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'''Signal generators''', also known variously as '''[[function generator]]s''', '''RF and microwave signal generators''', '''pitch generators''', '''[[arbitrary waveform generator]]s''', '''[[digital pattern generator]]s''' or '''frequency generators''' are [[Electronics|electronic]] devices that generate repeating or non-repeating electronic signals (in either the analog or digital domains). They are generally used in designing, testing, troubleshooting, and repairing electronic or electroacoustic devices; though they often have artistic uses as well.

There are many different types of signal generators, with different purposes and applications (and at varying levels of expense); in general, no device is suitable for all possible applications.

Traditionally, signal generators have been embedded hardware units, but since the age of multimedia-PCs, flexible, programmable software tone generators have also been available.

== General purpose signal generators ==
=== Function generators ===
[[Image:Leader LSG-15 signal generator.jpg|thumb|350px|[[Leader Instruments]] LSG-15 signal generator]]
{{main|Function generator}}

A '''[[function generator]]''' is a device which produces simple repetitive [[waveform]]s. Such devices contain an [[electronic oscillator]], a [[Electronic circuit|circuit]] that is capable of creating a repetitive [[waveform]]. (Modern devices may use [[digital signal processing]] to synthesize waveforms, followed by a [[digital to analog converter]], or DAC, to produce an analog output). The most common waveform is a [[sine wave]], but [[sawtooth wave|sawtooth]], step ([[pulse (signal processing)|pulse]]), [[square wave|square]], and [[triangle wave|triangular]] waveform oscillators are commonly available as are [[arbitrary waveform generator]]s (AWGs). If the oscillator operates above the [[audio frequency]] range (>20 [[kHz]]), the generator will often include some sort of [[modulation]] function such as [[amplitude modulation]] (AM), [[frequency modulation]] (FM), or [[phase modulation]] (PM) as well as a second oscillator that provides an [[audio frequency]] modulation waveform.

Function generators are typically used in simple electronics repair and design; where they are used to stimulate a circuit under test. A device such as an [[oscilloscope]] is then used to measure the circuit's output. Function generators vary in the number of outputs they feature, frequency range, frequency accuracy and stability, and several other parameters.

=== Arbitrary waveform generators ===
{{main|Arbitrary waveform generator}}      

'''Arbitrary waveform generators''', or '''AWG'''s, are sophisticated signal generators which allow the user to generate arbitrary waveforms, within published limits of [[frequency]] range, accuracy, and output level. Unlike function generators, which are limited to a simple set of waveforms; an AWG allows the user to specify a source waveform in a variety of different ways. AWGs are generally more expensive than function generators, and are often more highly limited in available bandwidth; as a result, they are generally limited to higher-end design and test applications.

=== RF and microwave signal generators ===
'''[[Radio frequency|RF (radio frequency)]] and [[microwave]] signal generators''' are used for testing components, receivers and test systems in a wide variety of applications including cellular communications, [[WiFi]], [[WiMAX]], [[Global Positioning System|GPS]], audio and video broadcasting, satellite communications, [[radar]] and [[electronic warfare]]. RF and microwave signal generators normally have similar features and capabilities, but are differentiated by frequency range. RF signal generators typically range from a few kHz to 6 GHz, while microwave signal generators cover a much wider frequency range, from less than 1 MHz to at least 20 GHz. Some models go as high as 70 GHz with a direct coaxial output, and up to hundreds of GHz when used with external waveguide source modules. RF and microwave signal generators can be classified further as analog or vector signal generators.

==== Analog signal generators ====

Analog Signal Generators are capable of producing [[Continuous wave|CW (continuous wave)]] tones. The output frequency can be tuned anywhere over their entire frequency range. In addition, many models offer various types of analog modulation, either as standard equipment or as an optional capability to the base unit. This could include [[Amplitude modulation|AM]], [[Frequency modulation|FM]], [[Phase modulation|ΦM (phase modulation)]] and [[pulse modulation]]. Another common feature is a built-in [[Attenuator (electronics)|attenuator]] which makes it possible to vary the signal’s output power. Depending on the manufacturer and model, output powers can range from -135 to +30 dBm. A wide range of output power is desirable, since different applications require different amounts of signal power. For example, if a signal has to travel through a very long cable out to an antenna, a high output signal may be needed to overcome the losses through the cable and still have sufficient power at the antenna. But when testing receiver sensitivity, a low signal level is required to see how the receiver behaves under low signal-to-noise conditions.  
Analog signal generators have been available for decades from various manufacturers. One of the earliest implementations was the [[HP200A]] Audio Oscillator, the first product sold by the [[Hewlett-Packard|Hewlett-Packard Company]] in 1939.

==== Vector signal generators ====

With the advent of digital communications systems, it is no longer possible to adequately test these systems with traditional analog signal generators. This has led to the development of vector signal generators, also known as digital signal generators. These signal generators are capable of generating digitally-modulated radio signals that may use any of a large number of [[digital modulation]] formats such as [[QAM]], [[QPSK]], [[Frequency-shift keying|FSK]], [[BPSK]], and [[OFDM]]. In addition, since modern commercial digital communication systems are almost all based on well-defined industry standards, many vector signal generators can generate signals based on these standards. Examples include [[GSM]], [[WCDMA|W-CDMA (UMTS)]], [[CDMA2000]], [[Long Term Evolution|LTE]], [[WiFi|Wi-Fi (IEEE 802.11)]], and [[WiMAX|WiMAX (IEEE 802.16)]]. In contrast, military communication systems such as [[JTRS]], which place a great deal of importance on robustness and information security, typically use very proprietary methods. To test these types of communication systems, users will often create their own custom waveforms and download them into the vector signal generator to create the desired test signal.

