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.