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 within the test instrument specifications is important.
Function generator specifications vary widely because there are several different types of this test instrument that are available ranging from analogue to digital, and according to their cost.
Main function generator specifications
Like any item of test equipment there are many different specifications
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 test instruments such as purpose designed sine wave generators that use different techniques internally to generate the sine wave.
Accordingly the function generator specification for sine wave distortion needs to be checked if this could be an issue. Typical levels may be < 2%. Specific sine wave generators may be able to offer much lower levels of distortion.
• 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.
In some instances the rise and fall times may not be fast enough for some logic circuits, especially edge triggered flip flops and it may be necessary to add a buffer to sharpen the edges. Even any test leads may slow the edges.
• Output symmetry
The function generator specification will give a range over which the output symmetry can be changed. This might be 20% - 80% ± 10%.
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.
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.
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.
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.
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 test instrument.
The stability of function generators can vary considerably. Analogue test instruments 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.
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.
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.
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.
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 one of these test instruments. Most of the specifications and performance parameters are relatively straightforward and have been detailed here as ideas for a check list.
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