, which could be in the reactive near-field, radiative near-field, far-field, or transition zones. Various models exist for processing information captured from test apparatuses in the Figure 2: A diagram of the test system setup for a vector free-space reflectometry test. (Source: NASA)

• Cover