System and method of digital tuning a voltage controlled oscillator

摘要

A voltage controlled oscillator in a clock multiply unit includes a plurality of varactors controlled by a plurality of respective control voltage signals. The plurality of varactors allows for a combination of efficient methods to set an output signal frequency for the voltage controlled oscillator. For example, a pair of independent varactors are controlled by a differential control voltage signal. Differential control improves noise immunity and fine tuning ranges of the voltage controlled oscillator. The voltage controlled oscillator determines an operating frequency range automatically at start-up or upon reset. For example, a digital coarse tuning circuit starts the voltage controlled oscillator oscillating at a frequency corresponding to a frequency in a lowest range of operating frequencies, compares an output generated by the voltage controlled oscillator with a reference signal, increases the oscillation frequency to a frequency in the next higher range if the frequency of the reference signal is higher than the frequency of the generated output, repeats the comparisons and possible increments for a predetermined number of times, and shuts down.