A signal generator is an instrument that provides a controlled output signal for use in testing or aligning, or in measurements on other circuits or equipment. Typical classification of signal generators includes audio, function and radio frequency generators. 

Audio Generators typically produce sine waves and symmetrical square waves using either output metering or a calibrated output attenuator, and produce controllable signal levels. 

Function Generators produce a wide variety of waveforms or periodic functions according to some basic function of time, such as sine, triangle and square waveforms and may additionally produce cosine, ramp or other waveforms. 

Radio Frequency (RF) Generators produce output frequencies and have a precision output atttenuator that allows setting of output levels. It can also produce either AM/FM modulation of the output signal so that receivers and other circuits may be tested or aligned.