Crystal oscillator is the abbreviation of quartz crystal oscillator (quartz crystal oscillator), also called active crystal oscillator, which can generate the clock frequency signal necessary for the central processor (CPU) to execute instructions. The execution of all CPU instructions is based on this basis. The higher the clock signal frequency, the faster the CPU generally runs.
As long as it is an electronic product containing a CPU, it contains at least one clock source. Even if the actual oscillation circuit is not visible outside, it is integrated within the chip, which is called the heart of the circuit system.
As shown in the figure below, the active crystal oscillator can output a preset periodic clock signal after applying an appropriate voltage externally.
The nominal frequency ( Normal Frequency ) of this periodic output signal is the frequency specified in the crystal component specifications, and is also the parameter that engineers are primarily concerned with in circuit design and component purchase. The nominal frequency of crystal is usually between 1 and 200MHz, such as 32768Hz, 8MHz, 12MHz, 24MHz, 125MHz, etc., and higher output frequency is also commonly used PLL (phase locked loop) to double the low frequency to more than 1GHz.
The frequency of the output signal will inevitably have a certain deviation. We use Frequency Error ( FrequencyTolerance ) or Frequency Stability ( Frequency Stability ) , expressed in units of ppm, that is, parts per million (1/106), It is the amount of change from the nominal frequency. The smaller the value, the higher the accuracy.
For example, the deviation of a 12MHz crystal oscillator is ± 20ppm, indicating that its frequency deviation is 12 × 20Hz = ± 240Hz, that is, the frequency range is (11999760 ～ 12000240Hz)
In addition, there is a temperature frequency difference ( Frequency Stability vs Temp ) that indicates the allowable deviation of the operating frequency from the reference temperature at a specific temperature range, and its unit is also ppm.
We often see some other parameters, such as load capacitance, resonance resistance, static capacitance and other parameters, is it the situation of Shenma? These are related to the physical properties of the crystal. Let ’s first understand the crystal, as shown in the figure below
Quartz crystals have a characteristic. If pressure is applied to a certain axis of the wafer, a certain potential will be generated in the direction of the corresponding force
Conversely, when an electric field is applied in certain axial directions of the crystal, the crystal will be mechanically deformed;
If an alternating voltage is applied to the quartz wafer, the crystal will produce mechanical vibration, and the mechanical deformation vibration will generate an alternating electric field. Although the voltage of this alternating electric field is extremely weak, its vibration frequency is very stable. When the frequency of the applied alternating voltage is equal to the natural frequency of the wafer (related to the size of the wafer after cutting, the thinner the crystal, the more difficult the cutting, and the higher the resonance frequency), the amplitude of the mechanical vibration will increase sharply. This phenomenon is called It is "piezo resonance".
After cutting the quartz wafer into a certain shape, and then sandwiching it with two electrode plates, a passive crystal oscillator is formed. The symbol diagram is as follows:
The figure below is a simplified circuit with the same impedance characteristics as the crystal resonator near the resonance frequency.
Among them: C1 is the dynamic equivalent series capacitance;
L1 is the dynamic equivalent series inductance;
R1 is the dynamic equivalent series resistance, which is the friction equivalent in the crystal
C0 is a static capacitance, equivalent to the capacitance between two electrode plates;
This equivalent circuit has a frequency response characteristic curve as shown in the following figure:
When the frequency at which the R1, L1, and C1 series branches resonate is the series resonance frequency (Fr), the capacitive reactance and the inductive reactance cancel each other. Therefore, the branch is equivalent to only the equivalent series resistance R1.
This frequency is the natural resonant frequency of the crystal. It is used as a design parameter for the stable operation of the crystal at the nominal frequency, determining the frequency adjustment range, setting the frequency fine-tuning device, etc. in the design of the highly stable crystal (but not the nominal frequency ), The expression is as follows:
The equivalent series resistance R1 determines the quality factor of the crystal element. The quality factor is also known as the mechanical Q value. It is an important parameter that reflects the performance of the resonator. It has the following relationship with L1 and C1:
The larger the R1, the lower the Q value will lead to unstable frequency. Conversely, the higher the Q value, the more stable the frequency. The characteristic of the crystal is that it has a high quality factor.
The equivalent circuit also has an anti-resonance frequency fL (parallel resonance frequency). At this time, the series branch appears as an inductive reactance, which is equivalent to an inductance, as shown in the following figure:
The frequency at this time is shown below:
Usually the nominal frequency given by the manufacturer's crystal component data sheet is not Fr or FL. When the actual crystal component is used in an oscillating circuit, it will generally be connected to the load capacitor to work the crystal between Fr and FL. A certain frequency of this frequency is determined by the phase and effective reactance of the oscillation circuit. By changing the reactance conditions of the circuit, the crystal frequency can be adjusted within a limited range.
