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The principle and function of crystal oscillator

Published time: 2018-09-03


A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a precise frequency.The crystal oscillator is the most important component of the clock circuit. Its main 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 related equipment to work at an unstable frequency, which is naturally prone to problems.Another effect of the crystal oscillator is to generate an oscillating current in the circuit and issue a clock signal.

It works in a resonant state with a crystal that converts electrical energy and mechanical energy to each other to provide stable, accurate single-frequency oscillation. Under normal working conditions, the ordinary crystal frequency has an absolute accuracy of 50 parts per million, and the advanced precision is higher. Some crystal oscillators can also adjust the frequency within a certain range by adding an external voltage, called a voltage controlled oscillator (VCO).

Jotrin Electronics limited said that the basic role of crystal oscillators is to provide a standard moment of timing control in digital circuits. The work of the digital circuit is based on the circuit design, and the specific task is completed at a certain moment. If there is no standard time for timing control, the whole digital circuit will become a "deaf" and do not know what to do at any time.

In order to obtain an AC signal in the circuit, it can be obtained by an RC or LC resonance circuit, but the oscillation frequency of these circuits is not stable. In circuits requiring high stable frequencies, a quartz crystal oscillator circuit must be used. Quartz crystal has a high quality factor. After the oscillation circuit adopts constant temperature and voltage regulation, the oscillation frequency stability can reach 10^(-9) to 10^(-11). it was used widely in communications, clocks, watches, computers... where high stability signals are required.Jotrin Electronics limited reminds that you should know the quartz crystal oscillator is not divided into positive and negative poles, and the outer casing is the ground wire.

The principle and function of crystal oscillator

The crystal oscillator is a commonly used clock component in the circuit. In the single-chip system, the role of the crystal oscillator is very large. It combines the internal circuit of the microcontroller to generate the clock frequency necessary for the microcontroller, and the execution of all instructions of the microcontroller is established. Base on it, the higher the clock frequency provided by the crystal oscillator, the faster the microcontroller will run.

The role of the crystal is to provide the system with a basic clock signal. Generally speaking, a system shares a crystal to keep the parts in sync. Some communication systems use different crystal oscillator for the fundamental and RF frequencies, and they are synchronized by adjusting the electronical frequency.The crystal oscillator is typically used in conjunction with a phase-locked loop circuit to provide the clock frequency required by the system. If different subsystems require clock signals of different frequencies, they can be provided by different phase-locked loops connected to the same crystal oscillator.

Next, I will specifically introduce the function of crystal oscillator and its working principle. The crystal oscillator generally adopts the capacitor three-terminal AC equivalent oscillation circuit as shown in Figure 1a; the actual crystal oscillator AC equivalent circuit is shown in Figure 1b, where Cv is used to adjust the oscillation frequency. Generally, the varactor diode is added with different reverse bias voltages, which is also the mechanism of voltage control; the equivalent circuit of the crystal is replaced by the crystal as shown in Fig. 1c. Where Co, C1, L1, RR are the equivalent circuits of the crystal oscillator.

crystal oscillator circuit diagram

Figure 1, crystal oscillator circuit diagram

Analysising of the entire oscillation tank shows that the use of Cv to change the frequency is limited: the entire tank capacitance C=Cbe, Cce, Cv, which determines the oscillation frequency, is connected in series with Co in parallel with C1. 

It can be seen that,the smaller C1 is, the larger Co is, and the smaller effect for Cv on the entire tank capacitance. Therefore, the frequency range in which "voltage control" can be made is smaller. In fact, since C1 is small (on the order of 1E-15), Co cannot be ignored (1E-12 magnitude, a few PF).

 When Cv becomes larger, the effect of lowering the frequency of the groove is getting smaller and smaller, and when Cv becomes smaller, the effect of increasing the frequency of the groove is getting larger and larger. 

This aspect causes the nonlinearity of the voltage control characteristics. The larger the voltage control range, the more the nonlinearity is. On the other hand, the feedback voltage (the voltage on the Cbe) that is distributed to the oscillation is getting smaller and smaller, and finally the vibration is stopped. 

The crystal oscillator with a higher overtone frequency has a smaller equivalent capacitance C1; therefore, the frequency variation range is smaller.

How to use a multimeter to measure whether the crystal oscillator works. 

Measuring whether the voltage of the two pins is half of the working voltage of the chip. For example, if the operating voltage is +5V of the 51 MCU, 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.

The effection of crystal oscillator in applications

Jotrin Electronics believes that the specific role of crystal oscillators in the clock source application of microcontrollers 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 oscillator and ceramic resonant tanks. The other one 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 they can be at a lower cost, but it 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 use the compenents, 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.

What is the environmental factors that affect the operation of the oscillator

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, the oscillator will stop working. 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 has 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.

Jotrin Electronics limited reminds everyone to consider the power consumption when choosing 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 power dissipation of a CMOS amplifier is proportional to the operating frequency and can be expressed as a 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 a large load capacitance and correspondingly require a larger 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 overview of MAX7375

The MAX7375 is a silicon oscillator, intended as a low-cost improvement replacing ceramic resonators, crystals, and crystal oscillator modules used as the clock source for microcontrollers and UARTs in 3V, 3.3V, and 5V applications.

The MAX7375 is a fully integrated oscillator, supplied at specific factory-trimmed frequencies with a rail-to-rail 50% duty cycle square-wave output. The oscillator frequency is generated directly without the use of a phase-locked loop (PLL). No additional components are used to set or adjust the frequency.

The MAX7375 datasheet and pdf


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