A
low frequency oscillator is an
oscillator - typically a
sine wave generator, although there is no inherent reason for it to be so - which produces frequencies typically in a range from less than 1hz to 200hz.
Although LFOs makes great desk ornaments, and they are used as scientific instruments, they are most commonly found in the field of electronic music, as a component - either a module, or integrated - of a synthesiser. The function of an LFO is sometimes performed by a regular oscillator - one which typically produces audible frequencies - a good example of this practice being the classic Arp 2600, in which each of the three oscillators can be used as an LFO. However, the majority of synthesisers in the analogue and digital eras use dedicated LFO circuitry. Although an LFO is of limited use on its own (sub-bass reinforcement and illegal psycho-acoustic noise experiments, essentially, as the human ear cannot determine pitch at such a low level), it is extremely useful as a control source.
For example, if the output of an LFO is routed into the modulation input of filter's cut-off frequency control, the cut-off frequency of the filter can be made to oscillate around a pre-set range at the frequency and amplitude of the LFO.
As another example, an LFO can be used to modulate the pitch of an oscillator, thus producing a vibrato effect, or it could be used to modulate the amplitude of an amplifier, thus producing tremolo. Filter modulation, vibrato and tremolo are the classic LFO effects, with pulse width modulation next on the list. Most synthesisers have several LFOs so as to modulate several parameters independently, and thet LFOs can be made to produce various different waveforms, usually including a 'random' wave.
With a modular sythesiser, an LFO can be routed to anything - other LFOs, a reverb unit, a slew limiter, a quadrastatic phase-shifting pitch-o-matic bloam-a-sträuft, and so forth. With a preset synthesiser the LFO can typically only modulate amplitude, pitch, and the level of the filter and its resonance control.