A low frequency oscillator, or LFO, is an oscillator designed to produce signals whose frequency is at the bottom of, or below, the audio range. Nearly all synthesizers have at least one LFO or a digital equivalent.It is useful for producing a repeating control signal for a wide variety of purposes; when added to the control input of a VCO it produces the effect known as vibrato, and if it is added to the control signal of a VCA, it produces tremolo. A typical LFO can generate a variety of useful waveforms, including sine, triangle, square, ramp-up, and ramp-down.The most useful LFOs have an upper frequency limit above 100 Hz and a lower limit below 1 Hz (such frequencies may be given as seconds/cycle; for instance, a frequency of 20 seconds/cycle corresponds to 0.05 Hz). An LFO may or may not have voltage control over its frequency or other wave parameters; an LFO with voltage control over the frequency is sometimes referred to as a "VCLFO".

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