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----- 1. matching capacitance load capacitance is the capacitance of the crystal to the desired normal shock. General external capacitor, are provided so that both ends of the oscillator equivalent capacitive load capacitance is equal or close. High requirements should also be considered where ic input capacitance to ground. Usually both ends of the capacitors on the crystal required is twice the load capacitance. Such a load capacitance connected in parallel to a close.
2. The total load capacitance is the effective capacitance across the outside across the crystal in the circuit. He is a test condition is a condition to use. Usually the application is given a value near the load capacitance can be accurately adjusted frequency. This mainly affects the magnitude of the capacitance and the resonant frequency of the load resonance resistance equivalent load.
3. Under normal circumstances, the oscillation frequency will increase the load capacitance is decreased, and the load capacitance causes the oscillation frequency is increased.
4. The crystal load capacitance is connected to two leads IC internal and external blocks all active and capacitance, can be regarded as crystal film series capacitance in the circuit. Load frequency different determinants different oscillation frequencies of the oscillator. Nominally the same frequency oscillator, not necessarily the same load capacitance. Since the quartz crystal oscillator has two resonant frequencies, a crystal oscillator in series arrangements of a low load capacitance of the crystal: the other is a high load capacitance in parallel with the crystal oscillator crystal arrangements. So, the same nominal frequency crystal load capacitance exchange must also be a requirement to not rashly interchangeable, otherwise it will cause appliance is not working properly.
Resistance next to the crystal (parallel and series)
At its output a series circuit of a resistor of 22K, 10M in an indirect resistance of its output and input terminal of which is connected to the crystal due to the internal end of the chip is a linear operational amplifier, an input reverse 180 of the output, the load capacitance resistance network at the crystal composition provide additional phase shift of 180 degrees, the overall loop phase shift of 360 degrees, the phase of the oscillation condition is satisfied, but also requires a closed loop gain is greater than or equal to 1, the crystal will work correctly.
Resistance effect crystal input and output connections is negative feedback to ensure that the amplifier operates at high gain linear region, typically in the resistance and the load capacitance of a network of M European level, the output terminal, providing 180 degree phase shift, also play a limiting role preventing overdriving the output of inverter crystal, crystal damage.
And a resistor in series with the crystal used to prevent the crystal from being excessively driven. Crystal driving over the consequences will gradually reduce the loss of the crystal in contact with the plating, which will cause the increase in frequency, and result in premature failure of the crystal, and can speak drive level adjustment. And for adjusting the drive level of the oscillation margin.
Internal Xin and Xout is generally a Schmitt inverter, the inverter can not drive the crystal oscillator. Thus, at both ends of a resistor in parallel with the inverter, the output from the resistor to complete 180 degrees reverse feedback signal a negative feedback to the input terminal, negative feedback amplifying circuit, and the crystal on the resistance, the impedance of the equivalent resistance of the crystal in parallel relationship, is what they want large resistance impedance or resistor little effect on large crystals of small?
Role of internal resistance of the inverter circuit plus a feedback loop, the amplifier is formed, and wherein when the crystal will AC equivalent feedback loop in accordance with the resonance frequency of the crystal, since the Q of the crystal is very high, so a large resistance the scope of the changes will not affect the output frequency. In the past, used to test the stability of this circuit, I tried to start the vibration can be normal from 100K ~ 20M, but will affect the pulse-width ratio.
Q of the crystal is very high, the value of Q What does it mean crystal series equivalent impedance is Ze = Re + jXe, Re << |? JXe |, crystal is generally equivalent to a very high Q inductors, equivalent to inductor conductor resistance very small. Q generally on the order of 10 ^ -4.
Crystals broke to avoid too strong a signal. Resistance is generally large, typically several hundred K.
String into the resistor is used to limit the amplitude of oscillation, and into the two capacitors according to several tens MHZ crystal LZ typically about 20 ~ 30P, and is mainly used to fine tune the frequency of the waveform, amplitude and impact, and resistance into depends on IC spec, and some are used for feedback, some of EMI countermeasures too
However, after conversion to parallel equivalent impedance, Re smaller, Rp of the greater, which is readily available formulas. Equivalent Rp very big crystals. Outside and resistance to this and is Rp, then, reducing the value of Rp -----> ----- increases Re> reduced Q
About Crystal
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.
