The working principle of crystal oscillator
Time：2022.01.12
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Central topic:
Basic Principles of Quartz Crystal Oscillator
Quartz crystal oscillator type characteristics
Main parameters of quartz crystal oscillator
Development Trend and Application of Quartz Crystal Oscillator
solution:
Quartz crystal oscillators have the trend of miniaturization, thin slice, chip, high precision, high stability, low noise and high frequency
Widely used in quartz clocks, TV industry, communication system products
Quartz crystal oscillators are high-precision and high-stability oscillators. They are widely used in various types of oscillating circuits such as color TVs, computers, and remote controllers, as well as frequency generators in communication systems, and clock signals for data processing equipment. Provide a reference signal for a specific system.
The basic principle of quartz crystal oscillator
The structure of a quartz crystal oscillator
Quartz crystal oscillator is a kind of resonant device made by using the piezoelectric effect of quartz crystal (crystal of silicon dioxide). Its basic composition is roughly as follows: a slice (referred to as a wafer for short) is cut from a piece of quartz crystal at a certain azimuth angle. It can be square, rectangular or round, etc.), silver layers are coated on its two corresponding surfaces as electrodes, each electrode is welded with a lead wire to the pin, and a package shell is added. Quartz crystal resonator, referred to as quartz crystal or crystal, crystal oscillator for short. Its products are generally packaged in metal shells, but also in glass, ceramic or plastic packages.
Piezoelectric effect
If an electric field is applied to the two electrodes of the quartz crystal, the wafer will be mechanically deformed. Conversely, if mechanical pressure is applied on both sides of the wafer, an electric field will be generated in the corresponding direction of the wafer. This physical phenomenon is called the piezoelectric effect. If an alternating voltage is applied to the two poles of the wafer, the wafer will generate mechanical vibration, and at the same time, the mechanical vibration of the wafer will generate an alternating electric field. In general, the amplitude of the mechanical vibration of the wafer and the amplitude of the alternating electric field are very small, but when the frequency of the applied alternating voltage is a certain value, the amplitude is obviously increased, which is much larger than the amplitude at other frequencies. This phenomenon is called piezoelectric resonance, which is very similar to the resonance phenomenon of the LC circuit. Its resonance frequency is related to the cutting method, geometry and size of the wafer.
Symbol and equivalent circuit
When the crystal is not vibrating, it can be regarded as a flat capacitor called electrostatic capacitance C. Its size is related to the geometric size of the wafer and the electrode area, generally about several PF to tens of PF. When the crystal oscillates, the inertia of the mechanical vibration can be equivalent to the inductance L. Generally, the value of L is tens of mH to hundreds of mH. The elasticity of the chip can be equivalent to the capacitance C, the value of C is very small, generally only 0.0002 ~ 0.1pF. The loss caused by friction when the wafer vibrates is equivalent to R, and its value is about 100Ω. Because the equivalent inductance of the chip is very large, and C is very small, and R is also small, the quality factor Q of the loop is very large, reaching 1000 to 10000. In addition, the resonant frequency of the wafer itself is basically only related to the cutting method, geometry, and size of the wafer, and it can be done accurately. Therefore, an oscillator circuit composed of a quartz resonator can obtain a high degree of frequency stability.
