Those of you who are regularly involved in electronics purchasing will often come across electronic components like crystal oscillators. So, what is the function of a crystal oscillator? How does it work? The crystal oscillator is a very critical part of digital circuit design. Usually in circuit design, the crystal oscillator is regarded as the core part of the digital circuit, because all the work of the digital circuit can not be separated from the clock signal. The crystal oscillator is the key button that directly controls the proper startup of the whole system. It can be said that as long as there is a digital circuit design, you can see the existence of crystal oscillator.

First, the definition of crystal oscillation

Crystal oscillations are often referred to as quartz crystal oscillators and quartz crystal resonators, or crystal oscillators. They are both generated by the piezoelectric effect of quartz crystals.

Crystal oscillators work on the following principle:When an electric field is applied to the two electrodes of a crystal, the crystal is mechanically deformed; on the contrary, if a mechanical pressure is applied to the two ends of the crystal, the crystal generates an electric field. This phenomenon is reversible,Double-sided PCB so we can utilize this property of the crystal to apply an alternating voltage at both ends of the crystal, so that the wafer generates mechanical vibration and alternating electric field at the same time. The vibration and electric field generated by the crystal is generally very small, but as long as a certain frequency, the amplitude will increase significantly, similar to our circuit designers often see the LC loop resonance.

Second, the classification of the crystal

① passive crystal oscillator

Passive crystal oscillator is a through the crystal, usually 2-pin non-polarized device (some passive crystal oscillator has no different polarity fixed a pin).

A passive crystal oscillator usually requires a clock circuit consisting of a load capacitor to generate an oscillating signal (sinusoidal signal).

②Active Crystal Oscillator

An active crystal oscillator is an oscillator that usually has four pins. An active crystal oscillator does not require the CPU's internal oscillator to generate a square wave signal. Active crystal oscillators can generate clock signals when power is applied.

Active crystal oscillator signal stability, good quality, simple connection, accuracy error is less than the passive quartz crystal resonator, but the price is higher than the passive crystal oscillator.

Third, the equivalent circuit of the crystal oscillator

In fact, the crystal oscillator is like a series RLC circuit.

The equivalent control circuit of a crystal oscillator shows a series RLC circuit, which represents the mechanical structure of the crystal oscillator vibrating with an electrical device connected in parallel with the crystal oscillator through a capacitor, while the crystal oscillator is managed towards series resonance operation.

Where R represents ESR equivalent series resistance, L represents equivalent inductance, C represents capacitance, CP represents parasitic capacitance.

The basic parameters of crystal oscillator

Generally speaking, the basic parameters of crystal oscillator include operating temperature, accuracy value, matching capacitance, package form and core frequency.

The core frequency of the crystal oscillator usually depends on the requirement of the frequency component. For microcontrollers, it is usually selected within a certain range, ranging from 4m to tens of M at most.

The accuracy of crystal oscillators is generally in the range of 5 ppm, 10 PPM, 20 PPM, and 50 ppm. High-precision clock chips are generally within 5 ppm, and general applications will choose about 20 PPM.

The matching capacitance of the crystal can be changed at the same time by constantly adjusting the value of the matching capacitance to change the core frequency of the crystal. Currently in the design of high-precision crystal oscillator, are we use this teaching method needs to be analyzed and adjusted.

Printed circuit board crystal oscillator design and layout

Crystal as the core part of the digital circuit, affecting the stability of the entire system. The choice of the system crystal determines the success or failure of the digital circuit.

Since the crystal oscillator contains quartz crystals, the crystals may be broken by external impact. Therefore, the reliable location of the crystal oscillator must be considered in the circuit design and avoided as much as possible to be close to the board edges, equipment housings and other places. The following points should be noted in the PCB layout.

① The crystal oscillator should not be too close to the board edge, and its case must be grounded to prevent the crystal from radiating stray waves. Especially we need to pay attention to this when designing the board. Case grounding can prevent the crystal from radiating outward, while shielding external signals on the crystal interference. If it must be arranged at the edge of the PCB, you can lay another GND line at the edge of the printed line of the crystal, and punch a hole at a certain distance from the overlay ground line to enclose the crystal.

② crystal below we can not cloth signal lines, so as not to affect the resulting data signal lines coupled to the crystal resonance noise. To ensure that the enterprise completely paved the ground, while the development of the crystal within 300mil do not wiring, so that one can effectively prevent crystal interference as well as other integrated wiring, components and layers of performance.

If the filtering device is placed under the quartz crystal resonator, and the capacitance and matching resistor of the filter are not arranged in the direction of signal flow, the filtering effect of the filter will deteriorate. The coupling capacitors should be placed as close as possible to the crystal power supply pins, and arranged in order from the largest to the smallest capacity value according to the power supply direction flow.

④ The wiring for the clock signal should be as short and wide as possible, and a balance should be found between the length of the wiring and the distance from the heat source. The following is an example of optimized layout.

Place the filter capacitors and matching circuits close to the microcontroller chip and away from the board edge.

② Arrange the filter capacitors and matching resistors according to the signal flow direction and place them neatly and compactly near the crystal.

③ Place the crystal near the China chip and make the alignment to the chip as short and straight as possible.

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