First, the basic principle of quartz crystal oscillators
1, the structure of the quartz crystal oscillator
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.
2, piezoelectric effect
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.
3, equivalent circuit symbols and
When the crystal is not vibrated, it may be referred to as a plate capacitor capacitance C, its size and geometry of the wafer, the electrode area, generally about several tens to several PF PF. When the crystal oscillator, the inertia of the available inductance L of the mechanical vibration equivalently. L values generally tens to hundreds mH mH. Flexible wafer capacitance C equivalently be used, the C value is small, typically only 0.0002 ~ 0.1pF. When the wafer due to friction loss caused by vibrations R equivalently, its value is about 100Ω. Due to the large wafer equivalent inductance, and C is small, R is small, and therefore the circuit quality factor Q large, up to 1,000 to 10,000. Coupled with the resonant frequency of the wafer itself and cutting essentially only the way of the wafer, geometry, size, but also to be done accurately, so the use of a quartz resonator oscillator circuit composed of a high frequency stability is obtained.
4, the resonant frequency
The equivalent circuit seen from the quartz crystal resonator, which has two resonant frequencies, i.e., (1) when L, C, R branch series resonance occurs, its minimum equivalent impedance (equivalent to R). Series resonance frequency is represented by fs arrangements, quartz crystal oscillator frequency fs for the series arrangements in pure resistive, (2) L when the frequency is higher than fs, C, R is inductive branch, with a capacitor C. Parallel resonance, which is represented by the parallel frequency fd.
The equivalent circuit of the quartz crystal, it can be shown qualitatively reactance - frequency characteristic. Visible frequencies below the resonance frequency fs series or parallel arrangements vibration frequency is higher than the frequency fd, quartz crystal substantially capacitive. Only fs
Second, the characteristics of the quartz crystal oscillator type
A quartz crystal oscillator is a high quality factor resonator quartz crystal (i.e. the resonator and the oscillation circuit. Quality crystal, the cut orientation, crystal oscillator circuit structure and the like form, together determine the performance of the International Electrotechnical Commission oscillator ( IEC) the quartz crystal oscillator is divided into four categories: common crystal oscillator (TCXO), a voltage controlled crystal oscillator (VCXO), a temperature compensated crystal oscillator (TCXO), temperature-controlled crystal oscillator (the OCXO) is currently developing there digital compensated crystal oscillation loss (a DCXO) and the like.
Common crystal oscillator (the SPXO) produces 10 ^ (- 5) to 10 ^ (- 4) the order of frequency accuracy, the standard 1-100MHz frequency, frequency stability is ± 100ppm. SPXO not using any frequency temperature compensation measures, inexpensive, it is generally used as the microprocessor clock device. Package sizes range from 21 × 14 × 6mm and 5 × 3.2 × 1.5mm.
Precision voltage-controlled crystal oscillator (VCXO) is 10 ^ (- 6) ~ 10 ^ (- 5) of the order, the frequency range 1 ~ 30MHz. Frequency stability of the oscillator is low tolerance ± 50ppm. Usually used for phase-locked loop. Package size 14 × 10 × 3mm.
Temperature compensated crystal oscillator (TCXO) temperature using a temperature sensitive device for frequency compensation, frequency accuracy of 10 ^ (- 7) to 10 ^ (- 6) magnitude, frequency range 1-60MHz, frequency stability of ± 1 ~ ± 2.5ppm, package size from 30 × 30 × 15mm to 11.4 × 9.6 × 3.9mm. Commonly used in portable telephone, a cellular telephone, two-way wireless communication device and the like.
Thermostatically controlled crystal oscillator (the OCXO) and the crystal oscillation circuit is placed in the incubator, to eliminate the effect of ambient temperature change on the frequency. OCXO frequency accuracy is 10 ^ (- 10) to 10 ^ (- 8) of the order, even for special applications to achieve higher. The highest frequency stability of the oscillators in four types.
