Function Generator Specifications

- key points about understanding function generator specifications, what they mean and how they affect performance.

There are a variety of specifications that are applicable to function generators.

When choosing a test instrument for a given application, it is necessary to assess the specification of the function generator to understand whether it is suitable.

Function generator specifications vary widely because of the number of different types available ranging from analogue to digital, and according to their cost.

TTi TG5011 Function Generator

Main function generator specifications

Although there are many different function generator specifications, the main ones are summarised below:

  • Waveforms:   Function generators generally produce sine wave, square wave, pulse, triangular and sawtooth or ramp waveforms. It is worth checking the specifications of these individual waveforms.

    • Sine wave distortion:   Analogue function generators create a sine wave from the triangular waveform using a pair of back to back diodes to shape the waveform. Although this produces a good representation of a sine wave, the distortion levels will be higher than sine waves produced by other means. Accordingly the function generator specification for sine wave distortion needs to be checked if this may be an issue. Typical levels may be < 2%
    • Triangular wave linearity:   There will be some departure from a straight line on the triangular wave. Typically linearity is better than 99% between levels of 10 and 90% of the waveform amplitude.
    • Square wave rise & fall times:   Another important function generator specification can be the square wave edge rise and fall times. This can be an issue when driving some logic chips. Chips that are synchronous and use a clock may require an edge of a certain speed. Typically a function generator may provide a rise time of 100ns between 10 and 90% of the waveform. The fall time may also be of the same order as well, although possibly different to the rise time
    • Output symmetry:   The function generator specification will give a range over which the output symmetry can be changed. This might be 20% - 80% ± 10%.
  • Output level:   The output level on most function generators will be continuously variable. Often it will be able to easily adjust to so that it is TTL compatible. However maximum limits will vary from generator to generator. Typical maximum levels may be 10 or 12 Volts peak to peak.
  • Output impedance:   In many instances the load that can be driven by the function generator is of importance. The figure is measured in ohms, Ω and is typically 50Ω. Any output level readings will assume this, and at this impedance the output will drop by half from its no load value.
  • DC offset:   One facility that some function generators provide is a DC offset. This enables the base voltage level of the signal to be varied over a given range. It may be variable over a range +5V to -5V for example.
  • Frequency range:   Function generators have a limited frequency range. There are a number of elements to the specification:

    • Lower frequency limit:   The lower frequency limits tend to be below 1 Hz, often 0.1 or 0.2 Hz. Often the lower limits are able to go well below normal requirements.
    • Upper frequency limit:   The upper frequency limit tends to be a headline specification for the function generator. Limits vary considerably from figures around 1 MHz up to 20 MHz or more.
    • Ranges:   There may be several switched ranges to the coverage. Often they tend to cover a decade in frequency, i.e. 1 - 10. However this specification is dependent upon the particular function generator.
  • Frequency stability:   The stability of function generators can vary considerably. Analogue versions tend to be much less stable, but digital ones will use a crystal for the clock in the generator. Typical figures may be around 0.1% per hour for analogue function generators, and 500 parts per million for digitally based test instruments. The specification may be given in terms of the time base stability
  • Phase lock capability:   Some generators may be able to phase lock the signal generator to an external clock signal. This would enable the function generator to provide a much more accurate, or synchronised output.
  • Modulation:   Some test instruments may have the capability for the output signal to be modulated, typically either amplitude or frequency modulation, but this is not true of many test instruments.

There will naturally be the usual function generator specifications for parameters such as size, weight and operating temperature. Power supply requirements and power consumption will also be given.

By Ian Poole

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