Meaning
Control system that generates an output signal whose phase is related to the phase of an input reference signal is the standard mechanism for frequency generation. A phase locked loop operates by continuously comparing the phase of its output with a reference signal and adjusting the output frequency to minimize the difference. This closed-loop feedback system is used for clock recovery and frequency demodulation.
It is a fundamental block in modern digital and radio-frequency integrated circuits. The system remains in a locked state as long as the input reference frequency stays within the tracking range of the loop.
Loop Filter
Dynamic response of the feedback loop is determined by the low-pass filter that sits between the phase detector and the voltage-controlled oscillator. This filter removes high-frequency components from the phase error signal, preventing them from modulating the oscillator frequency. The design of this filter determines the lock time and the loop bandwidth of the system.
Frequency Synthesis
Multiplication of the reference frequency is achieved by placing a frequency divider in the feedback path of the system. By dividing the feedback signal, the system forces the output oscillator to run at a multiple of the reference frequency. This technique allows a single stable low-frequency reference, such as a quartz crystal, to generate a wide range of high frequencies.
Sourcing Decision
Selecting this feedback system for an integrated circuit design involves choosing between fully integrated solutions and discrete components. High-performance RF systems often use discrete loop filters to optimize noise performance, while digital chips use fully integrated circuits to minimize board area. The choice affects the final development cost and board space of the system.