Resonant frequency
From the equivalent circuit of a quartz crystal resonator, it can be seen that it has two resonant frequencies, namely (1) When the L, C, and R branches are in series resonance, its equivalent impedance is the smallest (equal to R). The series vibration frequency is represented by fs. The quartz crystal is purely resistive to the series vibration frequency fs. (2) When the frequency is higher than fs, the L, C, and R branches are inductive and can be connected to the capacitor C. Parallel resonance occurs, and its parallel frequency is denoted by fd. According to the equivalent circuit of a quartz crystal, its reactance-frequency characteristic curve can be drawn qualitatively. It can be seen that when the frequency is lower than the series resonance frequency fs or the frequency is higher than the parallel resonant frequency fd, the quartz crystal is capacitive. Only in the extremely narrow range of fs
Quartz crystal oscillator type characteristics
The quartz crystal oscillator is composed of a quartz crystal oscillator (ie, a resonator and an oscillation circuit) with extremely high quality factors. The quality of the crystal, the cutting orientation, the structure of the crystal oscillator and the circuit form, etc., jointly determine the performance of the oscillator. International Electrotechnical Commission ( IEC) divides quartz crystal oscillators into 4 categories: ordinary crystal oscillators (TCXO), voltage controlled crystal oscillators (VCXO), temperature compensated crystal oscillators (TCXO), and oven controlled crystal oscillators (OCXO). Currently under development There are also digitally compensated crystal loss oscillations (DCXO), etc. Ordinary crystal oscillators (SPXO) can generate frequency accuracy of 10^(-5)～10^(-4), standard frequency 1-100MHZ, frequency stability The temperature is ±100ppm. SPXO does not use any temperature and frequency compensation measures, and is low in price. It is usually used as a clock device for microprocessors. The package size ranges from 21×14×6mm and 5×3.2×1.5mm. Voltage controlled crystal oscillator The accuracy of (VCXO) is in the order of 10^(-6)~10^(-5), and the frequency range is 1~30MHz. The frequency stability of the low-tolerance oscillator is ±50ppm. It is usually used in phase-locked loops. Package The size is 14×10×3mm. The temperature-compensated crystal oscillator (TCXO) uses temperature-sensitive devices for temperature and frequency compensation, with a frequency accuracy of 10^(-7)～10^(-6) and a frequency range of 1-60MHz. The frequency stability is ±1～±2.5ppm, and the package size ranges from 30×30×15mm to 11.4×9.6×3.9mm. It is usually used in handheld phones, cellular phones, two-way wireless communication equipment, etc. Thermostatically controlled crystal oscillator (OCXO ) Place the crystal and oscillation circuit in a constant temperature box to eliminate the influence of environmental temperature changes on the frequency. The frequency accuracy of OCXO is 10^(-10) to 10^(-8), and it can even be even better for some special applications. High. Frequency stability is the highest among the four types of oscillators.
The main parameters of the quartz crystal oscillator
The main parameters of the crystal oscillator are nominal frequency, load capacitance, frequency accuracy, frequency stability, etc. Different crystal oscillators have different nominal frequencies, and most of the nominal frequencies are marked on the crystal housing. For example, the nominal frequencies of common ordinary crystal oscillators are: 48kHz, 500 kHz, 503.5 kHz, 1MHz～40.50 MHz, etc. For special requirements, the crystal oscillator frequency can reach more than 1000 MHz, and some have no nominal frequency, such as CRB, ZTB, Ja, etc. . The load capacitance refers to the sum of all the effective capacitances inside and outside the IC block connected by the two leads of the crystal oscillator, which can be regarded as the series connection capacitance of the crystal oscillator in the circuit. The different load frequency determines the different oscillation frequency of the oscillator. For crystals with the same nominal frequency, the load capacitance may not be the same. Because the quartz crystal oscillator has two resonant frequencies, one is a low-load capacitance crystal with a series-connected oscillating crystal oscillator, and the other is a high-load-capacitance crystal with a parallel oscillating crystal. Therefore, when the crystal oscillators with the same nominal frequency are exchanged, the load capacitance must be required to be exchanged. They cannot be exchanged rashly, otherwise it will cause the electrical appliances to work abnormally. Frequency accuracy and frequency stability: Since the performance of ordinary crystal oscillators can basically meet the requirements of general electrical appliances, a certain frequency accuracy and frequency stability are required for high-end equipment. The frequency accuracy ranges from 10^(-4) to 10^(-10). The stability varies from ±1 to ±100ppm. It is necessary to select the appropriate crystal oscillator according to the specific equipment needs. For example, communication networks, wireless data transmission and other systems require more demanding quartz crystal oscillators. Therefore, the parameters of the crystal oscillator determine the quality and performance of the crystal oscillator. In practical applications, the appropriate crystal oscillator should be selected according to specific requirements. Because the price of crystal oscillators with different performances is different, the higher the requirements, the more expensive the price. Generally, the choice only needs to meet the requirements.
