Digital Storage Oscilloscope / Digital Scope 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.

Digital oscilloscopes are now the major form of oscilloscope that is available on the test equipment market.

These scopes may be referred to as digital oscilloscope or digital storage oscilloscope. Although these two names used to indicate separate types of instrument, they are often used interchangeably.

RIGOL DS2000 digital oscilloscope

Digital oscilloscope vs digital storage oscilloscope

The name digital storage oscilloscope dates back to the days when digital technology was entering the scope arena. Analogue storage scopes were not particularly effective, and therefore there was a need for effective storage technologies.

The original digital storage scopes had analogue input stages, and then converted the signals into a digital format to enable them to be stored in digital memory. They could then be processed before being converted back into an analogue format for display on a cathode ray tube.

As technology progressed, the storage facility was retained, but the whole scope became digital.

For a while, there was a difference between standard digital scopes and those with a storage capability, but this difference was eroded as the facility became incorporated on all digital oscilloscopes. Accordingly the two types are effectively the same, and the names are often used interchangeably to describe the same type of test instrument.

Digital oscilloscope technology

The basic concept behind digital oscilloscopes / DSOs is the conversion of the incoming analogue signal into a digital format where it can be processed using digital signal processing techniques.

When the signal enters the scope it is first pre-conditioned by some analogue circuits to ensure that the optimum signal is presented to the next stage.

This next stage involves the acquisition of the digital samples. To achieve this, an analog-to-digital converter, ADC, takes samples are discrete regular time intervals.

How a digital oscilloscope samples an input waveform at regular intervals determined by the timebase or clock
Digital oscilloscope sampling

The times and rate at which samples are taken is determined by the system clock. The rate at which samples are taken is often defined as part of the specification of the scope. This is measured in samples per second, and often quoted in Mega samples per second M samples per second.

The samples from the ADC are stored in memory and referred to as waveform points and together these points make up the overall waveform record. The number of waveform points within the record is referred to as the waveform length.

The waveform record is initiated by the trigger and again stopped by the timebase circuit after the given amount of time.

The waveform record is then processed by the processing circuitry and presented to the display for visual inspection by the user.

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

By Ian Poole

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