OSCILLATOR CIRCUIT

摘要

An oscillator circuit comprises a first, high-Q crystal oscillator and a second, low-Q oscillator arranged for kick-starting the crystal oscillator at switch-on by coupling the second oscillator to the first oscillator for a time period. The oscillator circuit is arranged to select the frequency of the second oscillator by placing the second oscillator in a phase locked loop with the first oscillator providing a reference frequency, and adjusting the frequency of the second oscillator towards the frequency of the first oscillator.

主权项

1. An oscillator circuit comprising:
a first oscillator comprising a crystal and arranged to generate a first oscillation signal at a first frequency, wherein the first oscillator has a first Q-factor; a second oscillator arranged to generate a second oscillation signal at a second frequency, wherein the second oscillator has a second Q-factor lower than the first Q-factor; a phase detector arranged to generate a difference signal indicative of a phase difference between the first and second oscillation signals; a filter arranged to generate a frequency control signal by filtering the difference signal; a frequency control storage device arranged to store a value of the frequency control signal; a selector switch (SW1) having selectable first and second selector states, wherein in the first selector state the selector switch (SW1) is arranged to deliver to a frequency control input of the second oscillator the stored value of the frequency control signal, and in the second selector state the selector switch (SW1) is arranged to deliver to the frequency control input of the second oscillator the frequency control signal generated by the filter whereby the second frequency is tuned towards the first frequency; a gating switch (SW2) having selectable first and second gating states, wherein in the first gating state the gating switch (SW2) is arranged to couple an output of the second oscillator to a terminal of the crystal and in the second gating state the gating switch (SW2) is arranged to de-couple the output of the second oscillator from the terminal of the crystal; and a controller arranged to select the states of the selector switch (SW1) and the gating switch (SW2), to switch-on the first and second oscillators and to select the states of the selector switch (SW1) and the gating switch (SW2) such that for a first time period commencing when both of the first and second oscillators have been switched on, the selector switch (SW1) has the first selector state and the gating switch (SW2) has the first gating state, and for a second time period commencing at completion of the first time period, the gating switch (SW2) has the second gating state, wherein a duration of the first time period is dependent on a difference between the first frequency and the second frequency and is within the range12F1-F2±25%,  where F1 is the first frequency and F2 is the second frequency.