- a summary of the different types of oscilloscope that are available
Oscilloscope tutorial includes:
An oscilloscope is one of the major tools available for testing electronic circuitry. The oscilloscope is able to display waveforms and as a result it gives a particularly useful view of what is happening in an electronic circuit. While the basic philosophy behind all oscilloscopes is the same, there are a number of different variants that are available, each possessing slightly different capabilities and being suited for a given application or set of applications.
Oscilloscopes fall into a variety of categories. The biggest distinction is whether they are analogue or digital, but within the digital oscilloscope arena there are ordinary digital oscilloscopes, digital storage oscilloscopes, digital phosphor oscilloscopes, and digital sampling oscilloscopes.
The analogue oscilloscope of the original type of oscilloscope. As the name implies it uses analogue techniques throughout to create the pattern on the display. Typically they use a cathode ray tube where the voltages on the x and y plats cause a dot on the screen to move. In the horizontal direction this is controlled by the time base, whereas in the vertical direction the deflection is proportional to the signal input. Essentially the signal is amplified and applied to the Y plates of the cathode ray tube.
A cathode ray tube consists of a number of elements. There is an electron gun that generates an electron beam that is fired along the length of the tube. This beam passes by deflection plates that are used to deflect the beam, as a result of electrostatic attraction and repulsion, and finally the beam hits a phosphor coating on the "screen" creating a small dot of light.
To assist in making the trace as clear as possible, intensity and focus controls are included. The focus ensures that the dot that scans the screen remains as sharp as possible and in this way it can deliver a clear trace. The intensity control is required because the intensity of the dot or trace varies according to the speed at which the scan is made. Controlling the intensity enables a clear trace to be obtained.
When the scan is very slow the dot is seen to traverse the screen and it is difficult to visualize the waveform. As the speed increases, it ceases to be seen as a dot, but instead it traces out a line and the signal waveform, which when triggered correctly remains static on the screen. The trace may be scanned across the screen many times a second. In many instances it my traverse the screen 100 000, 500 000 or more times a second.
However as the writing speed increases, the trace becomes steadily more dim, and ultimately becomes difficult to see despite the intensity control. For higher frequency signals faster writing speeds are required, and as a result analogue oscilloscopes have a limited frequency range. Typically the maximum frequency that can be seen by an analogue oscilloscope is around 1 GHz. Above this other types of oscilloscope are required.
The concept behind the digital oscilloscope is somewhat different to an analogue scope. Rather than processing the signals in an analogue fashion, the scope converts them into a digital format using an analogue to digital converter and then processes the signals digitally and then may convert them into an analogue format again for them to be displayed. With digital signal processing hardware and software becoming more powerful, this enables the processing of the signals to be undertaken in a far more flexible manner, and enables many additional features to be added.
Digital oscilloscopes, like analogue ones have limits on their performance, and in particular the frequency up to which they can operate. The upper limit of frequency for the oscilloscope is determined by two main factors, namely the analogue bandwidth of the front-end section. This is often referred to as the -3 dB point. Another limitation is the sample rate of the oscilloscope. Samples are taken at regular intervals, and the higher the sample rate, the higher the frequencies that can be seen on the screen.
Digital oscilloscopes can be put into three main categories: the digital storage oscilloscope; digital phosphor oscilloscope, and the digital sampling oscilloscope
Digital storage oscilloscope
The digital storage oscilloscope (DSO) is the conventional form of digital oscilloscope. It uses a raster type screen like that used on a computer monitor or television and in this way displays an image that fills the screen and may include other elements in addition to the waveform. These additional items may include text on the screen and the like.
To achieve the raster type display the waveform is stored in a digital format. As a result it can be processed either within the oscilloscope itself, or even by a PC connected to it. This enables a high degree of processing to be achieved, and the required display provided very easily and often with a very cheap processing platform. It also enables the waveform to be retained indefinitely, unlike the analogue scopes for which the waveform could only be stored for a very limited time.
To understand more about a digital storage oscilloscope it is necessary to understand what is inside the unit. The first stage the signal enters within the scope is the vertical amplifier where some analogue signal conditioning is undertaken to scale and position the waveform. Next this signal is applied to an analogue to digital converter (ADC). This takes samples are regular time intervals or sample points. The actual rate at which the samples are taken is important because this determines the time resolution to which the signal can be analysed later. Scope specifications quote the sample rate as a number of samples per second, or more usually mega samples per second (MS/s) or Giga samples per second GS/s)
The samples are stored in the memory within the oscilloscope as what are termed waveform points, and a single waveform point may be made up of several samples. The overall waveform is stored as a waveform record and its start is governed by the trigger, its finish being determined by the horizontal timebase time.
Being digital in format there is naturally a signal processor. This enables the signal to be processed in a variety of ways, before passing to the display memory and the display itself.
DSOs are widely used for many applications in view of their flexibility and performance. They excel when used as a single shot mode as the image can be captured, stored and manipulated as required. As a result they can be used for capturing transient conditions that may not be as easy to examine when using other forms of scope.
Digital Phosphor Oscilloscope
The digital phosphor oscilloscope (DPO) is a highly versatile form of oscilloscope that uses a parallel processing architecture to enable it to capture and display signals under circumstances that may not be possible using a standard DSO. The key element of a DPO is that it uses a dedicated processor to acquire waveform images. In this way it is possible 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.
The input to a digital phosphor oscilloscope (DPO) is similar to that of an analogue oscilloscope. It uses a vertical amplifier that feeds into an analogue to digital converter. However it is at this point that the architecture of a DPO differs from that of a DSO.
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.
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.
Digital Sampling Oscilloscopes
These oscilloscopes are used for analyzing very high frequency signals. They are used for looking at repetitive signals which are higher than the sample rate of the scope. They collect the samples by assembling samples from several successive waveforms, and by assembling them during the processing, they are able to build up a picture of the waveform. In this way these oscilloscopes may be able to view signals at frequencies up to 50 GHz and more.
The design of these scopes is optimised for very high frequency operation, and to achieve this the vertical amplifier topology is somewhat different. On entering the vertical amplifier chain, the signal is sampled prior to any amplification to ensure the maximum bandwidth is achieved. After the signal is sampled a lower frequency amplifier / attenuator combination can be used because the signal is effectively at a lower frequency at this stage. However this methodology does reduce the dynamic range of the instrument. Typically the maximum voltage that can be handled is around 3 volts peak to peak and it is not possible to place protection diodes ahead of the sampling diode ring as this would limit the frequency response.
While these oscilloscopes do have their limitations, they are able to display extraordinarily high frequencies. Where this type of frequency response is needed, a digital storage oscilloscope is the only alternative. Naturally they are also not cheap!
Oscilloscopes are widely used in industry and they are taking advantage of the huge levels of processing power that are now available. While most oscilloscopes are dedicated instruments, a growing number of scopes are being sold that incorporate a PC to provide the processing power. As PCs are widely available this considerably reduces the cost of buying a high performance oscilloscope. While this approach may not be suitable in many applications it is ideal for many others where a general purpose PC can be used. Even so dedicated oscilloscopes are being increasingly used as they now offer better performance levels than ever before.
Popular test equipment tutorials . . . . .
|• Arb / AWG||• Digital multimeter||• Oscilloscope||• Logic analyzer|
|• Logic probe||• Function generator||• Frequency counter||• RF sig gen|
|• Signature analyzer||• Spectrum analyzer||• RF network analyzer||• RF power meter|
|• Analogue multimeter||• TDR|