Third, the main parameters of the quartz crystal oscillator
The main parameters of the crystal has a nominal frequency of the load capacitance, the frequency accuracy, frequency stability and the like. Different crystal different nominal frequencies, most of the nominal frequency marked on the crystal housing. The common crystal used has a nominal frequency: 48kHz, 500 kHz, 503.5 kHz, 1MHz ~ 40.50 MHz, etc., for the particular requirements of the crystal frequency can reach more than 1000 MHz, a nominal frequency and some not, as CRB, ZTB, Ja series . Crystal load capacitance is connected to two leads IC internal and external blocks all active and capacitance, can be regarded as crystal film series capacitance in the circuit. Load frequency different determinants different oscillation frequencies of the oscillator. Nominally the same frequency oscillator, not necessarily the same load capacitance. Since the quartz crystal oscillator has two resonant frequencies, a crystal oscillator in series arrangements of a low load capacitance of the crystal: the other is a high load capacitance in parallel with the crystal oscillator crystal arrangements. So, the same nominal frequency crystal load capacitance exchange must also be a requirement to not rashly interchangeable, otherwise it will cause appliance is not working properly. Frequency accuracy and frequency stability: since the basic properties of ordinary quartz oscillator can meet the requirements of general electrical, high-end equipment required for a certain frequency accuracy and frequency stability. Frequency accuracy from 10 ^ - order of (4) to 10 ^ (- 10) ranging from the order. Stability range from 100ppm to ± 1 ±. To this selecting a suitable crystal device according to the specific needs, such as communication networks, wireless data transmission systems like the quartz crystal oscillator requires more demanding. Therefore, the parameters of the crystal determines the quality and performance of the crystal. In practical applications, to select the appropriate crystal according to specific requirements, different properties of the crystal vary its price, the more expensive the higher the price requirements, as long as the general requirements can be selected.
Fourth, the development trend of quartz crystal oscillators
1, compact, and thin chip type: mobile phone to satisfy represented portable light, thin, short of the requirements, the quartz crystal oscillator package from the traditional bare metal plastic coated metal shell to the ceramic package. Such devices e.g. TCXO volume reduced by 30 to 100 times. Less than 2mm thickness TCXO SMD package is now 5 × 3mm sized devices already on the market.
2, high accuracy and stability, the current uncompensated crystal oscillator can achieve overall accuracy ± 25ppm, the frequency stability of the VCXO generally up to ± 20 ~ 100ppm in the range of 10 ~ 7 ℃, while at the same temperature range OCXO Usually the frequency stability of ± 0.0001 ~ 5ppm, VCXO control ± 25ppm or less.
3, low noise, high frequency, the GPS communication system is not permitted shaking frequency, the phase noise is an important parameter characterizing the oscillator frequency shaking. Currently phase noise OCXO performance of mainstream products have greatly improved. In addition to the VCXO, other types of crystal oscillator does not exceed the maximum output frequency of 200MHz. For example, a voltage controlled oscillator UCV4 series GSM mobile phones and the like, a frequency of 650 ~ 1700 MHz, the power source voltage 2.2 ~ 3.3V, operating current of 8 ~ 10mA.
4, low, fast startup, low voltage, low current consumption and low driving has become a trend. Typically the power supply voltage is 3.3V. Many TCXO and VCXO products, current consumption does not exceed 2 mA. Quartz crystal oscillators Quick Start technology has also made a breakthrough. For example, Japanese Seiko produced VG-2320SC the VCXO type, under conditions ± 0.1ppm predetermined range, frequency stability time is less than 4ms. Tokyo, Japan ceramics produced by SMD TCXO, 4ms after the oscillation start rating can reach 90%. OAK's of 10 ~ 25 MHz OCXO products, after preheating for 5 minutes, then to achieve stability of ± 0.01 ppm.
Fifth, the quartz crystal oscillator applications
1, go quartz clock accurate, low power consumption, durability and its biggest advantage. Whether old or new multifunction quartz clock quartz are based on quartz crystal oscillator circuit as the core, its frequency accuracy determines the accuracy of travel of the electronic timepiece. From a schematic view of the principles of the quartz crystal oscillator in which the V1 and V2 constituting the CMOS inverter and the Q oscillating quartz crystal capacitors C1 and C2 constitute a resonance system tuning capacitor, where the inductance corresponding to the quartz crystal. Oscillating system component parameter determines the oscillation frequency. Usually Q, C1 and C2 are external elements. In addition to the feedback resistor R1, resistor R2 stable oscillation, which are integrated in the internal circuit. When it can not be individually adjusted by changing the accuracy of the value C1 or C2. But this time we still have a capacitance affixed method C, to change the oscillation parameters of the system to adjust the accuracy of travel. The electronic timepiece when walking speed, adjusting capacitors have two configurations: if walking too fast, and can be connected at both ends of the quartz crystal capacitance C, as shown in FIG. At this time, the total capacitance of the system increases, the oscillation frequency becomes low, while slowing down. Pianman crystal capacitor may be connected in series if the branch away C. As shown in FIG. At this time, the total capacitance of the system is reduced, the oscillation frequency becomes higher, faster travel. As long as the patient through trial and error, you can adjust the accuracy of travel time. Thus, the clock signal generator oscillator may be used.
