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Dec 13 2018

The Functions and Basic Working Principles of Crystal Oscillator

Warm hints: This article contains about 5000 words and reading time is about 20 mins.

Introduction

The crystal oscillator is an electromechanical device made of a quartz crystal with low electrical loss, which is precisely cut and plated with electrodes. This crystal has a very important characteristic. If it is energized, he will produce mechanical oscillation. On the contrary, if he is given mechanical force, he will generate electricity. This characteristic is called electromechanical effect. They have a very important feature, and their oscillation frequency is closely related to their shape, material, cutting direction and so on. Since the chemical properties of quartz crystals are very stable, the coefficient of thermal expansion is very small, and the oscillation frequency is also very stable. Since the control geometry can be very precise, the resonance frequency is also very accurate. According to the electromechanical effect of the quartz crystal, we can equivalent it to an electromagnetic oscillation circuit, that is, a resonant circuit. Their electromechanical effect is the constant conversion of machine-electric-machine-electric.... The resonant circuit composed of inductor and capacitor is the constant conversion of electric field-magnetic field. The application in the circuit is actually regarded as a high Q electromagnetic resonant circuit. Since the loss of the quartz crystal is very small, that is, the Q value is very high, when used as an oscillator, a very stable oscillation can be generated and used as a filter to obtain a very stable and steep band pass or band resistance curve.

Article Core

Crystal Oscillator

Purpose

Introduce the basic knowledge of crystal oscillator

Application

Graphics card, network card, motherboard and other accessories.

Keywords

Crystal oscillator

Catalog

Introduction


Crystal Oscillator Definition


Working Principle of Crystal Oscillator


Parameters of Crystal Oscillator


Type of Crystal Oscillators


Testing Method of Crystal Oscillator

Resistance Method

Self-made Tester

 

 

 

 

 

 

Crystal Oscillator stability Index

Total Frequency Difference

Frequency Stability

Frequency Temperature Stability

Power-on Characteristics (Frequency Stable Warm-up Time)

Frequency Aging Rate

Short-Term Stability

Reproducibility

Frequency Voltage Control Range

Voltage-controlled Frequency Response Range

Frequency Voltage Control Linearity

Single Sideband Phase Noise £(f)

Output waveform

Application of Crystal Oscillator


Two Common Methods of Crystal Oscillator In the Gate Circuit




Crystal Oscillator Definition 

The crystal oscillator is the most important component in the clock circuit. Its function is to provide the reference frequency to the parts of the graphics card, network card, motherboard and other accessories. It is like a ruler. The unstable working frequency will cause the operating frequency of the related equipment to be unstable and naturally easy. problem appear. Due to the continuous improvement of the manufacturing process, the important technical indexes such as frequency deviation, temperature stability, aging rate and sealing of the crystal oscillator are very good, and it is not easy to cause failure, but the quality of the crystal can still be noticed when selecting. What role does the crystal oscillator play in the application? The clock source of the microcontroller can be divided into two categories: clock sources based on mechanical resonant devices, such as crystal oscillators, ceramic resonant tanks, and RC (resistor, capacitor) oscillators. One is the Pierce oscillator configuration for crystal and ceramic resonant tanks. The other is a simple discrete RC oscillator. Oscillator based on crystal and ceramic resonant tanks typically provides very high initial accuracy and a low temperature coefficient. The RC oscillator can be started quickly and at a lower cost, but is typically less accurate over the entire temperature and operating supply voltage range and will vary from 5% to 50% of the nominal output frequency. However, its performance is affected by environmental conditions and circuit component selection. The component selection and board layout of the oscillator circuit must be taken seriously.

