Working Principle and Application of Quartz Crystal Oscillator

　　First, let's take a brief look at what a quartz resonator is. A quartz resonator is referred to as a crystal oscillator. It is made of a quartz crystal sheet with a piezoelectric effect. This quartz crystal sheet will produce mechanical vibrations when subjected to an external alternating electric field. When the frequency of the alternating electric field is the same as the natural frequency of the quartz crystal, the vibration becomes very strong. This is the response of the crystal resonance characteristic. This article will introduce the working principle and application of quartz crystal oscillators.

　　Working Principle of Quartz Crystal Oscillator

　　Computers have a timing circuit. Although the word "clock" is commonly used to refer to these devices, they are not actually clocks in the usual sense. It might be more appropriate to call them timers. A computer timer is usually a precision-machined quartz crystal that oscillates within its tension limits at a certain frequency, which depends on how the crystal itself is cut and the amount of tension it is subjected to. There are two registers associated with each quartz crystal, a counter and a holding register. Each oscillation of the quartz crystal decrements the counter by 1. When the counter reaches 0, an interrupt is generated and the counter is reloaded with the initial value from the holding counter. This method makes it possible to program a timer to interrupt 60 times per second (or at any other desired frequency). Each interrupt is called a clock tick.

Figure 1. Quartz oscillator

　　A crystal oscillator can be electrically equivalent to a two-terminal network consisting of a capacitor and a resistor in parallel and then a capacitor in series. In electrical engineering, this network has two resonant points, with the lower frequency being the series resonance and the higher frequency being the 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 a suitable capacitor is connected in parallel at both ends of the crystal oscillator, it will form a parallel resonant circuit. This parallel resonant circuit can be added to a negative feedback circuit to form a sinusoidal oscillation circuit. Since the frequency range in which the crystal oscillator is equivalent to an inductor is very narrow, the frequency of the oscillator will not change much even if the parameters of other components change greatly.

　　The crystal oscillator has an important parameter, which is the load capacitance value. By selecting a parallel capacitor equal to the load capacitance value, the nominal resonant frequency of the crystal oscillator can be obtained. The general crystal oscillator circuit is connected to the crystal oscillator at both ends of an inverting amplifier (note that it is an amplifier, not an inverter), and 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 capacitance value of these two capacitors in series should be equal to the load capacitance. Please note that the pins of general ICs have equivalent input capacitance, which cannot be ignored. The load capacitance of a general crystal oscillator is 15p or 12.5p. If the equivalent input capacitance of the component pins is considered, two 22p capacitors to form the crystal oscillator circuit are a better choice.

　　Applications of Quartz Crystal Oscillators

　　The biggest advantages of quartz clocks are accurate timekeeping, low power consumption, and durability. Whether it is an old quartz clock or a new multifunctional quartz clock, the core circuit is a quartz crystal oscillator, and its frequency accuracy determines the timekeeping accuracy of the electronic clock. From the schematic diagram of the quartz crystal oscillator principle, V1 and V2 constitute a CMOS inverter, and the quartz crystal Q, the oscillation capacitor C1, and the fine-tuning capacitor C2 constitute an oscillation system. Here, the quartz crystal is equivalent to an inductor. The component parameters of the oscillation system determine the oscillation frequency. Generally, Q, C1, and C2 are external components. In addition, R1 is a feedback resistor, and R2 is an oscillation stabilization resistor, which are integrated inside the circuit. Therefore, it is impossible to adjust the timekeeping accuracy by changing the value of C1 or C2. But at this time, we can still use a method to add a capacitor C to change the parameters of the oscillation system to adjust the timekeeping accuracy.

Figure 2: Quartz oscillator

　　There are two ways to adjust the capacitor according to the speed of the electronic clock: if the clock is too fast, capacitor C can be connected in parallel at both ends of the quartz crystal, as shown in Figure 4. At this time, the total capacitance of the system increases, the oscillation frequency becomes lower, and the clock is slower. If the clock is too slow, capacitor C can be connected in series in the crystal branch. As shown in Figure 5. At this time, the total capacitance of the system decreases, the oscillation frequency becomes higher, and the clock speed increases. As long as you patiently repeat the test, you can adjust the accuracy of the clock. Therefore, the crystal oscillator can be used as a clock signal generator.

　　With the development of television technology, color TVs recently use 500kHz or 503kHz crystal oscillators as the oscillation source of the horizontal and vertical circuits. After 1/3 division, the horizontal frequency is 15625Hz, which greatly improves its stability and reliability. In addition, the crystal oscillator is cheap and easy to replace.

　　Summarize

　　Quartz crystal oscillators are miniaturized, thinned and sliced ​​to meet the requirements of portable products such as mobile phones, which are light, thin and short. In communication system products, the value of quartz crystal oscillators has been more widely reflected and has also been developed more rapidly. Many high-performance quartz crystal oscillators are mainly used in communication networks, wireless data transmission, high-speed digital data transmission, etc.