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The crystal oscillator generates a clock signal by itself, which serves as a clock reference for various microprocessor chips. The crystal oscillator is equivalent to the heart of these microprocessor chips. Without the crystal oscillator, these microprocessor chips will not work.
The role of the crystal oscillator is to provide a basic clock signal for the system. Usually a system shares a crystal oscillator to facilitate synchronization of various parts. Some communication systems use different crystal oscillators for baseband and radio frequency, and synchronize them by electronically adjusting the frequency.
Crystal oscillators are mainly used in CPUs such as single-chip microcomputers, DSP, ARM, PowerPC, CPLD/FPGA, and communication interface circuits such as PCI interface circuits and CAN interface circuits.
Crystal oscillator is the abbreviation of active crystal oscillator, also called oscillator. The full professional term is called crystal oscillator, Crystal Oscillator, abbreviated as XO. Power supply is required, and the oscillation circuit is inside the crystal oscillator.
Crystal is the abbreviation of passive crystal oscillator, also called resonator. The full professional term is called crystal resonator, Crystal, abbreviated as XTAL. No power supply is required, and it needs to cooperate with the oscillation circuit to work.
1、Working principle of crystal oscillator:
Quartz crystal oscillator is a resonant device made by using the piezoelectric effect of quartz crystal. Its basic structure is roughly as follows: a thin slice is cut from a quartz crystal at a certain azimuth angle, a silver layer is applied on its two corresponding surfaces as electrodes, a lead is welded on each electrode and connected to the pin, and then a package shell is added to form a quartz crystal resonator, referred to as quartz crystal or crystal, crystal oscillator. Its products are generally packaged in metal shells, and there are also glass shells, ceramics or plastic packages.
Quartz crystal has a very important characteristic: if an alternating voltage is applied to it, it will produce mechanical oscillations, and vice versa, if it is mechanically vibrated, it will produce alternating voltage. This characteristic is called piezoelectric effect.
Under normal circumstances, the amplitude of the mechanical vibration of the chip and the amplitude of the alternating electric field are very small, and its vibration frequency is very stable. However, when the frequency of the applied alternating voltage is a certain value, the amplitude of the vibration will increase sharply. This phenomenon is called piezoelectric resonance. It is very similar to the resonance phenomenon of the LC circuit.
A quartz crystal oscillator can be equivalent to a two-terminal network with a capacitor and a resistor in parallel and a capacitor in series. This two-terminal network has two resonant points, corresponding to two resonant frequencies, namely fs and fp, with fp slightly greater than fs.
The lower frequency fs is the series resonant frequency, and the higher frequency fp is the parallel resonant frequency. Due to the characteristics of the crystal itself, the distance between these two frequencies is quite close. In this extremely narrow frequency range, the crystal oscillator can be further equivalent to an inductor.
Therefore, if we connect a suitable capacitor in parallel at both ends of the crystal oscillator, it will form a parallel resonant circuit. Adding this parallel resonant circuit to a negative feedback circuit can form a sinusoidal oscillation circuit.
Moreover, since the chemical properties of quartz crystals are very stable and the thermal expansion coefficient is very small, the frequency range in which the crystal oscillator is equivalent to an inductor is very narrow, and the oscillation frequency is very stable. Its resonant frequency is basically only closely related to the shape, material, cutting direction, etc. of the wafer, and the control of the geometric dimensions can be very precise, so even if the parameters of other components change greatly, the frequency of the oscillator will not change much. This is the principle of frequency stability of the crystal oscillator circuit.
2、Crystal oscillators are mainly divided into the following five categories:
1) Passive Crystal Resonator (XTAL)
It is a crystal resonator that does not require external power to drive directly. It does not have an oscillation function by itself and needs to cooperate with an external circuit (such as the oscillation circuit inside the chip) to generate a clock signal. The frequency stability of a passive crystal oscillator mainly depends on the characteristics of its internal crystal and the design of the external circuit. Usually, the frequency stability is in the order of 10⁻⁵ to 10⁻⁶. Its structure is simple, consisting only of a crystal and a package, and it relies on an external circuit to provide an excitation signal when working.