When in use, the ceramic resonant tank and the corresponding load capacitance must be optimized for a specific logic family. A crystal with a high Q value is not sensitive to the choice of amplifier, but it is prone to frequency drift (and possibly even damage) during overdrive. Environmental factors that affect the operation of the oscillator are: electromagnetic interference (EMI), mechanical shock and shock, humidity and temperature. These factors increase the output frequency variation, increase instability, and in some cases, cause the oscillator to stop. Most of the above problems can be avoided by using the oscillator module. These modules come with an oscillator, provide a low-impedance square wave output, and are guaranteed to operate under certain conditions. The two most common types are crystal modules and integrated RC oscillators (silicon oscillators). The crystal module provides the same accuracy as a discrete crystal. Silicon oscillators are more accurate than discrete RC oscillators and, in most cases, provide comparable accuracy to ceramic resonant tanks. Power consumption also needs to be considered when selecting an oscillator. The power consumption of the discrete oscillator is mainly determined by the supply current of the feedback amplifier and the capacitance value inside the circuit. The CMOS amplifier power consumption is proportional to the operating frequency and can be expressed as the power dissipation capacitor value. For example, the power dissipation capacitor value of the HC04 inverter gate is 90pF. When operating at 4MHz, 5V power supply, it is equivalent to 1.8mA supply current. Coupled with a 20pF crystal load capacitor, the entire supply current is 2.2mA. Ceramic resonant tanks typically have large load capacitances and correspondingly require more current. In contrast, crystal modules typically require a supply current of 10 mA to 60 mA. The power supply current of a silicon oscillator depends on its type and function, ranging from a few microamps of low frequency (fixed) devices to a few milliamps of programmable devices. A low-power silicon oscillator, such as the MAX7375, requires less than 2mA to operate at 4MHz. Optimizing the clock source for a specific application requires a combination of factors such as accuracy, cost, power consumption, and environmental requirements.

The crystal oscillator controls the clock frequency of the CPU, that is, the period in which the high and low levels are generated (generating a high level, and a low level is a period). Generally speaking, the higher the frequency, the speed at which the computer processes in unit time. The faster the crystal itself does not oscillate, but it will resonate with the external circuit at a fixed frequency, provided that the oscillation frequency of the external circuit must be consistent with the natural oscillation frequency of the crystal, at least very close, otherwise the circuit will stop Vibration. Regarding the test, in general, the use of a multimeter to measure the resistance (referring to the needle movement) is damaged (the needle with a low oscillation frequency will also be slightly swayed, but will immediately return to zero), the needle does not move (resistance infinity), it may be good, It is possible to open the lead.


Working Principle of Crystal Oscillator 

The crystal can be electrically equivalent to a capacitor and a resistor in parallel and then connected in series with a capacitor. The electrical network has two resonance points. The frequency is high and low, and the lower frequency is series resonance. The high frequency is a parallel resonance. Due to the characteristics of the crystal itself, the distance between the two frequencies is quite close. In this extremely narrow frequency range, the crystal oscillator is equivalent to an inductor, so as long as the crystal oscillator is connected in parallel with a suitable capacitor, it will form a parallel resonant circuit. . The parallel resonant circuit is added to a negative feedback circuit to form a sine wave oscillating circuit. Since the crystal oscillator is equivalent to a narrow frequency range of the inductor, even if the parameters of other components vary greatly, the frequency of the oscillator will not be change dramatically.



Parameters of Crystal Oscillator 

The crystal oscillator has an important parameter, that is, the load capacitance value. Selecting the parallel capacitance equal to the load capacitance value can obtain the nominal resonant frequency of the crystal oscillator.


Types of Crystal Oscillators

Resonant oscillators include quartz (or its crystal material) crystal resonators, ceramic resonators, LC resonators, etc. 

The crystal oscillator and the resonant oscillator have their own intersection active crystal resonant oscillator.

The reason why quartz wafer can oscillate circuit (resonance) is based on its piezoelectric effect. It is known from physics that if an electric field is added between the two plates of the wafer, the crystal will be deformed mechanically. On the contrary, if mechanical force is applied between the plates, the electric field will be generated in the corresponding direction. This phenomenon is called piezoelectric effect. If the alternating voltage is applied between the plates, the mechanical deformation vibration will occur, and the mechanical deformation vibration will produce alternating electric field. Generally speaking, the amplitude of this mechanical vibration is relatively small, and its vibration frequency is very stable. But when the frequency of the applied alternating voltage is equal to the natural frequency of the wafer (which depends on the size of the wafer), the amplitude of mechanical vibration will increase sharply. This phenomenon is called piezoelectric resonance, so quartz crystal is also called quartz crystal resonator. Its characteristic is high frequency stability. 


Testing Methods of Crystal Oscillator 

Crystal multimeter test method

Tip: How to measure whether the crystal oscillator starts to vibrate without an oscilloscope?

