Function Generator

Function Generator

Function and arbitrary waveform generators are among the most important and versatile pieces of electronic test equipment. In electronic design and troubleshooting, the circuit under scrutiny often requires a controllable signal to simulate its normal operation. The testing of physical systems and transducers often needs stable and reliable signals. The signal levels needed range from microvolts to tens of volts or more.

Modern DDS (direct digital synthesis) function generators are able to provide a wide variety of signals. Today's basic units are capable of sine, square, and triangle outputs from less than 1 Hz to at least 1 MHz, with variable amplitude and adjustable DC offset. Many generators include extra features:

• Higher frequency capability
• Variable symmetry
• Frequency sweep
• AM /FM modulation
• Gated burst mode
• More advanced models offer a variety of additional waveforms
• Arbitrary Waveform Generators can supply user-defined periodic waveforms

Function generators are used where stable and repeatable stimulus signals are needed.
Here are some common uses and users:

• Research and development
• Educational institutions
• Electronic and electrical equipment repair businesses
• Stimulus/response testing, frequency response characterization, and in-circuit signal injection
• Electronic hobbyists

There are a variety of function generators on the market spanning the cost range from a few tens of dollars to tens of thousands of dollars.

Arbitrary Function Generators (AFG) vs. Arbitrary Waveform Generators (AWG)

Arbitrary generators can be classified into arbitrary/function generators (AFG) and arbitrary waveform generators (AWG). The arbitrary/function generator (AFG) serves a wide range of stimulus needs; in fact, it is the prevailing signal generator architecture in the industry today. Typically, this instrument offers fewer waveform variations than its AWG equivalent, but with excellent stability and fast response to frequency changes. If the DUT requires the classic sine and square waveforms (to name a few) and the ability to switch almost instantly between two frequencies, the arbitrary/function generator (AFG) is the right tool. An additional virtue is the AFG’s low cost, which makes it very attractive for applications that do not require an AWG’s versatility.

The AFG shares many features with the AWG, although the AFG is by design a more specialized instrument. The AFG offers unique strengths: it produces stable waveforms in standard shapes, particularly the all-important sine and square waves – that are both accurate and agile. Agility is the ability to change quickly and cleanly from one frequency to another.
Most AFGs offer some subset of the following familiar wave shapes:

• Sine
• Square
• Triangle
• Sweep
• Pulse
• Ramp
• Modulation
• Haversine

While AWGs can certainly provide these same waveforms, today’s AFGs are designed to provide improved phase, frequency, and amplitude control of the output signal. Moreover, many AFGs offer a way to modulate the signal from internal or external sources, which is essential for some types of standards compliance testing.

In the past, AFGs created their output signals using analog oscillators and signal conditioning. More recent AFGs rely on Direct Digital Synthesis (DDS) techniques to determine the rate at which samples are clocked out of their memory.

The arbitrary waveform generator (AWG) can produce any waveform you can imagine. You can use a variety of methods – from mathematical formulae to “drawing” the waveform – to create the needed output. Fundamentally, an arbitrary waveform generator (AWG) is a sophisticated playback system that delivers waveforms based on stored digital data that describes the constantly changing voltage levels of an AC signal. It is a tool whose block diagram is deceptively simple. To put the AWG concept in familiar terms, it is much like a CD player that reads out stored data (in the AWG, its own waveform memory; in the CD player, the disc itself) in real time. Both put out an analog signal, or waveform.

Parameters typical to Arbitrary Waveform Generators (AWGs)

• Memory Depth (record length). Determines the maximum number of samples that can be stored. Higher memory depth permits reproducing more complex waveforms
• Sample Rate. Usually specified in terms of megasamples or gigasamples per second, denotes the maximum clock or sample rate at which the instrument can operate
• Bandwidth. An instrument’s bandwidth is an analog term that exists independently of its sample rate. The analog bandwidth of a signal generator’s output circuitry must be sufficient to handle the maximum frequency that its sample rate will support. In other words, there must be enough bandwidth to pass the highest frequencies and transition times that can be clocked out of the memory, without degrading the signal characteristics
• Vertical (Amplitude) Resolution. Vertical resolution pertains to the binary word size, in bits, of the instrument’s DAC, with more bits equating to higher resolution. The vertical resolution of the DAC defines the amplitude accuracy and distortion of the re-produced waveform
• Horizontal (Timing) Resolution. Horizontal resolution expresses the smallest time increment that can be used to create waveforms
• Region Shift. The region shift function shifts a specified edge of a waveform either right or left, toward or away from the programmed center value. If the specified amount of the shift is less than the sampling interval, the original waveform is re-sampled using data interpolation to derive the shifted values. Region shift makes it possible to create simulated jitter conditions and other tiny edge placement changes that exceed the resolution of the instrument
• Output Channels. Many applications require more than one output channel from the signal generator
• Digital Outputs. Some AWGs include separate digital outputs
• Filtering. Filtering allows you to remove selected bands of frequency content from the signal
• Sequencing. Permits repeating sections of the waveform saving valuable memory
• Tools to create and edit waveforms as well as import data created outside the tools