frekvenslåsløkke

A frekvenslåsløkke, often translated as a frequency-locked loop (FLL), is a control system that generates an output signal whose frequency is proportional to the frequency of an input signal. Unlike a phase-locked loop (PLL), which also locks to the phase of the input signal, an FLL primarily focuses on matching the frequency. This makes FLLs less sensitive to phase noise and jitter in the input signal.

The core component of an FLL is a frequency detector. This detector compares the input signal frequency with the frequency of a voltage-controlled oscillator (VCO). The output of the frequency detector is a voltage that is proportional to the difference between the two frequencies. This voltage is then used to adjust the VCO's frequency until it matches the input signal's frequency. A loop filter is typically included to smooth out the control voltage and improve stability.

FLLs are useful in applications where accurate frequency tracking is more critical than precise phase alignment. Examples include demodulating frequency-modulated (FM) signals, synchronizing oscillators in communication systems, and in sensor applications where a physical quantity is converted into a frequency. They can also be used for frequency synthesis, generating stable output frequencies derived from a reference input. The simplicity of an FLL compared to a PLL, when only frequency locking is required, can lead to simpler and more cost-effective designs.