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PLL frequency synthesizer

A frequency synthesizer is a circuit that generates one or more output signals whose frequency bears a prescribed relation to the frequency of an input reference signal. Such circuits are used as accurate frequency sources in a wide variety of electronic apparatus. Frequency synthesizers are widely used in modern radio communication systems. Such devices typically make use of a single quartz-controlled reference oscillator combined with a phase-locked loop to provide a multitude of output frequencies traceable to the highly stable reference from the oscillator. A phase looked-loop (PLL) frequency synthesizer uses a feedback loop for synchronization in frequency and phase with a certain reference frequency signal and generates a signal of a target frequency by multiplying the reference frequency signal or by combining the reference frequency signal with another reference frequency signal. A PLL frequency synthesizer comprises a reference signal generator, a voltage controlled oscillator (VCO), a variable frequency divider, a phase frequency comparator, low pass filter, and a control voltage supplying circuit. The reference signal generator generates a reference signal with a reference frequency. The phase comparator detects a phase difference between an output frequency of the reference oscillator and an output frequency of the voltage controlled oscillator. A signal corresponding with this phase difference is supplied as a control voltage to the voltage controlled oscillator through the low pass filter. The phase difference signal causes the output signal from the VCO to change in frequency so as to minimise the phase difference between the frequency divided signal and the reference signal.

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