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 Crystal oscillators can help electronic systems provide frequencies for synchronous operation, as frequency references or to achieve accurate timing. In microprocessor-based systems, there are several different frequency signals used to execute instructions, move data into and out of memory, and external communication interfaces. A simple embedded controller may have a clock frequency of several MHz, while microprocessors in personal computers usually expect an input frequency of 15 MHz. This will multiply internally to provide the frequency of the CPU and other subsystems. Other components in the system may have their own frequency requirements. In addition to providing the basic requirements of the specified frequency, the oscillator may have to meet other requirements depending on the application requirements of the product. For example, many product applications require extremely precisely defined frequencies. This is particularly important for systems that need to communicate with other devices through serial or wireless interfaces. Accuracy is usually measured in parts per million (PPM). At the same time, the trimming circuit can be based on resistance capacitance (RC) or inductance capacitance (LC) networks. These devices are relatively simple and can change the frequency in a wide range. However, designing an accurate RC oscillator or LC oscillator requires the use of expensive precise components. Even so, they cannot meet the highest accuracy and stability required by many product applications. Crystal oscillators (usually quartz) can also be used as resonant components. Cut the crystal into two parallel crystal planes and deposit metal contacts on them. Quartz has piezoelectric effect, which means that when the crystal is placed under pressure, voltage will be generated on its crystal surface. On the contrary, when voltage is applied to the crystal, the crystal will also change its shape.

 There are many kinds of potentiometers, including the commonly used wired potentiometers and non wired potentiometers and the electronic potentiometers widely used in audio circuits. However, no matter how they change their forms, their main working principles remain unchanged. The key components of potentiometer are resistor and brush. Potentiometers can be divided into several types according to the structural form between them and whether there is a switch. Potentiometers can also be classified according to the materials of resistors, such as wire winding, synthetic carbon film, metallic glass glaze, organic solid core and conductive plastic. The electrical properties mainly depend on the materials used. Then, what are the technical requirements of potentiometer products in terms of main product parameters? The main parameters of potentiometer include nominal resistance, rated power, resolution, sliding noise, resistance change characteristics, wear resistance, zero resistance and temperature coefficient. 1. Rated power The maximum power allowed to dissipate on the two fixed ends of the potentiometer is the rated power of the potentiometer. In use, it should be noted that the rated power is not equal to the power of the center tap and the fixed end. The rated power of potentiometer refers to the maximum power allowed to be consumed by long-term continuous load under the specified rated temperature in DC or AC circuit when the atmospheric pressure is 87~107kpa. 2. Nominal resistance These parameters are generally marked on the nominal resistance value of the product, and their series is similar to the series of resistors. 3. Allowable error level The error range between the measured resistance value and the nominal resistance value can be ± 20%, ± 10%, ± 5%, ± 2% and ± 1% according to different accuracy levels. The accuracy of precision potentiometer can reach 0.1% . 4. Resistance variation law It refers to the change relationship between the resistance value and the rotation angle (or sliding stroke) of the sliding contact. This change relationship can be in any functional form, and commonly used are linear, logarithmic and inverse logarithmic (exponential).

 Resistors are generally directly called resistors in daily life. It is a current limiting element. After connecting the resistor in the circuit, the resistance value of the resistor is fixed, usually two pins, which can limit the current through the branch connected to it. Those whose resistance cannot be changed are called fixed resistors. Those with variable resistance are called potentiometers or variable resistors. The ideal resistor is linear, that is, the instantaneous current passing through the resistor is proportional to the applied instantaneous voltage. Variable resistor for voltage division. On the exposed resistor, one or two movable metal contacts are pressed tightly. The contact position determines the resistance between either end of the resistor and the contact. According to the classification of materials, resistors can be basically divided into five types of resistors, including wire wound resistors, carbon synthetic resistors, carbon film resistors, metal film resistors and metal oxide film resistors. I will show you the internal structure of mainstream resistors through five kinds of resistors. Wire wound resistors are made by winding resistance wires. Resistors are made by winding high resistance alloy wires on the insulation skeleton, and are coated with heat-resistant glaze insulation layer or insulating paint. Winding resistance has low temperature coefficient, high resistance accuracy, good stability, heat resistance and corrosion resistance. It is mainly used as precision high-power resistance. Its disadvantages are poor high-frequency performance and large time constant. First of all, the wire wound resistor is to wind the nickel chromium alloy wire on the alumina ceramic substrate, and control the resistance circle by circle. The winding resistance can be made into a precision resistance with a tolerance of 0.005%. At the same time, the temperature coefficient is very low. The disadvantage is that the parasitic inductance of the winding resistance is relatively large and cannot be used in high frequency. The volume of the wound resistor can be made very large, and then add an external radiator, which can be used as a high-power resistor. Secondly, the carbon synthetic resistor is made of carbon and synthetic plastic. The concentration of carbon powder determines the resistance value. Tin plated copper leads are added at both ends, and finally packaged. The carbon film resistor is made by plating a layer of carbon on the ceramic tube and depositing the crystalline carbon on the ceramic rod skeleton. Carbon film resistor is the most widely used resistor at present because of its low cost, stable performance, wide resistance range and low temperature coefficient and voltage coefficient. In order to control the resistance more accurately, spiral grooves can be machined on the carbon film. The more spirals, the greater the resistance; Finally, metal leads are added, and resin packaging is formed. The process of carbon film resistor is a little more complicated, and it can be used as a precision resistor. However, due to the carbon quality, the temperature characteristic is still not very good. The metal film resistor is formed by coating a layer of metal on the ceramic tube, and the alloy material is evaporated on the surface of the ceramic rod skeleton by vacuum evaporation. Compared with carbon film resistance, metal film resistance has higher accuracy, better stability, lower noise and temperature coefficient. It is widely used in instruments and communication equipment. Finally, the metal oxide film resistor is formed by plating a layer of tin oxide on the porcelain tube and depositing a layer of metal oxide on the insulating rod. Because it is an oxide itself, it is stable at high temperature, resistant to heat shock, and has strong load capacity. According to its purpose, it has general purpose, precision, high frequency, high voltage, high resistance, high power and resistance network.