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A signal generator is piece of test equipment that produces an electrical signal in the form of a wave. This is used as a stimulus for the item being tested.
Signal generators in all their forms are widely used within test and development systems, being used with other test instruments.
When looking at what a signal generator is, it will be seen that they come in many forms - there are many signal generator types, each one being used to provide a different form of signal. Some provide RF signals, others audio signals, some can provide different shapes of waveform and others may provide just pulses.
Signal generators have been used for many years. Early types were very basic by the standard of today’s different types of signal generator. Performance levels as well as the variety of facilities that are available have increased and improved.
What is a signal generator
Signal generators come in various forms able to produce a variety of waveforms for different test applications. Some of these test instruments address the RF testing arena, whilst others are used for audio testing, possibly as a sine wave generator, etc and others for providing pulses, possibly for exciting digital circuits. There are thousands of different applications for signal generators.
However they differ from the measuring test instruments like oscilloscope, digital multimeters, spectrum analyzers, etc in that rather than measuring a signal, they generate a signal to be applied to a unit under test.
Accordingly it is worth defining a signal generator:
Signal generator definition:
A signal generator is an electronic test instrument that creates or generates either repeating or non-repeating waveforms. Thees waveform can be of different shapes and amplitude. Signal generators of all types are mostly used in designing, manufacturing, servicing and repairing electronic devices.
Summary of signal generator types
Looking at what a signal generator is, it will be seen that there are many different types of signal generator:
- Arbitrary waveform generator : The arbitrary waveform generator is a type of signal generator that creates very sophisticated waveforms that can be specified by the user. These waveforms can be almost any shape and can be entered in a variety of ways, even extending to specifying points on the waveform.
Essentially an arbitrary waveform generator can be thought of as a very sophisticated function generator.
Being considerably more complex, arbitrary waveform generators are more expensive than function generators, and often their bandwidth is more limited because of the techniques required in generating the signals.
- Audio signal generator: As the name implies this type of signal generator is used for audio applications. Signal generators such as these run over the audio range, typically from about 20 Hz to 20 kHz and more, and are often used as sine wave generators. They are often used in audio measurements of frequency response and for distortion measurements. As a result they must have a very flat response and also very low levels of harmonic distortion.
- Function generator: The function generator is a type of signal generator that is used to generator simple repetitive waveforms. Typically this signal generator type will produce waveforms or functions such as sine waves, sawtooth waveforms, square and triangular waveforms.
Early function generators tended to rely on analogue oscillator circuits that produced the waveforms directly. Modern function generators may use digital signal processing techniques to generate the waveforms digitally and then convert them from the digital into an analogue format.
Many function generators will tend to be limited to lower frequencies as this is where the waveforms created by this type of signal generator are often required. However it is possible for higher frequency versions to be obtained.
- Pulse generator: As the name suggests, the pulse generator is a form of signal generator that creates pulses. These signal generators are often in the form of logic pulse generators that can produce pulses with variable delays and some even offer variable rise and fall times.
Pulses are often needed when testing various digital, and sometimes analogue circuits. The ability to generate pulses enables circuits to be triggered, or pulses trains to be sent to a device to provide the required stimulus.
- RF signal generator: As the name indicates, this type of signal generator is used to generate RF or radio frequency signals.An RF signal generator may use a variety of methods to generate the signal. Analogue signal generator types used free running oscillators, although some used frequency locked loop techniques to improve stability. However most RF signal generators use frequency synthesizers to provide the stability and accuracy needed. Both phase locked loop and direct digital synthesis techniques may be used. The RF signal generators often have the capability to add modulation to the waveform. Lower end ones may be able to add AM or FM, but high end RF signal generators may be able to add modulation formats OFDM, CDMA, etc . so they can be used for testing cellular and wireless systems.
- Vector signal generator: The vector signal generator is a type of RF signal generator that generates RF signals with complex modulation formats such as QPSK, QAM, etc.
Vector signal generators tend to be used for the testing of modern data communications systems, everything from Wi-Fi to 4G, 5G mobile telecommunications systems and many other connectivity solutions that used advanced waveforms. As these waveforms use modulation schemes and waveforms that use phase information, a vector signal generator is often needed.
Signal generator formats
Like other forms of electronics test equipment, signal generators are available in a variety of different formats. The types of format available depend to a certain extent upon the exact type of signal generator, but there are several options that may be available.
- Traditional bench test equipment: The traditional bench test equipment is vision of what comes to mind when talking of test instruments. The self contained box which includes the generator itself as well as the power supply, control functions, the display and the external controls are what is normally thought of as test equipment. These test instruments cover the largest number volume, but are not always the most suitable as other options may also have their advantages.
- Rack test instrument card: There are test modules that can be slotted into a test rack Early rack systems included VXI, but today PXI is by far the most widely used. Based on the popular PC standard known as PCI, PXI is an open standard managed by the PXI Systems Alliance, PXISA, that has taken the PCI standard and updated it for test instrumentation applications. The rack consists of the basic 19 inch based rack system which includes a power supply, and the first slot is reserved for the controller or link to an external PC. The remaining card slots can be used for the test instruments. A good variety of signal generators, function generators, arbitrary waveform generators, etc are available. This approach is ideal for building an automated system with a number of units. Despite what may first be thought, very high performance PXI test instruments can be obtained, many of which equal the performance of bench test equipment.
- USB signal generator: Another option for many test instruments these days is to use the power of a PC to take on several of the functions of the test instrument. The test module provides the test equipment functionality, in this case generating the signal, but the power supply, controls and display are provided by the PC. This enables much lower cost instruments to be bought, whilst still retaining the capability and performance.
- Use computer generated waveform: In some cases it is possible to generate a waveform digitally within a computer using a signal generator app or programme. The resulting waveform can be sent out via the audio socket of an audio card. This route offers a very cheap route to creating a signal, but is limited by the output of the PC audio or audio card. It may be ideal for some applications, but the final output is very dependent upon the audio or output from the PC, and this route is possibly not the best option if an output with a guaranteed performance is needed.
There are many different formats for signal generators in terms of the physical format of the test instrument. If stand alone equipment is required, often the bench test equipment is the ideal route, but for systems and areas where PCs are available, other options may suit better.
The different types of signal generator are able to produce different types of waveform. These can be used in different applications, some testing RF equipment, others providing stimuli for logic boards and others used in a host of different areas to provide the different stimuli needed. When looking at what a signal generator is, it is necessary to determine the type of generator needed for the given job.