You can use a multimeter to measure whether the voltage of the two pins of the crystal oscillator is half of the working voltage of the chip. For example, if the working voltage is 5V, it is measured whether it is about 2.5V. In addition, if you touch the other foot of the crystal with a tweezers, this voltage has a significant change, which proves to be oscillating.

Tip: Just get a 1.5V battery and connect the crystal oscillator to the ear at the two ends of the crystal. Listen carefully. When you hear the humming sound, it means that it starts to vibrate. It is good!


Resistance Method 

Put the multimeter in the R×10K block and measure the resistance between the two pins of the quartz crystal to be infinite. If the measured resistance value is not infinite or even close to zero, it indicates that the crystal under test is leaking or breakdown.

This method can only measure whether the crystal is leaking. If there is an open circuit inside the crystal, the resistance method can't do anything. In this case, the method described below must be used.


Self-made Tester 

According to the circuit shown in the figure, soldering a simple quartz crystal tester can accurately test the quality of the crystal. In the figure, the two test sockets of XS1 and XS2 can be removed from the socket of the small seven-legged or small nine-legged tube socket. It is better to select high brightness of the LED tube. When detecting the quartz crystal, insert the two pins of the quartz crystal into the two sockets of XS1 and XS2, press the switch SB, if the quartz crystal is good, the oscillating circuit composed of the components such as the triode VT1, C1, C2, etc. Oscillation, the oscillating signal is coupled to the VD2 detection via C3, and the detected DC signal voltage turns VT2 on, so the LED connected in the VT2 collector circuit emits light, indicating that the quartz crystal to be tested is good, if the LED is not bright, then The quartz crystal being tested is bad. This tester tests quartz crystals at a wide frequency, but the optimum operating frequency is several hundred kilohertz to tens of megahertz.

An Introduction to Crystal Oscillators

A Simple Quartz Crystal Tester


Crystal Oscillator Stability Index

a. Total Frequency Difference: The maximum deviation of the crystal oscillator frequency from a given nominal frequency due to the combination of the specified working and non-working parameters within a specified time.

Note: The total frequency difference includes the maximum frequency difference caused by the frequency temperature stability, the deviation caused by the frequency aging rate, the frequency voltage characteristics and the frequency load characteristics. It is generally only used in the case of short-term frequency stability, but not in other frequency stability indicators. For example: precision guided radar.


b. Frequency Stability: Any crystal oscillator, the frequency instability is absolute, the degree is different. The curve of the output frequency of a crystal with time is shown in Fig. 2. The figure shows three factors of frequency instability: aging, drift and short stability.

Figure 2 Schematic diagram of crystal oscillator output frequency as a function of time

Figure 2 Schematic diagram of crystal oscillator output frequency as a function of time

Curve 1 is measured once in 0.1 seconds, showing the shortness of the crystal; curve 3 is measured once in 100 seconds, showing the drift of the crystal; curve 4 is measured once in 1 day. It shows the aging of the crystal.


c. Frequency Temperature Stability: under nominal power supply and load, the maximum allowable frequency offset working within a specified temperature range without an implied reference temperature or with an implied reference temperature.

ft=±(fmax-fmin)/(fmax+fmin)
ftref =±MAX[|(fmax-fref)/fref|,|(fmin-fref)/fref|]

ft:Frequency temperature stability (without implied reference temperature)

ftref:Frequency temperature stability (with implied reference temperature)

fmax :The highest frequency measured within the specified temperature range

fmin:The lowest frequency measured within the specified temperature range

fref:Specify the frequency measured by the reference temperature

Note: The crystal oscillator with ftref index is more difficult to produce than the crystal oscillator with ft index, so the crystal oscillator of ftref index is higher.


d. Power-on Characteristics (Frequency Stable Warm-up Time): Refers to the rate of change of the frequency of a period of time (such as 5 minutes) after power-on to another period (such as 1 hour) after power-on. Indicates the speed at which the crystal reaches a steady state. This indicator is useful for frequently switched instruments such as frequency meters.

