| Arbitrary Waveform Generator (AWG or ARB) |
Arbitrary Waveform Generator (AWG or ARB)
- overview, tutorial and information about about the basics of the
Arbitrary Waveform Generator or AWG and what and arbitrary waveform
generators can do, the how these peices of test equipment can be used, and
their specifications.
Arbitrary waveforms generators are a form of function or
signal generator that is able to produce an arbitrary waveform defined by a
set of values, i.e. "waypoints" entered to set the value of the waveform at
different times. As a result an arbitrary waveform generator is a form of
test equipment that is able to produce virtually any waveshape that is
required. An arbitrary waveform generator may also run in either a
repetitive or a single shot mode.
Arbitrary Waveform Generator techniques
There are a number of ways of designing arbitrary
waveform generators. They are based around digital techniques, and their
design falls into one of two main categories:
- Direct Digital Synthesis (DDS): This type of
arbitrary waveform generator is based around the DDS types of frequency
synthesizer. It uses integrated circuits intended for direct digital
frequency synthesizers, but enables an arbitrary waveform generator
circuit to be created relatively easily and for an economic price. This
disadvantage of this type of generator is that the clock frequency is
fixed and this means that the repetition rate of the stored waveform is
varied by changing the addressing interval. The result of this is that
the DDS type of arbitrary waveform generator is only able to accurately
reproduce a waveform at a repetition rate equal to the DDS clock
frequency divided by the waveform length (or sub-multiples of this). At
all other frequencies samples are either omitted or are duplicated an
uneven number of times. This can result in jitter that could be a
problem for some critical applications.
- Variable-clock arbitrary waveform generator
Arbitrary waveform generator resolution and speed
Two of the main specifications for an arbitrary waveform
generator are their resolution and also the speed. These two parameters
determine the precision with which the waveform can be reproduced. They are
governed by different elements within the arbitrary waveform generator
circuit.
The amplitude resolution is governed by the resolution of
the digital to analogue converter (D/A or D2A). This is described in terms
of the number of bits. A 12 bit resolution provides 4096 amplitude steps.
The speed of the arbitrary waveform generator is also
very important. The maximum repetition rate for the waveform is governed by
two factors: the length of the waveform in terms of the number of samples
required to simulate the waveform and the maximum clock frequency. For
example if the arbitrary waveform generator had a maximum clock frequency of
25 MHz and the waveform had 1000 points, then the maximum repetition rate
would be 25 kHz. If a higher repetition rate was required, then it would be
necessary to decrease the number of samples as it would not be possible to
increase the clock frequency in the arbitrary waveform generator!
Waveform capture
Before an arbitrary waveform generator can produce a
signal it is necessary to enter the points for the required output. There
are a number of ways in which the waveform for an arbitrary waveform
generator can be captured or generated:
- Capture a real waveform. This can be done using either a digitiser
of a digital oscilloscope.
- Use in-built waveform editing on the arbitrary waveform generator.
Many AWGs have some degree of waveform generation and editing capability
built on board, although this may not be as flexible as a full PC based
editing solution.
- Another way is to generate the waveform using software running on a
PC. Very sophisticated digital editing software is available for this
and allows for many variations to be incorporated.
Either of these methods can be used and then the digital
information can be downloaded onto the arbitrary waveform generator to
enable it to provide the required output.
Summary
The arbitrary waveform generator is a piece of test
equipment that can prove to be immensely useful in many applications. While
an arbitrary waveform gerneator is a very specialised piece of test
equipment and as a result it can be expensive, it is nevertheless almost
essential in any applications where a specific waveform needs to be
generated.
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