Richer data content more easily displayed with more intuitive meaning, and computer access are offered in an oscilloscope configuration recently introduced by Tektronix. The Digital Phosphor Oscilloscope (DPO) joins the analog real-time (ART) oscilloscope and digital storage oscilloscope (DSO) as an alternative when selecting a scope.
Unlike the ART scope and the DSO, the DPO displays, stores, and analyzes in real time, using three dimensions of signal information: amplitude, time, and the distribution of amplitude over time. Although, the DPO provides many features of the ART scope and DSO, it has a novel method of handling the input signal.
Experienced ART oscilloscope users have long used the persistence of the phosphor; that is, the tendency of the cathode ray tube screen to continue to emit light after it no longer is being excited by the electron beam. This technique, which integrates successive sweeps and emphasizes any variation from sweep to sweep, is valuable for detecting noise, modulation, or other variances.
This capability is not present in a DSO. On the other hand, DSOs provide a clean, stable presentation that can be studied at length. More importantly, the data is in digital form, ready to be stored or analyzed by a computer. To maximize signal integrity, both the ART scope and the DSO usually perform by minimizing the path from the signal input to the viewing screen.
The DPO is a significant departure from this philosophy. It inserts an integrating storage memory between the input and the viewing screen. At this point, the insertion of the memory completely separates the data-acquisition and video-generation functions.
The DPO uses this facility to display much more signal information on the screen, with greater control of the presentation than either the ART or DSO configuration. Although eclipsed by the difference in signal-path architecture, the digitizing speed of the front end that feeds data to the new architecture also is a major performance accomplishment.
Digital phosphor does not refer to the viewing screen, but rather to a specialized block of video RAM. In a DSO, a signal is sampled and stored. The stored data in the DSO becomes vertical deflection on the viewing screen. In any subsequent sweep, all data from the previous sweep is discarded, and only the last sweep is displayed.
In the DPO, the input is acquired continuously, limited to the triggering criteria, with each sweep stored in the 500×200 21-bit digital phosphor RAM array. Each sweep is stored without erasing previously acquired information. The high-speed processor that accomplishes this task is a proprietary 1.3 million transistor CMOS device named DPX™.
Totally separate from the acquisition activity, the contents of the digital phosphor RAM are converted to 4-bit gray-scale video data and displayed every 1/30 s. This produces a constant flicker-free display that is unaffected by the acquisition process.
The contents of the digital phosphor memory also can be accessed for graphical or statistical analysis on an external computer. Aliasing, a persistent problem in DSOs, is extremely unlikely in the DPO configuration.
Seven models offer bandwidths ranging from 500 MHz to 2 GHz, 2- or 4-GS/s sample rates, and color and monochrome display options. $9,760 to $34,995. Tektronix, (800) 426-2200 (press 3, code 1080).
Copyright 1998 Nelson Publishing Inc.
July 1998
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