Design Of A Low Phase Noise Voltage-Controlled Oscillator For The Cesium Atomic Frequency Standard

The Coherent Population Trapping(CPT) is a kind of quantum interference phenomenon, produced by the interaction between atoms and coherent light. The atomic frequency standards which are made based on the CPT technology have advantages of small volume, low power consumption and quick start. Besides, such atomic frequency standards are currently the only kind whose physical system can be made into a chip-scale size to achieve a miniaturization product in principle, and they can be implemented with different atoms among which the cesium atom is the best one to implement a CPT atomic frequency standard with the highest accuracy and stability. Hence, the CPT cesium atomic frequency standard possesses broader application prospects. According to the working principle of the cesium atomic frequency standard, a microwave signal source with low power consumption, small volume, low phase noise and a proper frequency regulation capability near the frequency of 4.596 GHz is a vital part in the system. The microwave signal source is used to scan by small step lengths at a certain frequency range to achieve the peak signal of the CPT, and then realize the closed-loop locking of the cesium atomic frequency standard. On the basis of this, a low phase noise 4.596 GHz voltage-controlled oscillator(VCO) which is used as the microwave signal source in the cesium atomic frequency standard is designed in this thesis. The designed VCO should meet some specifications: small footprint of less than 1cm2, low power consumption of at most 20 mW, proper output power of higher than-3dBm and low phase noise which is better than-55dBc/Hz@1KHz when running free. Besides,the phase noise of the loop should be better than-75dBc/Hz@300Hz when the VCO is locked by a phase-locked loop.To design the VCO, the negative resistance analysis method and the transmission analysis method with a virtual ground are used, and its performance is evaluated according to the corresponding simulation results in the time domain and frequency domain. In addition, in order to realize an accurate simulation of the designed circuit, parameters of the varactor which is a key component in the VCO are extracted under real operating environment and an equivalent circuit model of the varactor is built. Finally, the PCB footprint of the designed VCO is 0.60cm2, the power consumption is 19.6mW, the output power is-1dBm, the phase noise is-60.86dBc/Hz@1KHz when running free, the phase noise of the loop is-83.57dBc/Hz@300Hz when the designed VCO is locked by a phase-locked loop and the voltage-controlled tuning sensitivity is about 110MHz/V, all the performance is better than expected indexes. Furthermore, performance of the designed VCO can still maintain stable when tested in the high and low temperature system. At present, one thousand VCOs have been produced successfully and all their measured performance is better than design indexes.