frequency-locked loop

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A frequency-lock, or frequency-locked loop (FLL), is an electronic control system that generates a signal that is locked to the frequency of an input or "reference" signal.{{cite web |last1=Chaudhari |first1=Qasim |title=How a Frequency Locked Loop (FLL) Works |url=https://wirelesspi.com/how-a-frequency-locked-loop-fll-works/ |website=Wireless Pi |date=6 June 2016 |access-date=5 January 2024}} 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 negative feedback. 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 phase-locked loop (PLL), but only attempts to control the derivative 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).{{cite journal

|title=An iterative filter for FLL-assisted-PLL carrier tracking at low C/N₀ and high dynamic conditions

|journal=IEEE Transactions on Aerospace and Electronic Systems

|volume=58 |issue=1 |date=February 2022 |pages=275–289

|first1=Rong |last1=Yang |first2=Xingqun |last2=Zhan

|first3=Wantong |last3=Chen |first4=Yafeng |last4=Li

|doi=10.1109/TAES.2021.3100354 |bibcode=2022ITAES..58..275Y

|url=https://www.researchgate.net/publication/353595957

}}