Operating principle of Crystal Oscillator

Crystal Oscillator-Basic overview

Crystal Oscillator

The crystal is called a crystal oscillator, which is used to generate the original clock frequency. This frequency is amplified or reduced by the frequency generator and becomes a variety of bus frequencies in the computer. Taking the sound card as an example, to sample the analog signal 44.1khz or 48 kHz, the frequency generator must provide a 44.1khz or 48 khz clock frequency. If you need to support both types of audio, the sound card must have two crystal oscillator. However, entertainment sound cards generally use SRC to set the output sampling frequency to 48 khz to reduce costs. However, Src will damage the sound quality, in addition, the current entertainment sound card does not solve this problem well.

The crystal oscillator is generally called a crystal resonator. It is an electromechanical device. It is a Z crystal with a low power loss. It is made by precision cutting and grinding and plated with electrodes and welding leads. This kind of crystal has a very important feature. If it is powered on, it will produce mechanical oscillation. On the contrary, if it is powered on, it will generate electricity again, this feature is called electromechanical effect. They have a very important feature, and their oscillation frequency is closely related to their shape, material, and cutting direction. Because the chemical properties of Z crystals are very stable and the thermal expansion coefficient is very small, the oscillation frequency is also very stable. Because the controlled geometric size can be very precise, the resonance frequency is also very accurate. According to the mechanical and electrical effects of Z crystal, we can regard it as an electromagnetic oscillation loop, that is, a resonant loop. Their mechanical and electrical effects are the constant conversion of machine-electric-machine-electric. The resonant circuit consisting of inductance and capacitance is the constant conversion of the electric field-magnetic field. The application in the circuit actually regards it as an electromagnetic resonance loop with a high Q. Because the loss of the Z crystal is very small, that is, the Q value is very high. When the oscillator is used, a very stable oscillation can be generated for the filter, you can obtain a very stable and steep band-pass or band-resistance curve.

 

Crystal Oscillator-Main Parameters

Parameters	Basic description
Frequency Accuracy	When the nominal power supply voltage, nominal load impedance, reference temperature (252 ℃), and other conditions remain unchanged, the frequency of the crystal oscillator is equal to the maximum allowable deviation from the specified nominal value, that is, (fmax-fmin)/f0;
Temperature Stability	If other conditions remain unchanged, the maximum variation of the output frequency of the crystal oscillator within the specified temperature range is the allowable frequency offset value (fmax-fmin) relative to the sum of the output extreme values of the temperature range) /(fmax + fmin );
Frequency adjustment range	Adjust a variable element of the crystal oscillator to change the output frequency range.
Frequency Modulation (Pressure Control) Features	Including FM frequency offset, FM sensitivity, and FM linearity. ① Frequency modulation frequency deviation: the output frequency difference when the control voltage of the Voltage Controlled Crystal Oscillator changes from the nominal maximum value to the minimum value. ② Frequency Modulation sensitivity: The amount of output frequency changes caused by Voltage Controlled Crystal Oscillator variation units plus control voltage. ③ Frequency modulation linearity: it is a measure of the transmission characteristics of the modulation system compared with the ideal straight line (Least Square Method.
Load Characteristics	Other conditions remain unchanged, and the maximum allowable frequency deviation between the output frequency of the crystal oscillator and the output frequency under the nominal load within the specified range of variation.
Voltage Characteristics	Other conditions remain unchanged. the maximum allowable frequency deviation between the output frequency of the crystal oscillator and the output frequency under the nominal power supply voltage within the specified range of variation.
Clutter	The power ratio of the discrete spectrum component to the main frequency in the output signal that has no harmonic (except the sub-harmonic) Relationship with the main frequency, which is expressed by DBC.
Harmonic	The ratio of the Power pI of the harmonic component to the power P0 of the carrier, which is expressed by DBC.
Frequency Aging	The system drift process of the output frequency with time due to aging of components (mainly Z resonator) under the specified environment conditions. It is usually measured by the frequency difference within a certain time interval. For a high-stability crystal oscillator, the aging rate (relative frequency variation per unit time) is often used to measure the output frequency of a single-direction drift in an approximate linear manner during a long working period.
Daily fluctuation	After the specified preheating time, the oscillator is measured every hour for 24 consecutive hours. The test data is calculated in the S = (fmax-fmin)/f0 formula to obtain daily fluctuations.
Boot features	The maximum variation of the oscillator frequency value within the specified preheating time is represented by V = (fmax-fmin)/f0.
Phase Noise	Short-Term Stability Measurement in the frequency domain. The ratio of single-band noise to carrier noise is £ (f), which indicates the spectral density of £1 (f) to noise fluctuation, and the spectral density of frequency fluctuation, Sy (f) directly related, represented by the following formula: F2s (f) = f02sy (f) = 2f2 £( F) F-Fourier frequency or deviation carrier frequency; F0-carrier frequency.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Crystal Oscillator-Basic Classification

Crystal Oscillator

Crystal oscillator is also divided into two types: passive crystal oscillator and active crystal oscillator. The English name of the passive crystal oscillator is different from that of the active crystal oscillator. The Passive crystal oscillator is crystal, and the active crystal oscillator is oscillator ). The passive crystal oscillator uses a clock circuit to generate an oscillating signal. Therefore, the "passive crystal oscillator" is not accurate. The active crystal oscillator is a complete resonant oscillator. Both the Z crystal oscillator and the Z crystal resonator are electronic devices that provide stable circuit frequencies. The Z crystal oscillator uses the piezoelectric effect of the Z crystal to start vibration, while the Z Crystal Resonator works by the combination of the Z crystal and the built-in IC. The oscillator is directly applied to the circuit. The 3.3v voltage is generally required for the operating of the resonator. The oscillator has one more important technical parameter than the resonator: Resonant Resistance (RR). The resonator has no resistance requirements. The RR size directly affects the circuit performance. Therefore, this is an important parameter for the competition among sellers.