2, with the development of television technology, use more recently TV 500kHz or 503 kHz crystal oscillator as a line, the field circuit of the oscillation source, obtained by frequency division of 1/3 15625Hz line frequency, its stability and reliability of large In order to improve. Crystal surface and cheap, easy to replace.
3, in the communication system product, the value of the quartz crystal oscillator has been more widely reflected, but also by the faster development. Many quartz crystals are mainly used in high-performance communication networks, wireless data transmission, high-speed digital data transmission.
Crystal load capacitance
Crystal load capacitance elements is the total effective capacitance across the outside across the crystal in the circuit. It refers to the normal shock crystal capacitance required. General external capacitor, are provided so that both ends of the oscillator equivalent capacitive load capacitance is equal or close. High requirements should also be considered where ic input capacitance to ground. Usually the application is given a value near the load capacitance can be accurately adjusted frequency. This mainly affects the magnitude of the capacitance and the resonant frequency of the load resonance resistance equivalent load.
Crystal load capacitance = [(Cd * Cg) / (Cd + Cg)] + Cic + △ C where Cd, Cg is respectively connected to two feet of the crystal and a capacitor to ground, Cic (IC internal capacitance) + △ C (capacitor on the PCB). That is the load capacitance of 15pf, it then on both sides of almost a 27pf, and generally as a 6.5 ~ 13.5pF
Various logic chip pin crystal oscillator equivalent capacitance three internal oscillator pins typically an inverter, or an odd number of inverters in series. XI input pins and the crystal oscillator output pin XO a resistor connected between by, for CMOS chip is typically between several tens of M and M Europe. many internal pin of the chip already contains this resistance, do not pick up external pin. this resistor is provided to enable the inverter at the initial linear oscillation state inverter like a big gain of the amplifier, so that the vibrating quartz crystal is also connected between the input and output pins of the crystal, equivalent to a parallel resonant circuit, oscillation frequency should be the parallel resonant frequency of a quartz crystal. two grounded capacitors next to the crystal, in fact, three-point capacitance of the capacitive voltage division circuit, the ground point is the dividing point. i.e. with a ground voltage dividing point as a reference point, the oscillation primer input and output pins are inverted, i.e. parallel resonant circuit at both ends but quartz view, is formed to ensure a positive feedback circuit continues to oscillate. when the chip design, the two capacitors Has been formed, two generally equal capacity, the capacity depending on the size and layout different processes, but after all, is relatively small, not necessarily suitable for a wide range of frequencies when the number of external PF is about several tens to PF, depending on the frequency, and the characteristics of the quartz crystal may be important to note that: the capacitance values of the two series in parallel in the resonant loop, the oscillation frequency can affect the capacity when the two are equal, the feedback factor is 0.5, generally meet oscillation. conditions, but if the vibrating or oscillating easily unstable can be reduced input capacitance to ground, and the value added to increase the output of the feedback amount.
Design considerations:
1. on the crystal, external capacitors (if any) between the signal line and the IC holding short as possible. When a very low current IC through the crystal oscillator, if the line is too long, it will EMC, ESD and very sensitive to crosstalk effects. And long lines will increase the parasitic capacitance to the oscillator.
2. The other possible clock lines and signal lines disposed away from the crystal in the frequent switching of the position of the connection.
3. Beware of the crystal and the alignment
4. The crystal chassis ground
If the actual load capacitance improper configuration, the first line will cause the reference frequency error. As further transmission-reception circuit on the oscillation amplitude will decrease the crystal (not peaks), affect the signal strength of the signal and the mixed signal to noise.
Clipping occurs when the waveform distortion, the load can be increased to adjust the resistance (several tens to several hundreds K K). To stabilize waveform is approximately in parallel with a feedback resistor 1M.