In order to select the best function generator for any application, it is necessary to understand the performance parameters and specifications - there can be some hidden specifications, so knowing what to look for is important.
Function generator specifications vary widely because of the number of different types available ranging from analogue to digital, and according to their cost.
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.
- Power requirements: many items of test instrumentation can operate from a variety of power line voltages, but it is still worth checking. DC is an unlikely option, but may be available in some limited instances if needed.
- Environmental: For some applications, issues like the environmental considerations may be important. Storage and operating temperature, along with any humidity specifications will be quoted. Typically specifications for these aspects will indicate the equipment is unlikely to operate in a hostile environment - typically a laboratory room, although some ruggedized equipment may be available for some specialist applications.
- Mechanical: The size weight and general mechanical aspects may not be particularly important for most applications, but it is worth checking that there are no major issues.
Checking that the performance parameters and overall function generator specifications meet the requirements is important before investing in the purchase or hire of a function generator. Most of the specifications and performance parameters are relatively straightforward and have been detailed here as ideas for a check list.
More Test Topics:
Analogue Multimeter Digital Multimeter Oscilloscope Signal generators Spectrum analyzer Frequency counter LCR meter / bridge Dip meter, GDO Logic analyzer Power meter (RF & microwave) RF signal generator Logic probe Time domain reflectometer, TDR LabVIEW PXI GPIB / IEEE 488 Boundary scan / JTAG
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