Analog oscilloscopes rely on cathode ray tubes (CRTs) to visualize waveforms. The CRT's bandwidth matches that of the oscilloscope itself, meaning the electron beam's speed is directly related to the signal frequency. As the frequency increases, the electrons move faster, and the screen brightness changes accordingly. However, this relationship is inverse: lower frequency signals result in a brighter display, while higher frequencies appear dimmer. This variation in brightness or grayscale provides a third dimension of information, allowing users to observe how signal amplitude changes over time. For instance, the vertical axis shows amplitude, the horizontal axis represents time, and the screen's brightness reflects the signal's intensity at any given moment. This time-dependent phosphor persistence effect is especially useful for analyzing complex or intermittent waveforms. Analog Storage oscilloscopes are known for this feature, with the best models offering up to 800MHz bandwidth and the ability to capture fast transient events as short as 1ns.
In contrast, digital oscilloscopes do not support continuous phosphor-like persistence because they process signals digitally. They typically show waveforms in two states—high or low—making it difficult to replicate the smooth brightness transitions found in analog displays. To achieve multi-level brightness, special image processing chips are required. For example, Tektronix uses DPX processors, which integrate data acquisition, image processing, and storage into a single chip. These chips, made using a 0.65μm CMOS process, contain 1.3 million transistors and use a parallel architecture to handle high sampling rates. They function both as data acquisition units and raster scanners, simulating the phosphor's lighting properties. These processors support 16 levels of brightness and can display waveforms on a 500x200 pixel LCD screen, updating every 30 seconds. Unlike analog storage oscilloscopes, which relied on photographic film and were inconvenient for data storage, digital phosphor oscilloscopes offer much easier data handling. For example, they can highlight the most frequently occurring waveforms in red and less common ones in blue, making patterns immediately visible. With bandwidths exceeding 4GHz, digital oscilloscopes now outperform their analog counterparts in overall performance and versatility.
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