A commonly used crystal oscillator circuit in the semiconductor industry is an active crystal oscillator. Because the quartz crystal oscillator with its own power supply is easy to implement in IC, it has very good stability, high Q value (from ten thousand to several hundred thousand) and low manufacturing cost, so it is most popular with designers.

    Quartz crystal oscillators mainly consist of logic gate inverters, feedback resistors between the input and output of the (which act as amplifiers (gain stages)) and external crystal oscillator components. This layout is called a parallel resonant circuit. Any circuit that provides oscillation must satisfy Barkhausen's stability criterion: that is, the gain amplitude of the loop should be equal to an integer multiple of the oscillation frequency, and the phase shift around the loop should be an integer multiple of 0 or 2.

    Let's go through the individual modules and their functions one by one. Inverting Amplifier and Feedback Resistor: The inverting block is a simple CMOS logic NAND gate that can be used as a basic element of a digital integrated circuit. In conventional mode, its transfer characteristics are nonlinear. In general, CMOS inverters are designed to provide maximum gain at 1/2 the supply voltage. This can be achieved by adjusting the dimensions of the P-MOS and N-MOS during the design process. Within the above maximum gain range, the logic NAND gate begins to act as an amplifier. However, it cannot amplify any signal alone without the help of external components. The inverter itself can provide a phase shift of 180. Also, the 180 phase shift must be provided by external components.

    When the feedback resistor (Rf) is connected between the input and output of the inverter, it biases itself to where Vin=Vout. The inverter used in the oscillator circuit guarantees that the inverter bias point and its maximum gain are close to half the supply voltage. Therefore, the feedback resistors used can overcome the nonlinearity of the inverter and convert a simple logic gate into an analog amplifier. This amplifier can help overcome the losses of the crystal oscillator used in the circuit. The Rf value used depends on the operating frequency of the circuit design. Usually, the minimum and maximum oscillation frequency range of a chip crystal oscillator circuit is determined during the design stage. The Rf value at low operating frequency is higher than the Rf value at high operating frequency. The operating frequency between 20 MHz and 40 MHz generally adopts 450 k radio frequency. When the inverter is connected to the optimal feedback resistance value, if the inverter inputs a weak signal, the inverter amplifier can provide considerable conversion at the output end, thereby providing a loop gain greater than 1, reaching the Barkhausen standard. Resistor Rs is called the drive current limiting resistor because it is used to limit the output of the inverter to ensure that the crystal oscillator is not overdriven. However, a larger Rs value will prolong the start-up time of the crystal oscillator; the smaller the Rs value, the faster the oscillator will start. Therefore, the designer must choose the optimum value to provide enough current to start oscillation without overdriving the crystal oscillator.

    Additionally, it helps isolate the output driver from the complex impedances created by CL1, CL2, and the crystal oscillator. In some high frequency oscillators, its value can also be 0. Crystal oscillator manufacturers usually provide the highest drive capacitance for a crystal oscillator at that time. Caution should be exercised before using a crystal in a quartz crystal oscillator, as overexcitation may cause accelerated aging of the crystal oscillator.