A function generator is a specific form of signal generator that is able to generate waveforms with common shapes. Unlike RF generators and some others that only create sine waves, the function generator is able to create repetitive waveforms with a number of common shapes.
In particular it can be made to become a sine wave generator, square wave generator, and triangular wave generator.
Also a function generator may be able to vary the characteristics of the waveforms, changing the length of the pulse, i.e. the mark space ratio, or the ramps of the different edges of triangular or sawtooth waveforms, but it is only be able to create the waveforms that are built in to the function generator. It cannot be programmed to create additional waveforms - an arbitrary waveform generator, AWG is required for this.
Apart from just generating the waveforms themselves, this type of test instrument has the capability to add a DC offset to the signal. This can be very useful in a number of testing applications.
Typically function generators are only able to operate at relatively low frequencies, some only operating to frequencies of around 100kHz, although more costly test instruments can operate at higher frequencies, up to 20 or 30MHz.
Function generator capabilities
Function generators are capable of producing a variety of repetitive waveforms, generally from the list below:
Sine wave: A function generator will normally be able to act as a sine wave generator. This is the standard waveform that oscillates between two levels with a standard sinusoidal shape. Using the function generator as a sine wave generator is one of the more commonly used applications. Sine waves are widely used in testing applications.
Square wave: Another very widely used waveform is the square wave. It consists of a signal moving directly between high and low levels. Used as a square wave generator, this test instrument provides a very useful source of a basic digital waveform.
Pulse: A pulse waveform is another type that can be produced by a function generator. It is effectively the same as a square wave, but with the mark space ratio very different to 1:1. This form of waveform is again often used within digital applications.
Triangular wave: This form of signal produced by the function generator linearly moves between a high and low point. This form of waveform is often generated using an operational amplifier acting as an integrator. The triangular waveform generator typically also has a square wave output as well, and it is used as the basis for generating all the waveforms in a function generator test instrument.
The triangular waveform is often used in testing amplifiers - it is far easier to see distortion and clipping on a triangular waveform than it is on a sine waveform.
Sawtooth wave: Again, this is a triangular waveform, but with the rise edge of the waveform faster or slower than the fall, making a form of shape similar to a sawtooth. It is generated by the same circuit as the triangular waveform, but with the different rise and fall times created by changing the charge rate for the rise and fall elements of the integrator.
These are the basic waveforms that are produced within a function generator test instrument. These waveforms satisfy most of the needs for testing a number of items. Where specialised waveforms are required, then an arbitrary waveform generator is required.
Function generator controls
In addition to a selection of the basic waveforms that are available, other controls on the function generator may include:
· Frequency: As would be expected, this control alters the basic frequency at which the waveform repeats. It is independent of the waveform type.
· Waveform type : This enables the different basic waveform types to be selected:
Sine wave
Square wave
Triangular wave
· DC offset: This alters the average voltage of a signal relative to 0V or ground.
· Duty cycle: This control on the function generator changes the ratio of high voltage to low voltage time in a square wave signal, i.e. changing the waveform from a square wave with a 1:1 duty cycle to a pulse waveform, or a triangular waveform with equal rise and fall times to a sawtooth.
Function generator usage
Function generators are normally used within electronics development, manufacturing test and service departments. They provide a flexible form of waveform generation that can be used in many tests.
These test instruments are very flexible and not thought of as specialist instruments. Although they can often generate signals into the low end of the RF spectrum, normally a specific RF generator would be used, unless none were available.
Also they are generally not used for performance audio testing as the levels of distortion on the sine aves that would normally be used would have higher levels of distortion than these tests sometimes require. A typical figure for the sine wave distortion might be about 1%.
If very high frequency stability is required, ten some of these test instruments allow for the output signal to be phase locked to another source.
Types of function generator
There are several forms that the function generator can take. With modern digital technology there are many formats for this type of test equipment.
· Bench top test instrument: The most widely used form of function generator in the test laboratory is the test instrument contained within a box that sits on the laboratory bench. This test instrument contains the power supply, controls, display and of course the output connector.
· Rack based test instrument: Another format that this type of test equipment can take is a module within a rack system like PXI. Based on PCI, the PXI rack system has been developed specifically for test applications and includes a slot for either a controller or link to a computer. Test instrument cards slot into the chassis, enabling a test system to be created to meet the needs of the particular test application required. The test instrument cards can include any type of test instrument including volt meters, oscilloscopes, and of course a function generator
· USB function generator: A number of small function generators are available as USB based test instruments. They contain the core oft he function generator within the module that connects to a computer via a USB connection. This approach means that the power and control interfaces can use the PC rather than having the expense and space required to provide these within a larger box for the test instrument.
· Computer based function generator: A different approach is to use software based within a computer to provide the required waveforms and then use a digital card of the computer's audio output for the signal. Whilst very cheap, this may not have the output capability and accuracy of other types of test instrument. Also if the output is damaged as a result of the testing and a possible misconnection, etc it can result in costly repairs.
Function generators are normally very easy to operate. With modern processing technology often included this gives the possibility of many additional features including ease of operation, and remote control via one or more of the many standards available.
A good variety of function generators are available for a number of different manufacturers and also via a number of different distributors. Costs are usually quite reasonable as they are normally quite straightforward and do not require very high frequency components. These test instruments can be either analogue or digital. Analogue ones tend to use operational amplifiers as the basis of the design, but digital ones can use some form of direct digital synthesis.