Digital Oscilloscope & digital storage oscilloscope, DSO

The digital oscilloscope is the main type of oscilloscope used these days. Using digital technology it is able to provide much higher level of capability than analogues scopes could

Scope types includes:
Analogue scope Analogue storage scope Digital phosphor scope Digital scope USB / PC scope Mixed Signal Oscilloscope MSO Sampling scope

Oscilloscope Tutorial Includes:
Oscilloscope basics Oscilloscope types Specifications USB vs bench-top scope How to use an oscilloscope Scope triggering Oscilloscope probes Oscilloscope probe specifications

Using the advantages of digital technology, digital oscilloscopes are able to provide very high levels of performance. The levels of accuracy are higher, displays are cleared, and there are a shot of advantages to using digital scopes.

Today’s digital scopes are able to provide many facilities, including storage, that were not possible on all but the very high end analogue scopes. As a result of the storage capability, many digital oscilloscopes may also be referred to as a digital storage oscilloscope, DSO.

Digital oscilloscopes have taken over virtually completely from their analogue cousins. It is now difficult to buy new analogue scopes - the only ones that are available today are digital. Fortunately entry level digital oscilloscopes can be obtained for very reasonable provides and at the performance end of the market, the levels that can be achieved are exceedingly high.

Digital oscilloscope R&S RTB2000 — A typical modern digital oscilloscope

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.

Signals can enter the scope as analogue signals but they are converted into a digital format and this enables the power of the signal processing techniques that are available to provide much greater levels of functionality and this enables better insight into the signals that can be monitored.

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 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 MSa/s, or these days GSa/s, for Giga samples per second.

In addition to this, the resolution of the analogue to digital converter is important. The higher the number of bits from the ADC, the greater the accuracy with which the waveform is sampled. Typically very low resolution scopes may only have an 8 bit ADC and this can result in steps being seen not he displayed waveform as the level of the waveform moves up or down by one bit. Scopes typically have a greater resolution with scopes having 10, 12 and even up to 16 bit ADC resolution levels. The higher the resolution, the greater the details hat can be seen, especially as the scope is zoomed in on a vertical portion of a waveform.

Although microprocessors can be used to form the heart of the signal processing in the digital scope, it is more normal to use FPGS or sometimes CPLDs. These chips can be programmed to perform exactly the forms of manipulation that are required, and often in a parallel manner. This enables much higher levels of performance to be achieved.

Basic block diagram of a generic digital oscilloscope

When signals enter the digital oscilloscope, they pass through an analogue conditioning stage. This provides functions like attenuation, gain, AC / DC coupling, and impedance matching etc. The resulting waveform is then passed to the analogue to digital converter, ADC.

The ADC may have one or many cores - if it has multiple cores then the data is typically streamed in parallel to the FPGA and into memory. With data stored in this way, it is possible to process it in a variety of ways, recalling the data from memory as required.

Many digital oscilloscopes offer a logic analysis capability and incorporate some or digital input channels. These do not require the same analogue processing and they can be passed directly into the FPGA, obviously via protection circuitry. Scopes with this capability are typically referred to as MSOs or mixed signal oscilloscopes.

Once the waveform has been processed, the image to be displayed can be passed over the USB interface to the PC.

Analogue and digital triggering

It is possible to use the analogue waveform to trigger the scope, or use digital techniques.

Using digital techniques it is possible to obtain far better results from triggering.

Essentially ice the waveform is stored in memory, the trigger can be adjusted to trigger on any point onto e waveform - the trigger point can even be int he centre of the screen allowing the waveform before and after the trigger point to be viewed.Another advantage is that analogue triggering has a certain hysteresis within the system. This needs to be incorporated to ensure that the waveform is triggered at the correct point. Using digital techniques a much more accurate trigger can be obtained enabling the display to be much clearer even into e presence of noise. Also the trigger can be fired in a variety of other ways, even on a specific series waveform, etc.

Overall, digital triggering provides a much higher level of performance and a much greater degree of flexibility.

Advantages of digital oscilloscopes

Digital oscilloscopes provide a large number of advantages over their analogue predecessors. Using he power of current day digital processing techniques, the performance can be considerably enhanced.

Storage capability: As the waveforms are stored in memory to enable them to be processed, modern digital oscilloscopes are by their very nature also storage scopes and this enables even transient waveforms to be captured and displayed as needed.
Accuracy: Using digital technology, the accuracy of digital oscilloscopes is far higher than it was possible to achieve with analogue scopes. Often digital markers can be placed on a waveform to measure the exact voltage at a particular point.
Flexibility & functionality: Using digital techniques it is possible to programme in a very high level of functionality into a digital scope.

Modern digital oscilloscopes provide a huge level of performance and with the cost of their manufacture falling, many digital oscilloscopes are available for very reasonable prices. Cost is not the only item as the performance of digital oscilloscopes has improved over the years and the top end digital scopes provide a fantastic level of performance, especially when compared both e analogue scopes of many years ago.