Note: In most applications, crystal oscillators are powered for a long time. However, in some applications, crystal oscillators require frequent power-on and shutdown. At this time, the frequency stability warm-up time indicator needs to be taken into consideration (especially for harsh Military communication stations used in the environment, when frequency temperature stability is required to be ≤±0.3ppm (-45°C~85°C), OCXO is used as the local oscillator, the frequency stable warm-up time will be no less than 5 minutes, and only MCXO is needed. Ten seconds).


e. Frequency Aging Rate: The relationship between the oscillator frequency and time when measuring the oscillator frequency under constant environmental conditions. This long-term frequency drift is caused by the slow change of the crystal component and the oscillator circuit component. Therefore, the rate of the frequency offset is called the aging rate, and the maximum rate of change after the specified time limit can be used (eg, ±10 ppb/day, power-on 72). After the hour), or the maximum total frequency change within the specified time limit (eg ±1ppm/(first year) and ±5ppm/(ten years)).

Crystal aging is due to problems such as stress, contaminants, residual gases, structural process defects, etc. in the production of crystals. The stress has to be stabilized over a period of time. A crystal cutting method called "stress compensation" (SC cutting method) gives the crystal a good characteristic.

The molecules of contaminants and residual gases will deposit on the crystal chip or oxidize the crystal electrode. The higher the oscillation frequency, the thinner the crystal slice will be and the greater the effect will be. This effect is gradually stabilized over a long period of time, and the stability is repeated with the change of temperature or working state, which causes the contaminants to reconcentrate or disperse on the crystal surface. Therefore, the aging rate of the crystal oscillator with low frequency is better than that of the crystal oscillator with short working time, and the crystal oscillator with low frequency is better than the crystal oscillator with high frequency.

Note: The frequency aging rate of TCXO is: ±0.2ppm~±2ppm (first year) and ±1ppm~±5ppm (ten years) (except for special cases, TCXO rarely uses the daily frequency aging rate index, because even in the experiment Under the conditions of the chamber, the frequency change caused by the temperature change will also greatly exceed the frequency aging of the temperature compensated crystal oscillator every day, so this indicator loses its practical significance). The frequency aging rate of OCXO is ±0.5 ppb to ±10 ppb/day (after 72 hours of power-on), ±30 ppb to ±2 ppm (first year), and ±0.3 ppm to ±3 ppm (ten years).


f. Short-term Stability: short-term stability. Observation time is 1 Ms, 10 Ms, 100 Ms, 1 sec, 10 sec.

The output frequency of the crystal oscillator is affected by the internal circuit (Q value of the crystal, noise of the component, stability of the circuit, operating state, etc.), resulting in a wide spectrum of instability. After measuring a series of frequency values, it is calculated using the Allen equation. Phase noise can also reflect short-stable conditions (with special instrument measurements).


g. Reproducibility: Definition: After the crystal oscillator is stable for a long time, it will be shut down after a long period of time. It will stop for a period of time t1 (such as 24 hours), boot for a period of time t2 (such as 4 hours), measure the frequency f1, and then stop for the same period of time t1, then The power is turned on for the same period of time t2, and the frequency f2 is measured. Reproducibility = (f2-f1) / f2.


h. Frequency Voltage Control Range: The minimum peak value of the crystal oscillator frequency is adjusted from the reference voltage to the specified end voltage and the crystal oscillator frequency.

Note: The reference voltage is +2.5V, the specified end voltage is +0.5V and +4.5V, and the frequency-controlled crystal oscillator has a frequency change of -2ppm at +0.5V frequency control voltage, and the frequency is controlled at +4.5V frequency control voltage. The amount of change is +2.1ppm, then the VCXO voltage control frequency voltage control range is expressed as: ≥ ± 2ppm (2.5V ± 2V), the slope is positive, and the linearity is +2.4%.


i. Voltage-controlled Frequency Response Range: The relationship between peak frequency offset and modulation frequency when modulation frequency changes. It is usually expressed in dB with a specified modulation frequency lower than the specified modulation reference frequency.

Note: The frequency response of VCXO frequency voltage control range is 0 ~ 10 kHz.


j. Frequency Voltage Control Linearity: Output Frequency vs. Ideal (Linear) Function - A measure of the input control voltage transfer characteristic that is a percentage of the allowable nonlinearity of the entire range of frequency offsets.

