The output amplitude prediction of crystal oscillator

Crystal oscillator is an important component of the microwave and RF devices. One of the most important characteristics of crystal oscillators is phase noise spectrum. Phase noise within the half-bandwidth of the loop is closely related to the loaded quality factor QL. According to our research about Pierce oscillator circuit, we can derive that phase noise improves with increasing QL and an appropriate increase in the value of collector-to-ground capacitance C1 can improve QL. However, to change the value of C1 also affects the output amplitude of oscillation circuit. In the paper, the method of predicting output amplitude is presented and a prototype 50 MHz AT-cut 3rd overtone Pierce low phase noise crystal oscillator is designed. The output amplitude variation versus the value of C1 can be obtained with MATLAB. On the premise of specific value of C1, the values of other circuit parameters are changed to make sure that output amplitude is maintained in the required value range. A design of the prototype 50MHz Pierce crystal oscillator is presented and the experiments are carried out. The measurement phase noise results are -126 dBc/Hz@100Hz and -151 dBc/Hz@1KHz. Experimental result shows it is necessary to predict the output amplitude of crystal oscillators.