The Circuit Research Of Low Phase Noise, Low Dissipation, Integrated Digitally Controlled Crystal Oscillator

Crystal oscillators are integral part of radio frequency (RF) and digital devices. With the high Q of an electromechanical quartz crystal as the frequency selecting component, the crystal oscillators with very high frequency stability and low phase noise provide very accuracy clock signal references.Crystal oscillators are widely used in many electronic systems. Technological advancements in wireless communication made massive deployment and user adoption of cellular phones possible in the last decade. However the increase of subscribers asks for more efficient use of the limited spectrum resource. With the development of communication,navigation,radar,mobile communication,programmed telephone and measured instrument devices, the high performance of crystal oscillators is demanded. It includes high frequency stability wide variable frequency range, excellent linearity,low phase noise,low power dissipation and low cost.Designing crystal oscillators with high thermal,process and voltage stabilities have been a hot-topic in industry.The paper summarizes the principle and performances of quartz resonator and voltage controlled or digitally controlled crystal oscillators. Also, the paper is concerned with the condition of reliable start-up of the oscillation, the automatic gain controlled feedback loop and the phase noise are discussed in this paper in order to get all design guides for high stability ,low power dissipation and low phase noise. Based on these design guides, the analysis and design of a low phase noise and low power dissipation, with 0. 18μm CMOS process, integrated Colpitts crystal oscillator with automatic gain controlled feedback loop as the amplitude-limiting technique, low dropout voltage regulation with band gap reference voltage and frequency tuning monotonicity guaranteed by digitally controlled variable capacitor array is performed to revise influence caused by process, voltage and temperature(PVT) conditions for. handset. The oscillation with very stable amplitude value and frequency value is achieved. under the condition of wide range of power supply voltage (2.7v-3.7v) and working temperature (-40°C-125°C) and process changing (TT, FF, SS, SF, FS). As the oscillation circuit dissipation is. proportional to the oscillation signal amplitude, the controlled oscillation signal amplitude makes sure of the low oscillation circuit dissipation. Special layout design technique is applied to prevent the circuit failure from Latch-up, electrostatic discharge (ESD), substrate noise and power line noise.The crystal oscillator is fabricated in a 0.18μm CMOS process. Based on simple circuit model, the rough value of oscillator circuit component is achieved. Further, by taking the advantages of EDA circuit design software such as MATLAB, Advanced Design System (ADS) and Spectre in Cadence, the accurate value of oscillator circuit component is achieved. The layout design tool is Virtuos (DIVA) in Cadence. The validating tool of post-simulation is Calibre (supplied by Mentor Graphics) or Assura (Supplied by Cadence), by which the DRC, LVS, Antenna, Density and Latch-up cheeks are needed.The paper presents an integrated 26MHz digitally controlled crystal oscillator DCXO) for GSM standard that achieves a phase noise of -142.1dBc/Hz at 1 kHz and -146.3dBc/Hz at 10 kHz offset. Designed low dropout voltage regulation ensured the measured supply pushing at 26MHz is 0.6ppm/v. Frequency tuning is done by a pre-distorted 14-bit capacitor array digital to analog converter (DAC) where the top 9 bits are thermometer coded and the bottom 5 bits are∑△modulated. Monotonicity of～0.0075 ppm/step across the entire～123ppm tuning range is guaranteed by applying a sliding∑△modulation to the last exercised thermometer element.