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The radio frequency (RF) spectrum is more chaotic than ever, with more channels and increasingly complex signals crowding a limited frequency spectrum. As new applications use wireless transmission and digital RF systems proliferate, engineers need better tools to help them find and interpret intricate RF signal behaviors and interactions.

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Fortunately, digital phosphor technology, traditionally used in advanced oscilloscopes, has been applied to the RF domain and can now be found in pre-eminent real-time spectrum analyzers (RTSAs). In enabling users to view “live RF” signals for the first time, digital phosphor technology provides unmatched insight into RF signal behavior. In fact, full-motion digital phosphor displays show signals and details that are completely missed by conventional spectrum analyzers and vector signal analyzers (VSAs), greatly accelerating the discovery and diagnosis of problems relating to time-variant RF signals.

The name “digital phosphor” derives from the phosphor coating on the inside of cathode ray tubes (CRTs) used as displays in televisions, computer monitors and older test equipment. When an electron beam excites the phosphor, it fluoresces, lighting up the path drawn by the stream of electrons. Although, raster-scan technologies, such as liquid crystal displays (LCDs), eventually replaced CRTs in many applications due to depth and power advantages, the combination of phosphor coatings and vector drawing in CRTs provided several benefits that are useful for modern test and measurement applications.

First, this combination offers persistence. Phosphor continues to glow even after the electron beam has passed. Generally, the fluorescence fades quickly enough that viewers don't perceive it lingering, but even a small amount of persistence will allow the human eye to detect events that would otherwise be too short to see.

Second, phosphor coatings and vector drawing deliver proportionality. The slower the electron beam passes through a point on the phosphor-coated screen, the brighter the resulting light. Brightness of a spot also increases as the beam hits it more frequently. Users intuitively know how to interpret this z-axis information: a bright section of the trace indicates a frequent event or slow beam motion, and a dim trace results from infrequent events or fast-moving beams.

Persistence and proportionality do not come naturally to instruments with LCDs (or even raster CRTs) and a digital signal path. Digital phosphor technology was developed so the analog benefits of a vector CRT could be achieved, and even improved upon, with digital oscilloscopes and now RTSAs. Digital enhancements such as intensity grading, selectable color schemes and statistical traces communicate more information in less time.