Note:The typical VCXO frequency voltage control linearity is: ≤±10%, ≤±20%. The simple VCXO frequency voltage control linear calculation method is (when the frequency voltage control polarity is positive):

Frequency voltage control linearity = ± ((fmax - fmin) / f0) × 100%

Fmax: Output frequency of VCXO at maximum voltage control voltage

Fmin: Output frequency of VCXO at minimum voltage control voltage

F0: voltage control center voltage frequency


k. Single Sideband Phase Noise £(f): The ratio of the power density of a phase modulated sideband to the carrier power at carrier f.


l. Output Waveform: From a large class, the output waveform can be divided into two types: square wave and sine wave.

Square wave is mainly used in the clock of digital communication system. The other wave mainly has several index requirements such as output level, duty ratio, rise/fall time and driving capability.

With the rapid development of science and technology, high-quality signal sources are required as carriers for increasingly complex baseband information in similar systems such as communications, radar, and high-speed data transmission. Since a carrier signal with parasitic amplitude modulation and phase modulation (unclean signal) is modulated by the baseband signal carrying the information, the spectral components (parasitic modulation in the carrier) that should not exist in these ideal states will cause the transmitted The signal quality and the data transmission error rate are obviously deteriorated. Therefore, as the carrier of the transmitted signal, the cleanness of the carrier signal (spectral purity) has a direct impact on the communication quality. For sinusoidal waves, it is often necessary to provide indicators such as harmonics, noise, and output power.


Application of Crystal Oscillator 

The general crystal oscillator circuit is connected to the crystal oscillator at both ends of an inverting amplifier (note that the amplifier is not an inverter), and then two capacitors are respectively connected to the two ends of the crystal oscillator, and the other end of each capacitor is connected. Ground, the capacitance value of these two capacitors in series should be equal to the load capacitance. Please note that the pins of the general IC have equivalent input capacitance, which cannot be ignored.

The load capacitance of a typical crystal oscillator is 15p or 12.5p. If you consider the equivalent input capacitance of the component pins, it is better to have two 22p capacitors to form the crystal oscillator. Crystal oscillators are also classified into passive crystals and active crystals. The passive crystal oscillator is different from the English crystal name of the active crystal oscillator (resonance). The passive crystal oscillator is crystal, and the active crystal oscillator is called oscillator. The passive crystal oscillator needs to use the clock circuit to generate the oscillating signal, and it cannot oscillate itself. Therefore, the term "passive crystal oscillator" is not accurate; the active crystal oscillator is a complete resonant oscillator.

Figure 3 shows the infrared emission circuit

Figure 3 shows the infrared emission circuit


Figure 4 is a crystal oscillator circuit

Figure 4 is a crystal oscillator circuit

In the circuit, J, VD1, L1, C3~C5, and V1 form a crystal oscillation circuit. Since quartz crystal J has good frequency stability and is less affected by temperature, it is widely used in cordless telephones and AV modulators. V1 is a 29-36MHz crystal oscillatory triode. The emitter output is rich in harmonic components. After V2 amplification, the 3rd-frequency signal (ie 87-108MHz) is selected in the network where the collector is composed of C7 and L2 and is resonant at 88-108MHz. The signal is the strongest, and then amplified by V3, L3, C9 frequency selection to get the ideal FM band signal. The process of frequency modulation is such that the change of the audio voltage causes a change in the capacitance between the electrodes of VD1. Since VD1 is connected in series with the crystal J, the oscillation frequency of the crystal also changes slightly. After the triple frequency, the frequency offset is a crystal of 29 to 36 MHz. 3 times the frequency offset. In practical applications, in order to obtain a suitable degree of modulation, a quartz crystal or a ceramic vibrator having a large modulation frequency offset may be selected, or a circuit with a slightly complicated circuit of 6 to 12 frequency may be used. If the input audio signal is weak, a first-stage voltage amplifying circuit can be added.

FIG. 5 is an application of a crystal oscillator in a time base oscillating circuit 555

FIG. 5 is an application of a crystal oscillator in a time base oscillating circuit 555


Two Common Methods of Crystal Oscillator In the Gate Circuit

a. The advantage of this connection is that it is easy to start and adapt to a wide frequency range. I don't remember the specific frequency range.

Two Common Methods of Crystal Oscillator In the Gate Circuit

b. The advantages of this connection method are simple, the disadvantage is that it is not so easy to start, C1, C2 should be suitable.

Two Common Methods of Crystal Oscillator In the Gate Circuit


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