"Crystal oscillator, referred to as crystal. However, when the natural frequency of the applied alternating voltage and the wafer are equal, the amplitude of mechanical vibration will increase dramatically, a phenomenon known as a piezoelectric resonator, and therefore the quartz crystal is also known as quartz crystal resonator device which is characterized by a high degree of frequency stability quartz crystal oscillator using the quartz crystal vibrating piezoelectric effect, and the quartz crystal resonator is a quartz crystal and a built-in IC operates to cooperate Conversely, if the applying mechanical pressure on both sides of the wafer, the electric field is generated in a direction on the respective wafer, this physical phenomenon known as the piezoelectric effect.
Crystal oscillator, referred to as crystal. It can be electrically equivalent to a capacitor and a resistor in series with a capacitor in parallel with another two-terminal network, this network has the resonance points of electrical engineering, in which the high and low points of the lower frequency series resonance frequency is higher a parallel resonant frequency. Because of its characteristics such that Crystal distance corresponding to two frequencies close, within this very narrow frequency range, the crystal is equivalent to an inductor, so long as both ends of the parallel capacitor suitable crystal composition it will parallel resonant circuit . The parallel resonant circuit is applied to a negative feedback circuit, it can form a sine wave oscillator circuit, since the crystal is equivalent to the frequency range of the inductor is very narrow, so that even a significant variation of other parameter elements, the frequency of the oscillator will not be change dramatically.
The crystal has an important parameter that is equal to the load capacitance, select the parallel capacitance value of the capacitive load, the resonant frequency of the crystal can be obtained nominal.
The general crystal oscillator circuit is a crystal oscillator access across the inverting amplifier (note inverter amplifiers are not), then there are two capacitors connected to two ends of the crystal, the other end of each capacitor is then received the capacitance values of two capacitors in series should be equal to the load capacitance, note IC pin have generally equivalent input capacitance, this can not be ignored.
Usually crystal load capacitance 15p or 12.5p, if we take the equivalent input pin capacitance element, the capacitance of two 22p constituting the crystal oscillation circuit is a better choice.
Crystal oscillator is also divided into active and passive crystal oscillator types. Different active and passive crystal oscillator (resonator) English name is Crystal passive crystal (crystal), and is called the active crystal Oscillator (oscillator). Required by means of passive crystal clock circuit to generate an oscillation signal, it itself can not oscillate, so "passive crystal" is not accurate; active crystal oscillator is a resonant complete.
Resonant oscillator comprises a quartz (or crystal materials) crystal resonator, a ceramic resonator, the LC resonators and the like.
And crystal resonant oscillator have their common intersection active crystal resonant oscillator.
Quartz wafer so do the oscillation circuit (resonance) which is based on the piezoelectric effect, is known from physics, if an electric field is applied between the two plates of the wafer, the crystal will produce mechanical deformation; the other hand, if the plate between mechanical force applied, it will produce an electric field in the corresponding direction, a phenomenon known as the piezoelectric effect. As the applied between the plates is an alternating voltage, mechanical vibration will be deformed, with mechanical deformation of the vibration will produce an alternating electric field. In general, the amplitude of which mechanical vibration is relatively small, the vibration frequency is very stable. However, when the natural frequency of the applied frequency of the alternating voltage of the wafer (depending on the size of the wafer) are equal, the amplitude of mechanical vibration will increase dramatically, a phenomenon known as a piezoelectric resonator, and therefore also known as quartz crystal resonator . Which is characterized by a high degree of frequency stability.
The quartz crystal oscillator quartz crystal resonator and an electronic device are provided frequency stabilization circuit. A quartz crystal oscillator using the piezoelectric effect to vibrating quartz, and quartz crystal resonator is a quartz crystal and a built-in IC to cooperate to work. Directly applied to the oscillator circuit, the resonance is generally require work to maintain a voltage of 3.3V. More than oscillator resonator is an important technical parameters: resonance resistance (RR), the resistance of the resonator is not required. RR size directly affects the performance of the circuit, it is also an important parameter in the business competition.
Overview
A quartz crystal oscillator is an oscillator of high precision and high stability, is widely used in color television, computer, remote control and other types of oscillation circuit, and a communication system for the frequency generator generates a clock signal and a data processing device providing a reference signal for the particular system.
Oscillator is a quartz crystal resonator device manufactured using quartz (crystalline silica) piezoelectric effect, its basic structure is substantially: from a certain azimuth cut quartz sheet (simply referred to as wafer body, it may be square, rectangular or circular, etc.), corresponding to its two layer coating the surface of silver as an electrode on each electrode lead wire to each pad on a pin, constitutes together with the package housing a quartz crystal resonator, simply referred to as crystals or crystal oscillator. Its products are generally packaged with a metal shell, the shell also useful for glass, ceramic or plastic packages.
