Design Of High Stability Cyrstal Oscillator With Automatic Amplitude Control

With the development of science and technology,wearable devices and Internet of Things technologies have mushroomed.Therefore,low power consumption,low cost and high stability have become an urgent demand for portable mobile devices and even the whole integrated circuit chip.In the circuit system,the generation of clock is also an essential part,because it can not only be used to provide clock signal to the timer device,but also can be used as a real-time clock to wake up the low power circuit system in hibernation mode and so on.As everyone knows,quartz crystal oscillator is of high quality factor and the oscillation frequency stability features,it is widely used in various kinds of real-time clock control circuit.For crystal oscillator from starting to frequency stabilization,the generated different clock frequencies require different start time of oscillation,such as from hundreds of microseconds to a few seconds.Therefore,in the process of osillation,the longer it takes,the longer the chip needs to wait for power up,which means that the circuit system will generate more power consumption.In this paper,the crystal oscillator circuit structure is based on the design of amplitude detection control circuit module,through the amplitude detected by this module,the output signal control crystal oscillator negative resistance value.Once the amplitude stabilizes,the control circuit is turned off without consuming large amounts of current.Through the circuit design in this way,the start up time of crystal oscillation is obviously shortened,but also the overall power consumption of the circuit can be reduced.The circuit in this paper is designed to detect amplitude by automatic amplitude detection control circuit,so as to control the transconductance value of crystal oscillator.Once the detected amplitude is small,the circuit of control transconductance is switch on,increasing the value of the circuit transconductance.So that the circuit oscillate quickly,it can reduce the oscillation time effectively.When the start up of oscillation is completed,the detection range becomes larger,the control switch is turned off,the transconductance of the circuit is reduced,so that the power consumption is correspondingly reduced.For various reasons,the output frequency instability of crystal oscillator is greatly affected by the input voltage of the power supply.Therefore,it is necessary to design a Low Dropout Regulation(LDO)that can provide stable voltage for crystal oscillator.Based on the process of 0.13μm CMOS,the crystal oscillator designed in this paper at a voltage of 2～5.5V with a stable output frequency of 32.768 k Hz.The start-up time of oscillation is about 280 ms.When the crystal oscillator amplitude stabilizes,the enhancement circuit of negative resistance is turned off through an automatic amplitude control circuit.The average current of the crystal oscillator is less than 1μA,which effectively reduces the overall power consumption of the circuit.In this circuit,LDO provides a stable output voltage of the crystal oscillator.the crystal oscillator adopts Pierce structure,and uses AAC feedback method to monitor the oscillating amplitude of the oscillator continuously.