A Design Of 20MHz Low Phase Noise Crystal Oscillator

The crystal oscillator appeared in 1920s. In last few decades, crystal theory and production technology has great progress, and the crystal performance has been continuously improving. Presently, as a rather satisfying frequency source, the crystal oscillator is more and more taken into application in modern communication systems. Compared with other oscillators, the advantages of crystal oscillators are very obvious, including high natural frequency stability and an excellent phase noise performance, brought by its high Q value. In order to save the cost, improve the phase noise performance of the input reference clock of frequency synthesizer, and achieve the single-chip RF transceiver system, integrating the whole crystal oscillator except for the crystal onto single-chip becomes a trend.This paper first describes the characteristics and the equivalent model of the crystal resonator. The working principle of CMOS crystal oscillator is analyzed, and three basic crystal structures are introduced, whose advantages and disadvantages analyzed. Then important performance indicators, including phase noise and power consumption are introduced. In this paper, Santos structure is employed. The oscillation circuits of Santos structure are analyzed in great details, while both the small-signal model in initial stage and large-signal in stable stage are given. Critical gain, frequency pulling rate and oscillating amplitude are conducted. Finally, some basic theories of phase noise are introduced.In this paper, I designed a 20MHz, low phase noise crystal oscillator, serving as the frequency reference source in frequency synthesizer of UWB RF communication chip. The crystal consists of a quartz crystal resonator, the main oscillator circuits of Santos structure, the amplitude control circuit based on asymmetric differential pair, the band gap voltage reference and output buffer stage. Except for the quartz crystal resonator, all the oscillator circuits are implemented on single-chip. In this paper, given specific consideration in designing each part of the circuits, principle analysis, parameter selection, simulation analysis and phase noise performance optimization are given, respectively. Plenty of design margin are left, given actual non-ideal characteristics of the circuits.Cadence platform of the Spectre, Virtuos and other softwares are employed to accomplish circuit simulation and layout design. Crystal design is based on SMIC0.18um technique, while the approximate chip area is 550um*185um. Simulation results show that phase noise of the proposed crystal oscillator is -121dBc/Hz, -145dBc/Hz, -165dBc/Hz at off-center frequency 1kHz, 10kHz, 1MHz, respectively. The need of RF chip crystal oscillator for phase noise performance could be satisfied.