The basic principle and circuit design of quartz crystal oscillator

zkj2014

The basic principle and circuit design of quartz crystal oscillator [Copy link]

A quartz crystal oscillator is a high-precision, high-stability oscillator. It is a resonant component made using the piezoelectric effect of a quartz crystal. It replaces the LC frequency selection circuit and is widely used in oscillation circuits of various electronic products, such as frequency generators in communication systems. Function: Generate clock signals for data processing equipment, provide system oscillation pulses, stabilize frequency, and select frequency. The following is a brief introduction to the basic characteristics, frequency stabilization conditions, and circuit design of quartz crystal oscillators.

1. Basic characteristics of crystal materials

A quartz crystal oscillator is a resonant device made using the piezoelectric effect of a quartz crystal (a crystal of silicon dioxide). Its basic structure is as follows: a thin slice is cut from a quartz crystal at a certain azimuth angle, a silver layer is applied on its two corresponding surfaces as electrodes, a lead is welded on each electrode and connected to the pin, and then a package shell is added to form a quartz crystal resonator, referred to as quartz crystal or crystal, crystal oscillator. Its products are generally packaged in metal shells, and some are packaged in glass shells, ceramics or plastics.

When quartz crystal is deformed under pressure, it is also polarized. The polarization intensity is proportional to the pressure, and this phenomenon is called "direct piezoelectric effect". Conversely, when the crystal is deformed under the action of an electric field, the deformation is proportional to the electric field intensity, and this phenomenon is called "inverse piezoelectric effect".

Among more than 20 types of crystals with piezoelectric effect, quartz crystal is one of the most satisfactory materials for wireless communication equipment. It has high mechanical strength, stable physical and chemical properties, low internal loss, etc., and devices made of it are widely used in frequency control and frequency selection circuits. The piezoelectric properties, elastic properties and strength properties of quartz sheets are different in different orientations, and the performance of resonators made of it is also different. Dozens of useful cutting methods have been discovered.

2. Frequency stability conditions of quartz resonator

The frequency stability of a quartz crystal is related to the following aspects: load capacitance, excitation level

1. Load capacitance. The quartz resonator generally acts as an inductor component in the oscillation circuit to stabilize the frequency, and the other components of the circuit can be equivalent to a load capacitor and affect the equivalent parameters and frequency stability of the quartz resonator.

All effective capacitance from the two legs of the quartz crystal oscillator to the oscillation circuit is the load capacitance added to the quartz crystal by the oscillation circuit. The load capacitance and the quartz crystal oscillator together determine the operating frequency of the circuit.

By adjusting the load capacitance, the operating frequency of the oscillation circuit can be adjusted to the nominal value. The load capacitance should be selected reasonably. When the capacitance is too large, the influence of stray capacitance is reduced, but the fine-tuning rate decreases ; when the capacitance is too small, the fine-tuning rate increases, but the influence of stray capacitance increases, the load resonant resistance increases, and even oscillation is difficult.

2. Excitation level. Generally, 70-100uA is the best. When expressed in excitation power, 1-100uW is the best. The size of the excitation level directly affects the performance of the quartz resonator, so the circuit designer must strictly control the quartz resonator to work at the specified excitation level in order to give full play to the characteristics of the quartz resonator. Generally speaking, a small excitation level is beneficial for long-term stability, and a slightly larger excitation level is beneficial for short-term stability. However, if the excitation level is too large, the quartz plate will vibrate strongly, the temperature in the vibration area will rise, and a temperature gradient will be generated in the quartz plate, which will reduce the frequency stability; if the excitation level is too large, the mechanical deformation will exceed the elastic limit, causing permanent lattice displacement, causing permanent changes in the frequency, and sometimes even vibrating the quartz plate; if the excitation level is too large, the equivalent resistance will increase, the Q value will decrease, and the resistance temperature characteristics and frequency temperature characteristics will become irregular; if the excitation level is too large, it is easy to excite parasitic vibrations, and at the same time, it will increase aging. Of course, too low an excitation level will also reduce the signal-to-noise ratio and affect short-term stability. If the excitation level is too low, the resonator will not be easy to start, affecting the stability and reliability of the work. Therefore, the excitation level should be strictly controlled according to different requirements when using the resonator, and the excitation level should not be increased arbitrarily to increase the output.

3. Composition and design of quartz oscillation circuit

There are many forms of quartz oscillation circuits, but there are only two basic circuits:

Parallel crystal oscillator and series crystal oscillator. In the former, the quartz crystal appears in the form of parallel resonance, while in the latter, it appears in the form of series resonance. The quartz oscillator circuit is mainly composed of IC, quartz resonator XTAL, resistor, PCB, etc. The following is a brief description of each component:

1. IC: It can be selected according to practical needs, such as CMOS inverter IC; special oscillator IC; self-contained quartz oscillation gate in large-scale digital IC.

2. Load characteristics: The oscillation circuit (excluding CRYSTAL) has a negative resistance characteristic (using a network spectrum analyzer). The negative resistance value (absolute value) of the oscillation circuit must be greater than 3 times the resistance value of CRYSTAL (CRYS2TAL ESR/RR) so that the oscillation circuit can oscillate stably.

3. Capacitor: You can choose chip-type or traditional structure products according to the needs of users, but you should pay attention to choosing those suitable for high frequency and low loss, and choose NP0 series temperature coefficient products.

4. Resistors: When choosing the size, pay attention to power consumption.

5. PCB: Pay attention to meeting high-frequency usage and reducing the distribution parameters of the circuit as much as possible, adopt large-area grounding and one-point grounding solutions. Pay attention to EMI protection when necessary.

6. Quartz resonator XTAL: Resonators with a frequency greater than 1MHz should use ATMODE. You can choose the appropriate size and packaged crystal according to the needs of the user.


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