A signal generator produces ideal waveforms that act as a stimulus for electronic measurement. Most circuits require some kind of signal to be fed at the input to study and test their output using anacquisition system comprising various instruments like oscilloscope, multimeter and logic analyser. The output makes sense only when you have an ideal and controlled input.

(Image courtesy: www.ece.lsu.edu)
(Image courtesy: www.ece.lsu.edu)

Signal generators can provide a variety of signals for testing, which can be controlled as per your requirement. The signal can be a sine wave, a digital pulse, a binary pattern, an analogue function or any other arbitrary waveform.

One of the greatest advantages of a signal generator is that it can even add a controlled distortion to the signal. This is highly useful to study the response of your circuit in presence of the distorted signal, as you can check how much your design can handle and accordingly decide the correct performances range (refer Fig. 1).

Why do you need one?
An acquisition system together with a stimulus system like signal generator makes up a complete testing solution that can feed a device with real-world signals and acquire and analyse the resulting outputs. You can measure the output at various test points easily but it makes sense only when you can control what goes in, and that you can only do when a signal generator is connected at the input.

Fig. 1: Ideal vs waveform after adding controlled distortion
Fig. 1: Ideal vs waveform after adding controlled distortion

Signal generators find various applications, all of which fall under three main categories: margin testing, verification and characterisation.

Margin testing. Engineers need to test their designs in order to ensure that these meet the design specifications across the full range of operation. This is known as margin testing. This is a design challenge that requires a complete setup with stimulus and acquisition systems. With such a system you vary all the input parameters using the stimulus and check the limits until which you get the acceptable output. This testing also identifies how much stress the device can take while still doing its reliable operation.

Characterisation. Every device comes with characterisation information provided by its manufacturer. Characterisation information is actually a behaviour summary of the device. This information becomes available after the device has undergone complete characterisation tests and the data has been analysed. The characterisation data gives the designer a better understanding of the device characteristics, to better judge the acceptability of the device to the application. Again, to run such tests, you need both a stimulus system like signal generator and an acquisition system like oscilloscope.

Verification. Verification is done to check whether a product meets a set of initial design requirements, specifications and regulations. In the development phase, verification procedures involve special tests on various modules. The results are modeled and the modeled results are reviewed or analysed to see whether the product complies with specifications. In the post-development phase, verification procedures involve regularly repeating tests devised specifically to ensure that the product continues to meet the initial design requirements, specifications and regulations as time progresses. A lot of development phase and post-development phase verification tests require a stimulus system like signal generator.

Type of signal generators
Signal generators are broadly divided into mixed-signal generators and logic sources, covering the whole range of signal-generation needs. Each of these types has its unique strengths that make it more or less suitable for specific applications.

Mixed-signal generators. These are designed to output waveforms with analogue characteristics. They produce various types of waveforms like sine, square/rectangular, sawtooth/triangular, step/pulse and various complex waves. These waveforms show the rounding and imperfections that are part of every real-world signal. Various parameters of these waveforms, such as amplitude, frequency, phase, DC offset and rise time/fall time, can be controlled in a mixed-signal generator. You can even create aberrations such as overshoot and add edge jitter, modulation and more.

Fig. 2: Amplitude of an AC waveform
Fig. 2: Amplitude of an AC waveform

Logic/digital sources. True digital sources, on the other hand, are designed to drive digital systems. Unlike mixed-signal generators, these cannot generate analogue waveforms like sine or triangular waves. Such signal generators output binary pulse streams. Their features are optimised for computer bus needs and similar applications.

Analogue or digital signal generator
Most signal generators today are based on digital technology. Many can fulfil analogue requirements as well. Arbitrary waveform generators (AWGs) and function generators are primarily designed for analogue and mixed-signal applications. These instruments use sampling techniques to build and modify waveforms of almost any imaginable shape.

What to look for while buying one
This is actually a difficult question, and the selection is completely driven by your specific needs. A detailed understanding of your requirements and specifications mentioned in the datasheet can help you select the right signal generator for you. A signal generator produces various waveforms and its specifications are directly related to waveform properties.

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