 

 

Crystal Oscillator-Working Principle

Crystal Oscillator

Computers all have a timing circuit. Although the word "Clock" is generally used to represent these devices, they are not actually the common clock, and they are called timers) it may be more appropriate. A computer timer is usually a precision-processed Z crystal. A Z crystal oscillates at a certain frequency within its tension limit, depending on how the crystal itself is cut and how much tension it receives. Two registers are associated with each Z crystal, one counter and one holdingregister ). Each oscillation of the Z crystal reduces the counter by 1. When the counter is reduced to 0, an interruption occurs. The counter reloads the initial value from the keep counter. This method makes programming a timer, making it possible to generate 60 interruptions per second (or interrupt at any other desired frequency. Each interruption is called a clocktick ).

The crystal oscillator can be electrically equivalent to a two-end network in which a capacitor is connected in parallel with a resistor and a capacitor is connected. In electroengineering, the network has two resonance points, the low frequency is the series resonance, and the high frequency is the parallel resonance. Because of the characteristics of the crystal, the distance between the two frequencies is very close. In this very narrow frequency range, the crystal oscillator is equivalent to an inductor, as long as the two ends of the crystal oscillator are connected in parallel with an appropriate capacitor, it will constitute a parallel resonance circuit. This parallel resonant circuit can be added to a negative feedback circuit to form a sine wave oscillator circuit. Because the frequency range of the crystal oscillator equivalent to inductance is very narrow, even if the parameters of other components change greatly, the frequency of this oscillator will not change significantly. An important parameter of the crystal oscillator is the load capacitance value. You can obtain the nominal resonance frequency of the crystal oscillator by selecting a parallel capacitor equal to the load capacitance value. Generally, the oscillator circuit is connected to the crystal oscillator at both ends of a reversed-phase amplifier (note that the amplifier is not a inverter). Then, two capacitors are connected to the two ends of the crystal oscillator respectively, and the other end of each capacitor is connected to the ground, the capacity value of these two capacitors in series should be equal to the load capacitor. Note that the general IC pins have the equivalent input capacitor, which cannot be ignored. Generally, the load capacitor of the crystal oscillator is 15 P or 12.5 P. If the equivalent input capacitor of the element pin is considered, the two 22p capacitors constitute the oscillator circuit of the crystal oscillator.

 

 

Crystal Oscillator-Function

Crystal Oscillator

The crystal oscillator plays a specific role in the application. The microcontroller clock source can be divided into two types: the clock source based on the mechanical resonator, such as the crystal oscillator, ceramic resonant slot; RC (resistance, capacitor) oscillator. One is the Pierce oscillator configuration, which is suitable for crystal oscillator and ceramic resonant channels. The other is a simple discrete RC oscillator. The oscillator Based on the crystal oscillator and the ceramic resonant trough can provide very high initial accuracy and low temperature coefficient. RC oscillator can be quickly started and the cost is relatively low, but the accuracy is usually poor throughout the temperature and operating power supply voltage range, will change within the nominal output frequency of 5% to 50%. However, its performance is affected by environmental conditions and the selection of circuit components. Take the component selection and circuit board layout of the oscillator circuit seriously. In use, the ceramic resonant slot and the corresponding load capacitance must be optimized according to the specific logic series. The high-Q crystal oscillator is not sensitive to the amplifier selection, but it is prone to frequency drift (or even damage) during over-driving ). Environmental factors that affect the operation of the oscillator include electromagnetic interference (EMI), mechanical vibration and shock, humidity and temperature. These factors increase the change in the output frequency, increase instability, and in some cases, cause the oscillator to stop vibration. Most of the above problems can be avoided by using the oscillator module. These modules have their own oscillator, provide low-resistance square wave output, and can run under certain conditions. The two most common types are the crystal oscillator module and the integrated RC oscillator (silicon Oscillator ). The crystal oscillator module provides the same precision as the discrete crystal oscillator. The accuracy of the silicon oscillator is higher than that of the discrete RC oscillator. In most cases, it can provide the same precision as that of the ceramic resonant channel.

Power Consumption also needs to be considered when selecting an oscillator. The power consumption of the discrete oscillator is mainly determined by the power supply current of the feedback amplifier and the capacitance value inside the circuit. The power consumption of the CMOS amplifier is proportional to the operating frequency, which can be expressed as the power dissipation capacitance value. For example, the power dissipation capacitance value of the hc04 inverter door circuit is 90pf. When the power supply is 4 MHz or 5 V, it is equivalent to 8mA of the power supply current. With the load capacitor of 20pf crystal oscillator, the overall power supply current is 2mA. The ceramic resonant trough generally has a large load capacitance, which also requires more current. In contrast, the power supply current of the crystal oscillator module is generally 10mA ~ 60mA. The power supply current of the silicon oscillator depends on its type and function, which can range from several micro-security of the low frequency (fixed) device to several milliamps of the programmable device. A low-power silicon oscillator, such as max7375, operates at 4 MHz with less than 2mA of the current. The following factors must be taken into account to optimize the clock source in specific applications: precision, cost, power consumption, and environmental requirements.