| Oscilloscope types |
Oscilloscope types
- a summary of the different types of oscilloscope that are available
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.
Analogue oscilloscope
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.
Digital oscilloscopes
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!
Other approaches
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.
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