Introduction to the classification of quartz crystal oscillators and common detection methods

Quartz crystal oscillator is abbreviated as quartz crystal, commonly known as crystal oscillator, it is made of quartz crystal with piezoelectric effect. This kind of quartz crystal sheet will produce mechanical vibration when subjected to an applied 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, which is the response of the crystal resonance characteristics. Using this characteristic, it can be used to stabilize the frequency and select the frequency, replacing the LC (coil and capacitor) resonant circuit, filter, etc.

Quartz crystal oscillator is a high-precision and high-stability oscillator. It is widely used in various types of oscillator circuits such as color TVs, computers, and remote controls, as well as in communication systems for frequency generators and for data processing. Clock signal and provide reference signal for specific system.

Quartz resonators have extremely high frequency stability, so they are mainly used as resonant components or frequency references in oscillation circuits that require very stable frequencies (using quartz crystal as an oscillating circuit component to form a crystal oscillator to generate the fundamental frequency of the clock signal Devices), such as the color subcarrier oscillator of color TVs, the time base oscillator of electronic clocks and the clock pulse oscillator of game consoles, etc. The cost of quartz crystal is relatively high, so ceramic resonant components are generally used in circuits that are not too demanding.

1 Classification of quartz crystal oscillators

Quartz crystal oscillators can be divided into ordinary quartz crystal oscillators, precision quartz crystal oscillators, medium-precision quartz crystal oscillators and high-precision quartz crystal oscillators according to accuracy (or frequency stability); they can be divided into metals according to the package structure and shape Shell crystal oscillators, glass shell crystal oscillators, bakelite shell crystal oscillators and plastic shell crystal oscillators. Metal shell encapsulated quartz crystal oscillators are available in three types: tin welding, cold pressure welding and resistance welding; according to the number of lead electrodes Divided into two-electrode (two-terminal) crystal oscillator, three-electrode (three-terminal) crystal oscillator and four-electrode (four-terminal) crystal oscillator; according to the purpose can be divided into color TV crystal oscillators, DVD players Crystal oscillators, crystal oscillators for wireless communication, crystal oscillators for electronic clocks and other types are used; according to the basic resonant circuit, it can be divided into two types: parallel crystal oscillators and series crystal oscillators. The appearance of a common quartz crystal oscillator is shown in Figure 1.

Figure 1 The appearance of common quartz crystal oscillators

2 Detection of quartz crystal oscillator

To inspect the quartz crystal, firstly check the appearance. The surface of the normal quartz crystal is clean and free of cracks, the pins are firm and reliable, and the resistance value is infinite. If the resistance measured with a multimeter is very small or even close to zero, it means that the measured crystal is leaking or punctured and has been damaged. If the measured resistance is infinite, it means that the quartz crystal has no breakdown and leakage, but it cannot be determined whether the crystal is damaged. At this time, it can be further tested according to the different capacitances of crystals of different frequencies. Use the capacitance block of a digital multimeter to measure the capacitance of the quartz crystal, and then compare the data to determine whether the quartz crystal is damaged. Several commonly used methods for detecting crystal oscillators are as follows.

2.1 Resistance method measurement

Use a multimeter to measure the forward and reverse resistance values ​​of the quartz crystal oscillator with the R×10k block. Normally, it should be ∞ (infinity). If the quartz crystal oscillator has a certain resistance value or is 0, it means that the quartz crystal oscillator has leaked or broken down. But the reverse is not true, that is, if the resistance measured by a multimeter is infinite, then the quartz crystal cannot be completely judged as good; in this case, another method can be used to further judge.

2.2 Capacitance method measurement

By measuring the capacitance of the quartz crystal oscillator with a capacitance meter or a digital multimeter with capacitance measurement function, it can be roughly judged whether the quartz crystal oscillator has changed value. For example, the approximate capacitances of 45kHz, 480kHz, 500kHz and 560kHz quartz crystal oscillators commonly used in remote control transmitters are 296-310pF, 350-360pF, 405-430pF, and 170-196pF, respectively. If the measured capacity of the quartz crystal oscillator is greater than the approximate value or there is no capacity, it can be determined that the quartz crystal oscillator has changed value or is damaged by an open circuit.

2.3 Detect with test circuit

The general method for quartz crystal testing is to make a simple oscillator circuit, and connect the quartz crystal to be tested (note that the two leads of the quartz crystal oscillator should not be too close together) to see if it starts to oscillate. If it starts to vibrate, the crystal is good, otherwise it is bad. The detection circuit of commonly used quartz crystal is shown as in Fig. 2. The quartz crystal under test and the transistor V1 form an oscillator. If the crystal is good, the oscillator starts. The signal is coupled to VD1, VD2, rectified by C3 and filtered by C4, and then amplified by the amplifier circuit to drive the LED to emit light; if the crystal is damaged, then The oscillator cannot start, and the LED cannot emit light. In the figure, BX represents the crystal under test.

Figure 2 Quartz crystal detection circuit

There is also the simplest way to roughly judge the quality of quartz crystals. The method is: insert a test pencil tip into the live wire hole of the mains, hold one foot of the quartz crystal with your finger, and touch the top of the test pencil with the other foot. If the neon bulb is red, it generally means that the quartz crystal is good; if the neon bulb is not bright, it means that the crystal is broken.