When an electric field is applied across the two electrodes of the quartz crystal wafer will produce mechanical deformation. Conversely, when mechanical pressure is applied on both sides of the wafer, the electric field is generated in a direction on the respective wafer, this physical phenomenon known as the piezoelectric effect. If the alternating voltage applied at the poles of the wafer, the wafer will produce mechanical vibration, mechanical vibration while the wafer will produce an alternating electric field. In general, the amplitude of mechanical vibration of the wafer and the amplitude of the alternating electric field is very small, but the frequency of the applied alternating voltage is a certain value, significantly increased amplitude, the amplitude is much larger than the other frequencies, which phenomenon is known as the piezoelectric resonator, the resonant LC circuit and the phenomenon it is very similar. It resonance frequency of wafer cutting pattern, geometry, size and so on.
What are the common types of crystal
Many people do not know much about the crystal, the crystal has the heart, known in the circuit, generating a pulse signal by mixing with other components, issued a directive to achieve the work. Crystal widely used in different fields mainly for automotive electronics, smart home, digital electronics, security equipment, communications equipment and so on.
Many crystal type, depending on the performance can be divided into active and passive crystal oscillator, TCXO divided active oscillator, voltage controlled crystal oscillator, a voltage-controlled temperature-compensated crystal oscillator. Two types of passive crystal quartz crystal and ceramic crystal materials.
Depending on the equipment needs of different areas, many manufacturers choose to use imported from Japan and Taiwan imported crystal crystal brand. Taiwan-based Crystal Brand: Asia Taojing Zhen, Hongxing Jing Zhen, TXC crystal, crystal and so heightened, Japanese Crystal Brand: KDS big vacuum crystal, crystal Seiko Epson, Citizen crystal, crystal Kyocera, NDK Crystal, and so on.
Quartz crystal is divided into two kinds of patch and plug, the most common plug body is a cylindrical quartz crystal and a crystal oval. Patch crystal large package size, mainly 7.0x5.0mm, 5.0x3.2mm, 3.2x2.5mm, 2.0x1.6mm, 1.6x1.2mm, we use different package size SMD crystal properties exhibited also in different areas different.
Selection of the most critical parameters in that 6:00 crystal, crystal frequency package size, crystal (some use a frequency less than the partial) load capacitance deviation precision crystal, the crystal, the crystal temperature range requirements, as well as the active crystal supply voltage requirements. According to the customer in different environments used to choose different products and different parameters.
The microcontroller clock source can be divided into two categories: Clock Source mechanical resonant devices, such as crystals, ceramic resonators; phase shift circuit based on the clock source, such as: the RC (resistance, capacitance) oscillator. Silicon oscillators typically are fully integrated RC oscillator, in order to improve the stability, comprising a clock source, resistor and capacitor matching, temperature compensation. Figure 1 shows two clock sources. Figure 1 shows two separate oscillator circuits, wherein Figure 1a is a Pierce oscillator configured for mechanical resonant devices, such as crystals and ceramic resonators. FIG. 1b feedback oscillator simple RC.
The main difference between the mechanical resonator and the RC oscillator
Typically provide very high initial accuracy and low temperature coefficient and crystal-based ceramic resonator oscillator (mechanical) a. Relatively speaking, the RC oscillator fast startup, relatively low cost, but generally suffer from poor accuracy over temperature and operating voltage range, varies in the range of 5-50% of the nominal output frequency. The circuit shown in Figure 1 can produce reliable clock signal, but by environmental conditions affect the performance and the circuit element selection and layout of the oscillator circuit. Need seriously oscillator circuit board layout and component selection. In use, the ceramic resonators and their respective load capacitance must be optimized according to a particular logic family. Crystal oscillator having a high Q value is not sensitive to the selection of the amplifier, but it is susceptible to frequency drift overdrive (even damage). Environmental factors affecting the oscillator work are: electromagnetic interference (EMI), mechanical vibration and shock, humidity and temperature. These factors will increase the output frequency changes, increased instability, and in some cases, can also cause the oscillator to stop.
The oscillator module
Most of the above problems can be avoided by using the oscillator module. The module has its own oscillator to provide low impedance square wave output, and to ensure the operation under certain conditions. The two most common types of modules and are integrated silicon crystal oscillators. Crystal modules provide the same discrete crystal accuracy. Silicon oscillator accuracy than discrete RC oscillator is high, can be provided in most cases with ceramic resonator reasonable precision.
Power consumption
Consideration of oscillator selection power. Discrete power oscillator is primarily determined by the capacitance value of the internal supply current and a feedback amplifier circuit. CMOS power amplifier proportional to the operating frequency, power dissipation can be expressed as the capacitance value. For example, the capacitance value of the power dissipation of the inverter gate circuit HC04 is 90pF. When working in 4MHz, 5V power supply current of 1.8mA equivalent. 20pF load capacitance coupled with the crystal, the entire supply current of 2.2mA.
Usually ceramic resonator having a large load capacitance, accordingly requires more current.
In contrast, crystal module generally requires a supply current of 10mA to 60mA.