The development trend of quartz crystal oscillator
Miniaturization, thinning and chipization
In order to meet the requirements of light, thin, short and small portable products represented by mobile phones, the packaging of quartz crystal oscillators has changed from a traditional bare metal shell covered with plastic metal to a ceramic package. For example, the volume of devices such as TCXO has been reduced by 30 to 100 times. The thickness of the TCXO packaged in SMD is less than 2mm, and devices with a size of 5×3mm are already on the market.
High precision and high stability
At present, the total accuracy of uncompensated crystal oscillators can also reach ±25ppm. The frequency stability of VCXO is generally ±20-100ppm in the range of 10～7℃, while the frequency stability of OCXO in the same temperature range is generally ±0.0001～ 5ppm, VCXO is controlled below ±25ppm.
Low noise, high frequency
Frequency tremor is not allowed in GPS communication systems. Phase noise is an important parameter that characterizes oscillator frequency tremor. The phase noise performance of mainstream OCXO products has been greatly improved. Except VCXO, the highest output frequency of other types of crystal oscillators does not exceed 200MHz. For example, the UCV4 series voltage-controlled oscillator used in mobile phones such as GSM has a frequency of 650-1700 MHz, a power supply voltage of 2.2-3.3V, and a working current of 8-10mA.
Low function, quick start
Low-voltage operation, low-level drive and low current consumption have become a trend. The power supply voltage is generally 3.3V. At present, many TCXO and VCXO products do not consume more than 2 mA of current. The rapid start-up technology of the quartz crystal oscillator has also made breakthrough progress. For example, the VG-2320SC VCXO produced by Japan Seiko, under the condition of ±0.1ppm specified value range, the frequency stabilization time is less than 4ms. The SMD TCXO produced by Tokyo Ceramics Company in Japan can reach 90% of the rated value 4ms after the oscillation starts. OAK's 10-25 MHz OCXO products can reach a stability of ±0.01 ppm after being warmed up for 5 minutes.
Application of Quartz Crystal Oscillator
Quartz clocks have the biggest advantages, such as accurate timekeeping, low power consumption, and durability.
Whether it is an old-fashioned quartz clock or a new-style multi-function quartz clock, the core circuit of the quartz crystal oscillator is used, and its frequency accuracy determines the timekeeping accuracy of the electronic clock. From the schematic diagram of the principle of the quartz crystal oscillator, V1 and V2 constitute a CMOS inverter. The quartz crystal Q, the oscillating capacitor C1 and the trimming capacitor C2 constitute the oscillation system, where the quartz crystal is equivalent to an inductor. The component parameters of the oscillating system determine the vibration frequency. Generally, Q, C1 and C2 are external components. In addition, R1 is a feedback resistor, and R2 is an oscillating stable resistor, they are all integrated in the circuit. Therefore, the travel time accuracy cannot be adjusted by changing the value of C1 or C2. But at this time, we can still add a capacitor C to change the parameters of the oscillation system to adjust the travel time accuracy. According to the speed of the electronic clock, there are two ways to adjust the capacitor: if the time is too fast, the capacitor C can be connected in parallel at both ends of the quartz crystal, as shown in Figure 4. At this time, the total capacitance of the system increases, the oscillation frequency becomes lower, and the travel time slows down. If the travel time is slow, a capacitor C can be connected in series in the crystal branch. As shown in Figure 5. At this time, the total capacitance of the system decreases, the oscillation frequency becomes higher, and the travel time increases. You can adjust the timekeeping accuracy after patient trial and error. Therefore, the crystal oscillator can be used as a clock signal generator.
With the development of TV technology, color TVs have recently adopted 500kHz or 503kHz crystal oscillators as the oscillation source of the line and field circuits. The line frequency of 15625Hz is obtained by dividing by 1/3, and its stability and reliability have been greatly improved. . The crystal oscillator is cheap and easy to replace.
In communication system products, the value of quartz crystal oscillators has been more widely embodied, and it has also been developed faster. Many high-performance quartz crystal oscillators are mainly used in communication networks, wireless data transmission, and high-speed digital data transmission.