2) Simple Packaged Crystal Oscillator (SPXO)
It is a crystal oscillator that does not take temperature compensation measures. In the entire temperature range, the frequency stability of the crystal oscillator depends on the performance of the crystal used inside it. The frequency stability is in the order of 10-5. It is generally used in ordinary places as a local oscillator source or intermediate signal. It is a product with lower value among crystal oscillators. Its work is completely completed by the free oscillation of the crystal. This type of crystal oscillator is mainly used in occasions where stability requirements are not high.
From the original quartz crystal blank to the final crystal oscillator packaged
3) Temperature Compensated Crystal Oscillator (TCXO)
The temperature compensated crystal oscillator is a crystal oscillator that compensates for the temperature characteristics of the crystal frequency inside the crystal oscillator to meet the stability requirements within a wide temperature range of -40℃~105℃; the frequency stability is in the order of 5*10-7~5*10-8.
Generally, the analog temperature compensated crystal oscillator uses a thermistor compensation network, which consists of a temperature sensor and a frequency compensation circuit. The voltage change of the temperature sensor controls the VCO or varactor diode to calibrate the frequency. Due to its good start-up characteristics, superior cost performance, low power consumption, small size, strong environmental adaptability and other advantages, it has been widely used.
4) Voltage Controlled Crystal Oscillator (VCXO)
The voltage controlled crystal oscillator is a crystal oscillator that can change the output frequency of the crystal oscillator by adjusting the external voltage. Its frequency control range and linearity mainly depend on the combination of the varactor diode and crystal parameters used in the circuit. It is mainly used for phase-locked loop or frequency fine-tuning.
5) Oven Controlled Crystal Oscillator (OCXO)
Uses precision temperature control to make the circuit components and crystals work at the temperature of the zero temperature coefficient point of the crystal. The frequency stability of medium-precision products is 10-8~10-9, and the frequency stability of high-precision products is above 10-10.
Use a constant temperature bath to keep the temperature of the quartz crystal resonator in the crystal oscillator constant, and minimize the amount of change in the oscillator output frequency caused by ambient temperature changes. OCXO is composed of a constant temperature bath control circuit and an oscillator circuit. Usually people use a differential series amplifier composed of a thermistor "bridge" to achieve temperature control. It is mainly used as a frequency source or standard signal.
OCXO is called the crown of the crystal industry, especially the crystal used for OCXO, which requires excellent Q and aging. The most advanced technologies in the crystal industry are used in this industry. This industry can be said to be a collection of cutting-edge crystal technologies. It is essentially a small electronic system. OCXO is as important as the tamed crystal oscillator (GPSDO) and the rubidium atomic clock, and it is more cost-effective and has a wider range of uses. With the popularization of 5G applications, the demand for OCXO is rising rapidly. A 5G small base station requires at least one OCXO, while a macro base station may require more than ten OCXOs.
Different characteristics determine the application scenarios of the four types of crystal oscillators: If the device needs to be ready to use, SPXO, VCXO and TCXO should be selected. If the clock signal is required to have a higher stability, TXCO and OCXO are recommended.
3. Application of crystal oscillators
The application of crystal oscillators is extremely wide, and can be combined with the main parameters of the crystal oscillator for specific applications, such as output frequency, frequency stability, operating temperature range, input voltage and power, output waveform, package size and appearance, etc. The following are common fields of crystal oscillators:
· Scientific research and measurement: atomic clocks, measuring equipment, telemetry, remote sensing, remote control and other applications;
· Industrial field: communications, telecommunications, mobile/cellular/portable terminals, aviation, navigation, instrumentation, computers, digital devices, displays, disk drives, modems, sensors and other applications;
· Consumer field: wearable devices, smart homes, audio equipment, cable TV systems and TVs, PCs, cameras, wireless communications, toys, medical devices and equipment and other applications;
· Automotive field: ADAS, central control screens, instrument panels, streaming media rearview mirrors, car cameras, millimeter wave radars, lidars, Internet of Vehicles, tire pressure monitoring, car navigation and other automotive electronics applications.
