Frequency-locked loop

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.^[1] 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).^[2]

References

^ Chaudhari, Qasim. "How a Frequency Locked Loop (FLL) Works". Wireless Pi. Wireless Pi. Retrieved 5 January 2024.
^ Yang, Rong; Zhan, Xingqun; Chen, Wantong; Li, Yafeng (February 2022). "An iterative filter for FLL-assisted-PLL carrier tracking at low C/N₀ and high dynamic conditions" (PDF). IEEE Transactions on Aerospace and Electronic Systems. 58 (1): 275–289. Bibcode:2022ITAES..58..275Y. doi:10.1109/TAES.2021.3100354.