Silicon oscillator supply current depends on its type and function, and can range from a low of a few microamps to milliamps programmable devices (fixed) devices. A low-power silicon oscillator, such as the MAX7375, only work when a current of 2mA than 4MHz.
in conclusion
In particular microcontroller applications, select the best clock source needs to consider the following factors: accuracy, cost, power consumption, and environmental requirements. The following table summarizes the common type of oscillator, and analyzes their advantages and disadvantages.
The basic principles and characteristics of the crystal
Crystal generally use the three-terminal capacitor (Colpitts) oscillator circuit AC equivalent of FIG. 1a; the actual crystal AC equivalent circuit in FIG. IB, where Cv is used to adjust the oscillation frequency, generally different varactor diode plus reverse bias voltage is achieved, which is the mechanism of action of the voltage control; FIG. 1c after the equivalent circuit of a crystal instead of a crystal. Where Co, C1, L1, RR is an equivalent circuit of a crystal.
Analysis found throughout oscillating tank circuit, the frequency of use is limited to change the Cv of: oscillation frequency is determined throughout the tank capacitor C = Cbe, Cce, Cv three capacitors in series and parallel re-Co and C1 in series. It can be seen: C1 smaller, Co larger, more hours Cv changes throughout the tank capacitor effect. Thus can "VCO" frequency range is smaller. In fact, due to the C1 is small (1E-15 of the order), Co can not be ignored (1E-12 on the order of a few PF). So, Cv becomes large role in reducing the frequency of the tank circuit smaller, Cv becomes small, the increasing effect of the tank circuit frequency is growing. This aspect causes the voltage controlled non-linear characteristics, the greater the voltage control range, the more severe nonlinearity; on the other hand, give the feedback voltage (the voltage on Cbe) oscillations are getting smaller, leading to stop vibration.
The higher harmonic frequency using the crystal, the smaller the equivalent capacitance of a C1; therefore the smaller the frequency range.
Crystal indicators
The total frequency difference: within a predetermined time, since the crystal oscillator frequency and the predetermined operating parameters of all combinations of inoperative due to a given maximum deviation from the nominal frequency.
Description: The total frequency difference comprises a frequency-temperature stability, the maximum frequency difference between the frequency deviation caused by aging rate, frequency of the voltage and frequency characteristics of the load caused by the common characteristics. Generally only in the short-term frequency stability concerns, and other occasions frequency stability index is not strictly required for adoption. For example: Precision-guided radar.
Frequency stability: any crystal frequency instability is absolute, just different degrees. A crystal oscillator output frequency versus time curve shown in Figure 2. The figures show three factors frequency instability: aging, drift and short-term stability.
Figure frequency instability
Curve 1 is measured with a case where 0.1 seconds, short-term stability of the performance of the crystal oscillator; curve 3 measured is 100 seconds a case, the performance of the drift of the crystal oscillator; curve 4 shows a case where a measurement days. The performance of the aging of the crystal.
Frequency temperature stability: at nominal power supply and the load, without working at a temperature within a predetermined temperature range or implied reference temperature with implicit reference maximum allowable deviation.
ft = ± (fmax-fmin) / (fmax + fmin)
ftref = ± MAX [| (fmax-fref) / fref |, | (fmin-fref) / fref |]
ft: frequency temperature stability (without implicit reference temperature)
ftref: frequency temperature stability (with implicit reference temperature)
fmax: maximum predetermined frequency measured over the temperature range
fmin: measured over a predetermined temperature range of the lowest frequency
fref: a predetermined reference temperature measured frequency
Description: The index ftref crystal oscillator which is difficult to produce than the use of a crystal oscillator ft index, so the higher index crystal oscillator ftref price.
Power characteristics (frequency stabilization preheat time): refers to the rate of change of the frequency of another period of time (e.g., 1 hour) after startup of the period of time (e.g., 5 minutes) to the power frequency. Shows the crystal oscillator is stable speed. This switching indicator instrument as often useful frequency meter.
Description: In most applications, a crystal oscillator
Long-term power-up, but in some applications that require frequent crystal oscillator startup and shutdown, then the warm-up time frequency stability indicators need to be taken into account (especially for military radio communications used in harsh environments, when required frequency temperature stability of ≤ ± 0.3ppm (-45 ℃ ~ 85 ℃), employed as a local oscillator OCXO, the frequency stability warm-up time less than 5 minutes, while the use of MCXO requires only ten seconds).
Frequency aging rate: the relationship between the oscillator frequency and the time when the oscillator frequency, measured at constant environmental conditions. This long-term frequency drift is caused by slowly changing the crystal element and the oscillator circuit element, therefore, the rate of aging is called frequency offset, the maximum rate of change can be used after a specified time limit (e.g., ± 10ppb / day, power 72 after hours), or within the time limit specified maximum total frequency change (eg: ± 1ppm / (first) and ± 5ppm / (ten)) is represented.
Aging of the crystal due to the presence of stress, contaminants, residual gas, and other structural defects in the production process problems at the time of the crystal. To go through a stress
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