Low Phase Noise Crystal Oscillator Design

Since the first crystal oscillator came out in the early 1920s, the technique of crystal oscillator has been greatly improved and has been widely used. But as we know, almost all the crystal oscillators are based on the three-point oscillators. They have simple structures so it's very easy for us to design, and in most time they work very fine. Although they have such advantages, as more and more systems need differential reference frequency sources, the three-point oscillators can't meet the demands. So it is very urgent to design differential output crystal oscillators. In digital TV tuner chip, in order to reject the common-mode noise to improve its performance, all the signals are differential. So the phase-locked loop which provides local oscillate signals for the chip needs differential reference frequency source.In this paper, firstly base on the cross-coupled oscillators which are widely used in VCO designs, we design a differential output crystal oscillator. As we know, the low noise of circuits will be upconverted to near carrier phase noise. To reduce the phase noise as much as possible, we also add additional RC high-pass filters to filter it.Then because all the existed phase noise models can't tell how to optimize phase noise of oscillators effectively, we improve the Leeson model to tell how to optimize the phase noise of this circuit. Simulation results show that after optimizing using the methods we introduced, the phase noise of this circuit can achieve as low as -130dBc/Hz@100Hz while using most ordinary crystal resonator's model (Q=20000). In order to meet the input demands of crystal oscillator divider in the next stage and reduce the effects from later signals to oscillator stage, we use two stages of cascode buffers to adjust the shape of the waveform and increase the isolation of the circuit. We also analysis the impact on the signal's phase noise from buffers and give a way to optimize it.At last, we introduce the key points in the analog layout design and presents layout of the whole circuits. After the chip is manufactured and packaged, we design the PCB and test the crystal oscillator. The testing results show that the performance of the cross-coupled crystal oscillator meets the system demands.