A crystal oscillator and a phase noise reduction method thereof are provided. The crystal oscillator may include a crystal oscillator core circuit, a first bias circuit and a phase noise reduction circuit, the first bias circuit is coupled to an output terminal of the crystal oscillator core circuit, and the phase noise reduction circuit is coupled to the output terminal of the crystal oscillator core circuit. In operations of the crystal oscillator, the crystal oscillator core circuit is configured to generate a sinusoidal wave. The first bias circuit is configured to provide a first voltage level to be a bias voltage of the sinusoidal wave. The phase noise reduction circuit is configured to reset the bias voltage of the sinusoidal wave in response to a voltage level of the sinusoidal wave exceeding a specific voltage range.

A crystal oscillator and a phase noise reduction method thereof are provided. The crystal oscillator may include a crystal oscillator core circuit, a bias circuit coupled to an output terminal of the crystal oscillator core circuit, a pulse wave buffer coupled to the output terminal of the crystal oscillator core circuit, and a phase noise reduction circuit coupled to the output terminal of the crystal oscillator core circuit. The crystal oscillator core circuit may generate a sinusoidal wave. The bias circuit may provide a bias voltage of the sinusoidal wave. The pulse wave buffer may generate a pulse wave according to the sinusoidal wave. The phase noise reduction circuit may generate a reset signal including at least one reset pulse for resetting the bias voltage. In addition, the reset signal is generated without calibrating the at least one reset pulse to a zero-crossing point of the sinusoidal wave.

A clock generator can include a Fin Field Effect Transistor (FinFET) oscillator and a phased-locked loop (PLL). The FinFET oscillator can generate a FinFET signal. The PLL can generate an output clock signal based on a reference clock signal and the FinFET signal.

A method for startup of a crystal oscillator (XO) with aid of external clock injection, associated XO and a monitoring circuit therein are provided. The XO includes an XO core circuit, an external oscillator, and an injection switch, where a quality factor of the external oscillator is lower than a quality factor of the XO core circuit. The method includes: utilizing the external oscillator to generate an injected signal; turning on the injection switch to make energy of the injected signal be injected into the XO core circuit, where an amplitude modulation (AM) signal is generated according to combination of the injected signal and an intrinsic oscillation signal from the XO core circuit; and controlling the external oscillator to selectively change an injection frequency of the injected signal according to the AM signal. More particularly, the injection switch is not turned off until the startup process is completed.

An ultra-low noise crystal oscillator uses two crystal unit; an oscillation element of an oscillation circuit section and a crystal filter of a subsequent filter section. A Butler circuit in which the capacitors (C1, C2) and the inductor (L) connected in series is connected in parallel to the oscillator circuit section. This is the crystal oscillator that simplifies the manufacturing process, improves the manufacturing quality, and has good floor noise characteristics.

An oscillation circuit includes: a detection circuit configured to detect input of a first predetermined signal; a control circuit configured to control switching from the second operation mode to the first operation mode; and a non-volatile memory storing mode switching permission information, in which the control circuit is configured to: when the input of the first predetermined signal is detected by the detection circuit and the mode switching permission information indicates that the switching from the second operation mode to the first operation mode is permitted in the second operation mode, switch from the second operation mode to the first operation mode, and when the input of the first predetermined signal is not detected by the detection circuit and the mode switching permission information indicates that the switching from the second operation mode to the first operation mode is not permitted in the second operation mode, continue the second operation mode.

The circuit device includes a current generation circuit and a current-voltage conversion circuit. The current generation circuit generates a temperature compensation current based on a temperature detection voltage from the temperature sensor and temperature compensation data. The current-voltage conversion circuit converts the temperature compensation current into the temperature compensation voltage. The current generation circuit performs a fine adjustment of the temperature compensation current based on lower bits of the temperature compensation data, and performs a coarse adjustment of the temperature compensation current based on higher bits of the temperature compensation data.

An ultra-low noise crystal oscillator uses two crystal unit; an oscillation element of an oscillation circuit section and a crystal filter of a subsequent filter section. A Butler circuit in which the capacitors (C1, C2) and the inductor (L) connected in series is connected in parallel to the oscillator circuit section. This is the crystal oscillator that simplifies the manufacturing process, improves the manufacturing quality, and has good floor noise characteristics.

An apparatus injects a start clock to a crystal at the beginning to increase an overall start up speed of the crystal. The apparatus relies on an impedance change inside the crystal itself instead of searching for a synchronization on the yet small crystal oscillation. The apparatus includes an oscillator (separate from the crystal) to search for the crystal's resonance frequency by detecting the crystal's impedance change. Once the frequency of the oscillator matches the crystal's resonance, there is significant change in the crystal's impedance. Using that information, the apparatus can lock the oscillator frequency at the crystal resonance frequency and inject the clock with high efficiency.

The circuit device includes a current generation circuit configured to generate a temperature compensation current based on a temperature detection voltage, and a current-voltage conversion circuit configured to perform current-voltage conversion on the temperature compensation current to output a temperature compensation voltage. The current-voltage conversion circuit includes an operational amplifier, and a feedback circuit. The operational amplifier includes a differential section having a current mirror circuit and differential pair transistors, an output section configured to output the temperature compensation voltage, and an RC low-pass filter configured to output a signal obtained by performing a low-pass filter process on an output signal of the differential section to an input node of the output section.