Reducing Duration of Start-up Period for a Crystal Oscillator Circuit

Abstract

A crystal oscillator circuit comprises a crystal; oscillator circuitry for generating a crystal oscillation signal at an oscillation frequency; and a kick-start circuit for injecting pulses into the crystal during a start-up period. The oscillator circuitry comprises a differential pair of transistors and can operate in an oscillating mode or a start-up mode. In the oscillating mode, the differential pair of transistors is cross-coupled so that a gate terminal of one transistor is coupled to a drain terminal of the other transistor, and vice versa, and the drain terminals are coupled to the crystal to generate the crystal oscillation signal. In the start-up mode, the kick-start circuit drives the gate terminals of the transistors with said pulses. This crystal oscillator circuit has a decreased start-up time compared to prior art solutions and a reduced influence of parasitic oscillations.

What is claimed is: 
     
         1 . An oscillator system, comprising:
 a crystal;   oscillator circuitry connected to the crystal and configured to:
 generate a crystal oscillation signal at an oscillation frequency; and 
 operate in a selectable one of at least an oscillating mode and a start-up mode; and 
   a kick-start circuit configured to inject a number of pulses into the crystal during a start-up period of the oscillator system;   wherein the kick-start circuit comprises an oscillator, said oscillator configured to:
 generate said number of pulses during the start-up period of the oscillator system; and 
 be phase locked to said oscillator circuitry during a time period where the oscillator circuitry operates in the oscillating mode. 
   
     
     
         2 . The oscillator system of  claim 1  wherein the oscillator of the kick-start circuitry is configured to be fee running during the start-up period of the oscillator system. 
     
     
         3 . The oscillator system of  claim 1 , wherein:
 the oscillator circuitry comprises a first differential pair of transistors;   in the oscillating mode, the first differential pair of transistors are cross-coupled so that:
 a gate terminal of a first transistor of the first differential pair is coupled to a drain terminal of a second transistor of the first differential pair; and 
 a gate terminal of the second transistor of the first differential pair is coupled to a drain terminal of the first transistor of the first differential pair; 
   each one of the drain terminals of the first and second transistors of the first differential pair further being coupled to a terminal of the crystal to generate the crystal oscillation signal at the oscillation frequency; and   in the start-up mode, the kick-start circuit is configured to drive the gate terminals of the first and second transistors of the first differential pair with said number of pulses during the start-up period of the oscillator system.   
     
     
         4 . The oscillator system of  claim 3  wherein the oscillator system is configured to, in the start-up mode of the oscillator circuit:
 disconnect the gate terminal of the first transistor of the first differential pair from the drain terminal of the second of the first differential pair; and 
 disconnect the gate terminal of the second transistor of the first differential pair from the drain terminal of the first transistor of the first differential pair. 
 
     
     
         5 . The oscillator system of  claim 3  wherein:
 the oscillator circuitry further comprises a second differential pair of transistors, the second differential pair of transistors being complementary to the first differential pair of transistors; 
 the second differential pair of transistors, in the oscillating mode, are cross-coupled so that:
 a gate terminal of a first transistor of the second differential pair is coupled to a drain terminal of a second transistor of the second differential pair; and 
 a gate terminal of the second transistor of the second differential pair is coupled to a drain terminal of the first transistor of the second differential pair; 
 each one of the drain terminals of the first and second transistors of the second differential pair further being coupled to a terminal of the crystal to generate the crystal oscillation signal at the oscillation frequency. 
 
 
     
     
         6 . The oscillator system of  claim 5 , wherein the kick-start circuit, in the start-up mode, is configured to drive the gate terminals of the first and second transistors of the second differential pair with the number of pulses during the start-up period of the oscillator system. 
     
     
         7 . The oscillator system of  claim 5 , wherein the oscillator system is configured to, in the start-up mode of the oscillator circuit:
 disconnect the gate terminal of the first transistor of the second differential pair from the drain terminal of the second transistor of the second differential pair; and   disconnect the gate terminal of the second transistor of the second differential pair from the drain terminal of the first transistor of the second differential pair.   
     
     
         8 . The oscillator system of  claim 3 , wherein each transistor of the first differential pair of transistors comprises a field effect transistor. 
     
     
         9 . The oscillator system of  claim 1  further comprising a capacitor configured to be:
 connected in parallel with the crystal for adjustment of the oscillation frequency when the oscillator circuitry is operating in the oscillating mode; and 
 disconnected from the crystal in the start-up mode. 
 
     
     
         10 . The oscillator system of  claim 1 , wherein the oscillator system is further configured to determine the number of pulses based on a difference in frequency between a frequency of the pulses and an oscillation frequency of an oscillation generated in the crystal by injecting the pulses into the crystal. 
     
     
         11 . The oscillator system of  claim 10 , wherein the oscillator system is configured to determine the number of pulses so that a phase difference between each one of the pulses and the oscillation signal generated in the crystal by injecting that one and previous pulses of the number of pulses into the crystal is between −π/2 and π/2. 
     
     
         12 . The oscillator system of  claim 11  wherein:
 the oscillator is further configured to inject at least a further number of pulses into the crystal during a further start-up period of the oscillator system, wherein a phase difference between pulses of the further start-up period and the crystal oscillation signal is between −π/2 and π/2; and 
 the start-up period and the further start-up period are separated by a time period where the phase difference exceeds π/2. 
 
     
     
         13 . An electronic apparatus, comprising an oscillator system comprising:
 a crystal;   oscillator circuitry connected to the crystal and configured to:
 generate a crystal oscillation signal at an oscillation frequency; and 
 operate in a selectable one of at least an oscillating mode and a start-up mode; and 
   a kick-start circuit configured to inject a number of pulses into the crystal during a start-up period of the oscillator system;   wherein the kick-start circuit comprises an oscillator, said oscillator configured to:
 generate said number of pulses during the start-up period of the oscillator system; and 
 be phase locked to said oscillator circuitry during a time period where the oscillator circuitry operates in the oscillating mode. 
   
     
     
         14 . The electronic apparatus of  claim 13 , wherein the electronic apparatus comprises a wireless communications device comprising a receiver and/or a transmitter for radio frequency signals. 
     
     
         15 . The electronic apparatus of  claim 13 , wherein the wireless communications device comprises a base station for a wireless communications system. 
     
     
         16 . The electronic apparatus of  claim 13 , wherein the wireless communications device comprises a mobile phone for use in a wireless communications system.