FLL (frequency-locked loop)

A frequency-locked loop (FLL) is a type of electronic circuit that is used to generate a stable output frequency that is synchronized with a reference frequency. FLLs are widely used in communication systems, signal processing, and digital circuits.

The basic idea behind an FLL is to compare the phase and frequency of a reference signal with those of a feedback signal, and then use a control mechanism to adjust the frequency of the feedback signal so that it matches the reference signal. The feedback signal is generated by a voltage-controlled oscillator (VCO), which is typically a free-running oscillator with an output frequency that is proportional to the input voltage. The reference signal, on the other hand, is a stable signal with a fixed frequency.

FLLs operate in a closed-loop feedback configuration, which means that the output frequency of the VCO is constantly adjusted until it matches the frequency of the reference signal. This is done by comparing the phases of the two signals and generating an error signal that is proportional to the phase difference between them. The error signal is then used to adjust the input voltage of the VCO, which in turn adjusts the frequency of the feedback signal.

The control mechanism used in an FLL is typically a low-pass filter or a phase detector. A low-pass filter is used to remove any high-frequency noise or jitter from the input signal, while a phase detector is used to compare the phase of the reference signal with that of the feedback signal. The phase detector generates an output voltage that is proportional to the phase difference between the two signals, which is then filtered and used to adjust the VCO input voltage.

The performance of an FLL depends on several factors, including the accuracy of the reference signal, the stability of the VCO, and the quality of the control mechanism. The accuracy of the reference signal is critical, as any errors in the reference frequency will be propagated to the output frequency of the FLL. The stability of the VCO is also important, as any fluctuations in the VCO output frequency will affect the accuracy of the output frequency of the FLL.

FLLs can be used in a wide range of applications, including frequency synthesis, phase-locked loops, and frequency modulation. In frequency synthesis, an FLL is used to generate a stable output frequency that is a multiple of the reference frequency. In phase-locked loops, an FLL is used to synchronize the phase of a signal with that of a reference signal. In frequency modulation, an FLL is used to generate a modulated output signal with a carrier frequency that is synchronized with the reference signal.

In summary, a frequency-locked loop is a type of electronic circuit that is used to generate a stable output frequency that is synchronized with a reference frequency. FLLs operate in a closed-loop feedback configuration, and use a control mechanism to adjust the frequency of a feedback signal generated by a voltage-controlled oscillator so that it matches the reference signal. FLLs can be used in a wide range of applications, including frequency synthesis, phase-locked loops, and frequency modulation.