Editing Function generator (section)

== Electronic instruments ==

=== Principles of Operation ===
Simple function generators usually generate triangular waveform whose frequency can be controlled smoothly as well as in steps.<ref name="Introduction to System Design Using Integrated Circuits">{{cite book |last=Sonde |first=B. S. |title=Introduction to System Design Using Integrated Circuits |year=1992 |publisher=New Age International |isbn=978-81-224-0386-2 |pages=244–246 |url=https://books.google.com/books?id=-ICzJGftw8gC&q=function+generator&pg=PA244}}</ref>  This triangular wave is used as the basis for all of its other outputs. The triangular wave is generated by repeatedly charging and discharging a [[capacitor]] from a constant [[current source]]. This produces a [[linear]]ly ascending and descending voltage ramp. As the output voltage reaches upper or lower limits, the charging or discharging is reversed using a [[comparator]], producing the linear triangle wave. By varying the [[current (electricity)|current]] and the size of the capacitor, different [[frequency|frequencies]] may be obtained. [[Sawtooth wave]]s can be produced by charging the capacitor slowly with low current, but using a diode over the current source to discharge quickly - the polarity of the diode changes the polarity of the resulting sawtooth, i.e. slow rise and fast fall, or fast rise and slow fall.

A 50% [[duty cycle]] [[Square wave (waveform)|square wave]] is easily obtained by noting whether the capacitor is being charged or discharged, which is reflected in the current switching comparator output. Other duty cycles (theoretically from 0% to 100%) can be obtained by using a comparator and the sawtooth or triangle signal. Most function generators also contain a non-linear [[diode]] [[Waveform shaping|shaping circuit]] that can convert the triangle wave into a reasonably accurate [[sine wave]] by rounding off the corners of the triangle wave in a process similar to [[Clipping (audio)|clipping]] in audio systems.

A [[walking ring counter]], also called a [[Johnson counter]], and a (linear) resistor-only shaping circuit is an alternative way to produce an approximation of a sine wave.
This is perhaps the simplest [[numerically-controlled oscillator]].
Two such walking ring counters are perhaps the simplest way to generate the [[continuous-phase frequency-shift keying]]
used in [[dual-tone multi-frequency signaling]] and early [[modem]] tones.<ref>
Don Lancaster.
[https://www.tinaja.com/ebooks/tvtcb.pdf "TV Typewriter Cookbook"]. ([[TV Typewriter]]).
1976.
p. 180-181.
</ref>

A typical function generator can provide frequencies up to 20&nbsp;MHz. RF generators for higher frequencies are not function generators in the strict sense since they typically produce pure or modulated sine signals only.

Function generators, like most [[signal generator]]s, may also contain an [[Attenuator (electronics)|attenuator]], various means of [[modulation|modulating]] the output waveform, and often the ability to automatically and repetitively "sweep" the frequency of the output waveform (by means of a [[voltage-controlled oscillator]]) between two operator-determined limits. This capability makes it very easy to evaluate the [[frequency response]] of a given [[electronic circuit]].

Some function generators can also generate [[white noise|white]] or [[pink noise]].{{Citation needed|date=March 2012}}

More advanced function generators are called [[arbitrary waveform generator]]s (AWG). They use [[Direct digital synthesizer|direct digital synthesis]] (DDS) techniques to generate any waveform that can be described by a table of amplitudes and time steps.

=== Specifications ===
Typical specifications for a general-purpose function generator are:
* Produces sine, square, triangular, sawtooth (ramp), and pulse output. [[Arbitrary waveform generator]]s can produce waves of any shape.<ref name=bakshi/>
* It can generate a wide range of frequencies. For example, the Tektronix FG 502 (ca 1974) covers 0.1&nbsp;Hz to 11&nbsp;MHz.<ref name="FG502">{{Citation |title=FG 502 Function Generator |year=1973 |series=Instruction Manual |publisher=Tektronix |location=Beaverton, OR }}, pp=1-7&ndash;1-8</ref>
* Frequency stability of 0.1 percent per hour for analog generators<ref name="FG502"/> or {{nowrap|500 ppm}} for a digital generator.
* Maximum sinewave [[distortion]] of about 1% (accuracy of diode shaping network) for analog generators.<ref>FG 502 distortion is {{nowrap|0.5 percent}}</ref> Arbitrary waveform generators may have distortion less than {{nowrap|-55 dB}} below {{nowrap|50 kHz}} and less than {{nowrap|-40 dB}} above {{nowrap|50 kHz.}}
* Some function generators can be phase locked to an external signal source, which may be a frequency reference or another function generator.
* Amplitude modulation (AM), frequency modulation (FM), or phase modulation (PM) may be supported.
* Output amplitude up to {{nowrap|10 V}} [[peak-to-peak]].
* Amplitude can be modified, usually by a calibrated [[Attenuator (electronics)|attenuator]] with decade steps and continuous adjustment within each decade.
* Some generators provide a DC offset voltage, e.g. adjustable between -5V to +5V.<ref name=bakshi/>
* An output impedance of {{nowrap|50 Ω}}.

=== Software ===
A completely different approach to function generation is to use [[software]] instructions to generate a waveform, with provision for output. For example, a general-purpose [[digital computer]] can be used to generate the waveform; if frequency range and amplitude are acceptable, the [[sound card]] fitted to most computers can be used to output the generated wave.