Frequency-locked loop

A frequency-lock, or frequency-locked loop (FLL), is an <a href="/facts/Electronics/E4JhgISq">electronic</a> <a href="/facts/Control_system/8CEjIDlS">control system</a> that generates a signal that is locked to the frequency of an input or "reference" signal. This circuit compares the frequency of a controlled oscillator to the reference, automatically raising or lowering the frequency of the oscillator until its frequency (but not necessarily its phase) is matched to that of the reference.
A frequency-locked loop is an example of a control system using <a href="/facts/Negative_feedback/G9byVgpN">negative feedback</a>. Frequency-lock loops are used in radio, telecommunications, computers and other electronic applications to generate stable frequencies, or to recover a signal from a noisy communication channel.
A frequency-locked loop is similar to a <a href="/facts/Phase-locked_loop/sgmca8aQ">phase-locked loop</a> (PLL), but only attempts to control the <a href="/facts/Derivative/7Ito70Dh">derivative</a> of phase, not the phase itself. Because it tries to do less, an FLL can acquire lock faster and over a wider range than a PLL. Sometimes the two are used in combination, with a frequency-locked loop used initially until the oscillator frequency is close enough to the reference that a PLL can take over.
Advanced applications can use both simultaneously, creating what is called an "FLL-assisted PLL" (FPLL).

Frequency-locked loop open-in-new

Frequency-locked loop