Analog oscilloscopes rely on cathode ray tubes (CRTs) to visualize waveforms. The bandwidth of the CRT is essentially the same as that of the analog oscilloscope itself, meaning the speed at which electrons move inside the tube corresponds directly to the frequency of the input signal. As the signal frequency increases, the electron beam moves faster, and the brightness of the waveform on the screen changes accordingly. However, for lower frequency signals, the electron beam moves more slowly, resulting in a brighter display. This variation in brightness allows the oscilloscope to represent a third dimension of the signal—such as amplitude over time—through brightness or grayscale levels. For example, the vertical axis typically shows amplitude, the horizontal axis represents time, and the screen’s brightness reflects how often a particular part of the signal appears. This time-dependent phosphor persistence effect is especially useful for analyzing complex or intermittent waveforms. Analog Storage oscilloscopes are known for this feature, with top models offering up to 800MHz bandwidth and the ability to capture transient events as fast as 1ns.
In contrast, digital oscilloscopes do not naturally support persistence displays because they process signals digitally, using only two states: high or low. This binary nature means the waveform is displayed in a "present" or "not present" manner. To replicate the multi-level brightness seen in analog scopes, digital oscilloscopes require specialized image processing chips. For instance, Tektronix uses DPX processors, which combine data acquisition, image processing, and storage in one unit. These chips contain 1.3 million transistors and are built using a 0.65μm CMOS process with a parallel architecture, allowing for high sampling rates. They function both as data acquisition tools and raster scanners that mimic the light emission properties of a CRT phosphor. The DPX chip supports 16 levels of brightness and can be displayed on a 500x200 pixel monochrome or color LCD screen, updating every 30 seconds. Unlike analog storage oscilloscopes, which required photographic film for recording, digital phosphor oscilloscopes offer much easier data handling, saving, and analysis. For example, they can highlight the most frequently occurring waveforms in red and the least frequent ones in blue, making it easy to identify patterns at a glance. With bandwidths now exceeding 4GHz and enhanced fluorescent display capabilities, digital oscilloscopes generally outperform their analog counterparts in overall performance and versatility.
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