==== Logic signal generators ====
Also known as 'data pattern generator' or more often '[[digital pattern generator]]', this type of signal generators produces logic types of signals - that is logic 1's and 0's in the form of conventional voltage levels. The usual voltage standards are: [[LVTTL]], [[LVCMOS]].
As such, they must be distinguished from 'pulse/pattern generators', which refers to signal generators able to generate logic pulses with different analog characteristics (such as pulse rise/fall time, high level length, ...).

Logic signal generators (digital pattern generators) are used as stimulus source for digital integrated circuits and embedded systems - for functional validation and testing.

== Special purpose signal generators ==
[[File:Tone-generator-and-wire-tracker-0a.jpg|thumb|right|A pitch generator and a probe for locating a specific pair of wires amongst many, for example in a [[punch block]].]]

In addition to the above general-purpose devices, there are several classes of signal generators designed for specific applications.

=== Pitch generators and audio generators ===

A '''pitch generator''' is a type of signal generator optimized for use in [[sound reproduction|audio]] and [[acoustics]] applications. Pitch generators typically include sine waves over the [[audio frequency]] range (20 [[Hertz|Hz]]&ndash;20&nbsp;kHz). Sophisticated pitch generators will also include '''sweep generators''' (a function which varies the output frequency over a range, in order to make frequency-domain measurements), '''multipitch generators''' (which output several pitches simultaneously, and are used to check for [[intermodulation distortion]] and other non-linear effects), and '''tone bursts''' (used to measure response to transients). Pitch generators are typically used in conjunction with [[sound level meter]]s, when measuring the acoustics of a room or a sound reproduction system, and/or with oscilloscopes or specialized audio analyzers. 

Many pitch generators operate in the [[digital]] domain, producing output in various digital audio formats such as [[AES-3]], or [[SPDIF]]. Such generators may include special signals to stimulate various digital effects and problems, such as [[clipping]], [[jitter]], [[bit error]]s; they also often provide ways to manipulate the [[metadata]] associated with digital audio formats.

The term [[synthesizer]] is used for a device that generates audio signals for music, or that uses slightly more intricate methods.

==== Computer programs ====

Whilst professional signal generators can be expensive, casual hobbyists can make use of computer programs which generate signals and use the sound card to output the signal as audio. These programs can be fun for experimentation but are often limited by the hardware capabilities of the sound card to generate signals only within the aural band.

=== Video signal generators ===
{{main|Video signal generator}}
A '''video signal generator''' is a device which outputs predetermined [[video]] and/or [[television]] waveforms, and other signals used to stimulate faults in, or aid in parametric measurements of, television and video systems. There are several different types of video signal generators in widespread use. Regardless of the specific type, the output of a video generator will generally contain [[synchronization]] signals appropriate for television, including horizontal and vertical sync pulses (in analog) or sync words (in digital). Generators of ''composite'' video signals (such as [[NTSC]] and [[PAL]]) will also include a [[colorburst]] signal as part of the output. Video signal generators are available for a wide variety of applications, and for a wide variety of digital formats; many of these also include audio generation capability (as the audio track is an important part of any video or television program or motion picture).

==Technical Trends Driving the ARB Industry==
New high-speed DACs provide up to 16-bit resolution at sample rates in excess of 1 GS/s. These devices provide the foundation for an AWG with the bandwidth and dynamic range to address modern radio and communication applications. In combination with a quadrature modulator and advanced digital signal processing, high-speed DACs can be applied to create a full-featured vector signal generator with very high modulation bandwidth. Example applications include commercial wireless standards such as Wi-Fi (IEEE 802.11), WiMAX (IEEE 802.16) and LTE, in addition to military standards such as those specified in the Joint Tactical Radio System (JTRS) initiative. Also, broad modulation bandwidth allows multi-carrier signal generation, necessary for testing receiver adjacent channel rejection.

==See also==
*[[A/N URM-25D Signal Generator]], Cold War-era hardware still in use today.
*[[Digital Pattern Generator]], for generating digital (logic) type of signals

==External links==
*[http://onlinetonegenerator.com Free online audio tone generator]
*[http://www.bkprecision.com/support/downloads/guides/Function_and_Arbitrary_Waveform_Generator_Guidebook/BK-Function-Generator-and-AWG-Guidebook.pdf Function Generator & Arbitrary Waveform Generator Guidebook]
*[http://www.radio-electronics.com/info/t_and_m/generators/rf-signal-generator-specs-specifications.php Understanding signal generator specifications]
*[http://www.byteparadigm.com/digital-pattern-generator-101.html Digital Pattern Generator (Digital Signal Generator) - Background Information]
*[http://www.ztecinstruments.com/waveform-generator-fundamentals Waveform Generator Fundamentals]
*[http://www.ztecinstruments.com/zconnect/?p=1213 Blog Post: All About ABRS (Waveform Generators)]
{{Electrical and electronic measuring equipment}}
[[Category:Laboratory equipment]]
[[Category:Electronic test equipment]]

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