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Analog Integrated Circuit Design, 2nd Edition by Kenneth W. Martin, David A. Johns, Tony Chan Carusone

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Practically all digital and radio frequency circuits and most analog circuits require a precision oscillator. Unfortunately, integrated circuit oscillators are not, on their own, suitable for use as frequency- or time-references in high-performance circuits. A major problem is that their frequency of oscillation is not known precisely. Furthermore, the jitter of integrated circuit oscillators (which can be thought of as random fluctuations in frequency) is too high for many applications. Therefore, integrated circuit oscillators are most often inserted into a phase locked loop where their operating frequency and phase is continuously compared against a precise external time-reference and automatically tuned to maintain a constant frequency and phase. The process is analogous to generating a precise on-chip voltage or current based on an externally-supplied reference. In summary, a phase-locked loop is a circuit that uses an external timing reference to measure and generate on-chip frequencies and times accurately.

It is natural to wonder why, if a precise time-reference is available, a phase locked loop is required at all. Why not simply use the time-reference (usually itself a clock) directly? One reason is that the required frequency may be different than that of the reference. This is frequently the case since timing references are commonly derived from precision resonators ...

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