Function Generator Types

- the different types of function generator, their differences and their various applications, including analogue, digital, sweep function generators.

There are several different types of function generator.

The different function generator types offer different levels of performance and different elvels of cost.

Accordingly each type of function generator will be more suitable for some applications rather than others.

Types of function generator

There are a number of ways of designing function generator circuits. However there are two main approaches that may be used:

  • Analogue function generator:   This type of function generator was the first type to be developed. First models appeared in the early 1950s when digital technology was not widely used.

    Despite the fact that they use analogue technology, these analogue function generators offer a number of advantages:

    1. Cost effective:   Analogue function generators are very cost effective, being at the lower end of the function generator price range.
    2. Simple to use:   Analogue function generators provide an effective test instrument that is able to meet most user needs, while remaining simple and easy to use.
    3. Maximum frequencies:   The analogue function generators do not have the high frequency limitations on non-sinusoidal waveforms such as triangles and ramps as do the digital function generators.
  • Digital function generator:   As the name indicates, digital function generators utilise digital technology to generate the waveforms. There are a number of ways in which this can be done, but the most versatile and most widely used technique for digital function generators is to use direct digital synthesis, DDS.

    DDS uses a phase accumulator, a look-up table containing a digital representation of the waveform, and a DAC. The phase accumulator moves another position each time it receives a clock pulse. The next position in the look-up table is then accessed giving the digital value for the waveform at that point. This digital value is then converted into an analogue value using a digital to analogue converter, DAC.

    Digital function generators are able to offer high levels of accuracy and stability because the clock for the system is crystal controlled. Also digital function generators provide a high spectral purity and low phase noise. A DDS based digital function generator can also can be swept over a much wider frequency range than an analogue function generator. It can also perform a number of other functions such as phase continuous frequency hopping because of the action of the direct digital synthesizer.

    The disadvantage of the digital function generators is that they are more comprehensive than their analogue cousins, they require a high performance DAC and other digital circuitry and this means they are more costly and also more complicated to sue as a result of their additional functionality
  • Sweep function generator:   A sweep function generator is simply one that can sweep its frequency. Typically the more versatile sweep function generators utilise digital technology, but it is also possible to use analogue versions as well.

    Sweep function generators may be able to sweep over ranges of up to 100:1 or more, although this is very dependent upon the actual generator type in question. Speed of the sweep may also be important. Another feature that may be of importance is whether the sweep is linear or logarithmic. Some function generators may have a switch for this.

Today, most function generators adopt a digital approach, analogue function generators will still be encountered as they are still able to provide levels of performance that are more than satisfactory for the exacting requirements needed in many development laboratories. Also analogue function generators tend to be much cheaper.

By Ian Poole

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