When the load capacitance is connected in series with the crystal, as shown in the figure below:
The small capacitor CL connected in series can adjust the resonance frequency of the quartz crystal within a small range. At this time, the new load resonance frequency is as follows:
Among them, C1 is much smaller than C0 + CL
When the load capacitance is connected in parallel with the crystal, as shown in the figure below:
Similarly, the parallel load CL can also adjust the resonance frequency in a small range. The corresponding load resonance frequency is as follows:
From a practical point of view, for a given load capacitance value, the two frequencies F'r and F'L are the same. This frequency is the actual frequency exhibited by most crystal applications, and it is also the manufacturer's The test index parameter for the product that meets the nominal frequency requirements, which is the nominal frequency of the crystal oscillator introduced at the beginning of this article,
When the crystal element is connected to an external capacitor (parallel or series), the resistance at the load resonance frequency is the load resonance resistance RL, which is always greater than the resonance resistance of the crystal element itself.
The crystal itself cannot generate an oscillating signal and must be implemented with the help of a corresponding external oscillator circuit. The following figure is a series oscillator circuit , in which a two-stage amplifier composed of transistors Q1 and Q2, a quartz crystal X1 and a capacitor CL LC circuit. In this circuit, the quartz crystal is equivalent to an inductance, CL is a variable capacitor, and the capacitance can be adjusted to make the circuit enter a resonance state, and the output waveform is a square wave.
The parallel oscillator circuit is shown in the following figure. Readers of this form may see more. Generally, the single chip microcomputer will have such a circuit. The two pins of the crystal oscillator are connected to the internal inverter of the chip (such as a single-chip microcomputer), and then combined with external matching capacitors CL1, CL2, R1, R2 to form a Pierce oscillator
In the figure above, U1 is an inverting amplifier with a large gain, CL1 and CL2 are matching capacitors, which are the voltage divider capacitors of the three-point capacitor circuit, and the ground point is the voltage divider point. Taking the grounding point or the voltage dividing point as the reference point, the input and output are reversed, but from the perspective of the parallel resonant circuit, that is, both ends of the quartz crystal, a positive feedback is formed to ensure that the circuit continues to oscillate. Used to fine-tune the frequency and waveform, and affect the amplitude. X1 is a crystal, equivalent to the inductance in the three-point type
R1 is the feedback resistance (generally ≥1MΩ), which makes the inverter in the linear working region at the beginning of oscillation, R2 and the matching capacitor form a network, provide a 180-degree phase shift, and at the same time limit the oscillation amplitude and prevent the output of the inverter from The crystal oscillator is damaged by overdriving.
A very important parameter involved in the crystal oscillator is the load capacitance CL (Load capacitance ) , which is the total effective capacitance (not the matching capacitor external to the crystal oscillator) connected across the crystal in the circuit, which mainly affects the load resonance frequency and equivalent The load resonance resistance, together with the crystal, determines the operating frequency of the oscillator circuit. By adjusting the load capacitance, the operating frequency of the oscillator can be fine-tuned to the nominal value.
The formula for the load capacitance is as follows:
Among them, CS is the parasitic capacitance between the two pins of the crystal ( Shunt Capacitance )
CD represents the total capacitance from the output pin of the crystal oscillation circuit to ground, including PCB trace capacitance CPCB, chip pin parasitic capacitance CO, and matching capacitor CL2, that is, CD = CPCB + CO + CL2
CG represents the total capacitance of the crystal oscillation circuit input pin to ground, including PCB trace capacitance CPCB, chip pin parasitic capacitance CI, plus matching capacitor CL1, that is, CG = CPCB + CI + CL1
Generally, CS is about 1pF, CI and CO are generally several picofarads, please refer to the chip or crystal data sheet
(It is assumed here that CS = 0.8pF, CI = CO = 5pF, and CPCB = 4pF).
For example, the load capacitance value in the specification is 18pF, then there is
Then CD = CG = 34.4pF, the calculated matching capacitance value CL1 = CL2 = 25pF
It's so complicated, I can't understand it. I want to use simpler, more stable and more accurate devices. Have!
Active crystal oscillator encapsulates all passive crystal oscillators and related oscillation circuits in a "box", there is no need to manually match the peripheral circuits accurately. When different output frequencies are used, only a "box" of corresponding frequency needs to be purchased Then use complicated formula calculation to calculate, can save a lot of brain cells to do other more meaningful work.
The schematic diagram of the encapsulated "box" is shown below:
