Digital Phosphor Oscilloscope, DPO Tutorial

- each different type of oscilloscope has its own advantages and disadvantages. Digital oscilloscopes are now the most common types in use, offering high levels of performance.

The digital phosphor oscilloscope, DPO is another form of digital oscilloscope.

The DPO scope has a different architecture to that of the more traditional digital / digital storage types and this enables it to process signals more quickly.

To achieve this, the DPO adopts a parallel processing architecture rather than the more straightforward serial technology.

Digital phosphor scope basics

Using parallel processing techniques and a dedicated processor, the DPO is able to capture transient events that occur in digital systems more easily. These may include spurious pulses, glitches and transition errors. It also emulates the display attributes of an analogue oscilloscope, displaying the signal in three dimensions: time, amplitude and the distribution of amplitude over time, all in real time.

In terms of the digital phosphor oscilloscope architecture, the signal first enters an analogue vertical amplifier. This feeds into an analogue to digital converter in a similar fashion to a digital storage scope. However it is from this point that the architecture of a DPO differs from that of a DSO.

Basic block diagram for a digital phosphor oscilloscope showing how it gathers the vertical waveform and how samples are gathered and displayed
Digital phosphor oscilloscope, DPO, basic block diagram

For any oscilloscope there is a time delay between the end of one scan and when the trigger is ready to initiate the next one. During this period the scope does not see any activity that may occur on the signal line For a DSO this time can be relatively long because the scope processes information serially and this can form a bottleneck. However the DPO uses a separate parallel processor and this enables it to capture and store waveforms despite the fact that the display may be acting much slower. By using the parallel processing the DPO is not limited by the speed of the display, signals may be captured independently of the activity of the display.

Although the name of the DPO may indicate that it relies on a chemical phosphor, this is not necessarily the case as more modern displays are used. However it possesses many of the aspects of a phosphor oscilloscope, displaying a more intense image the more often the waveform passes a certain point.

Each time a waveform is captured it is mapped into the DPO memory. Each cell represents a screen location. The more times data is stored into a location, the greater the intensity attached to it. In this way intensity information builds up in cells where the waveform passes most often. The overall result is that the display reveals intensified waveform areas, in proportion to the frequency of occurrence of the signal at each point. This has the same appearance as those displayed on an analogue phosphor oscilloscope, and this gives rise to the name.

Effectively the processor within the DPO operates in parallel with the acquisition system for display management, measurement control, and overall instrument control. In this way its operation does not affect the acquisition speed of the overall scope.

The advantage of this approach is that it achieves a virtually 'real time' display that is able to capture transient events as well as the repetitive waveforms.

Additionally , only a DPO provides the Z (intensity) axis in real time, and this is a feature that is missing from conventional digital storage oscilloscopes.

By Ian Poole

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