Principles of Operation Simple function generators usually generate triangular waveform whose frequency can be controlled smoothly as well as in steps. 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
linearly 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 and the size of the capacitor, different
frequencies may be obtained.
Sawtooth waves 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 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 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 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. A typical function generator can provide frequencies up to 20 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 generators, may also contain an
attenuator, various means of
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 or
pink noise. More advanced function generators are called
arbitrary waveform generators (AWG). They use
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 generators can produce waves of any shape. • Frequency stability of 0.1 percent per hour for analog generators Arbitrary waveform generators may have distortion less than below and less than above • 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
peak-to-peak. • Amplitude can be modified, usually by a calibrated
attenuator with decade steps and continuous adjustment within each decade. • Some generators provide a DC offset voltage, e.g. adjustable between -5V to +5V. • An output impedance of .
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. == Circuit elements ==