What is crystal oscillator in electronics | Xtalong
1. What is a crystal oscillator?
Definition: Crystal oscillators generally refer to quartz crystal oscillators, also called crystal oscillators.
A crystal oscillator is an electronic oscillator circuit that uses the inverse piezoelectric effect, i.e. when an electric field is applied across certain materials, it undergoes mechanical deformation. Therefore, it uses the mechanical resonance of a vibrating crystal of piezoelectric material to generate an electrical signal with a very precise frequency.
Crystal oscillators have high stability, quality factor, small size, and low cost, which make them superior to other resonators such as LC circuits, ceramic resonators, rotating forks, etc.
Circuit symbol: Crystal oscillator is one of the most commonly used electronic components in electronic circuits. It is generally represented by the letters "X", "G" or "Z", and the unit is Hz.
2. How is the crystal oscillator made? --How to change from a quartz blank to a crystal oscillator?
A quartz blank is used as a resonant element in an oscillating circuit, and when subjected to a voltage potential, it will start to vibrate and oscillate at its "fundamental frequency", which is a correlation: the circuit supports mechanical resonance and vice versa. Crystals are used in the feedback loop of an oscillator to limit the frequency of the oscillator.
3. How Crystal Oscillators Work
Quartz crystal oscillator is a resonant device made by using the piezoelectric effect of quartz crystal. Its basic composition is roughly: cut a thin slice from a quartz crystal at a certain azimuth angle, and coat silver on its two corresponding surfaces. The layer is used as an electrode, and a lead wire is welded to each electrode to connect to the pin, and a package shell is added to form a quartz crystal resonator, which is referred to as a quartz crystal or crystal or crystal oscillator for short. Its products are generally packaged in metal shells, but also in glass shells, ceramics or plastics.
Piezoelectric effect: If an electric field is applied to the two electrodes of the quartz crystal, the wafer will be mechanically deformed. Conversely, if mechanical pressure is applied on both sides of the wafer, an electric field will be generated in the corresponding direction of the wafer. This physical phenomenon is called piezoelectric effect.
If an alternating voltage is applied to the two poles of the wafer, the wafer will generate mechanical vibration, and at the same time, the mechanical vibration of the wafer will generate an alternating electric field.
Under normal circumstances, the amplitude of the mechanical vibration of the wafer and the amplitude of the alternating electric field are very small, but when the frequency of the applied alternating voltage is a certain value, the amplitude increases significantly, which is much larger than that at other frequencies. This phenomenon is called piezoelectric resonance, which is very similar to the resonance phenomenon of the LC circuit. Its resonant frequency is related to the cutting method, geometry and size of the wafer.
When the crystal does not vibrate, it can be regarded as a plate capacitance called electrostatic capacitance C. Its size is related to the geometric size of the chip and the electrode area, generally about several picofarads to tens of picofarads. When the crystal oscillates, the inertia of mechanical vibration can be equivalent to the inductance L.
4. Equivalent Circuit of Crystal Oscillator
In effect, the crystal behaves like a series RLC circuit, consisting of the components:
Low value resistor R S
Large value inductance L S
Small value capacitor C S
It is then connected in parallel with the capacitance of its electrode Cp.
The equivalent circuit for a crystal shows a series RLC circuit, which represents the crystal's mechanical vibration, in parallel with a capacitor, Cp, which represents the electrical connection to the crystal. Quartz crystal oscillators tend to operate towards their "series resonance".
5. Crystal impedance frequency
The equivalent impedance of the crystal has a series resonance, where Cs resonates with the inductor Ls at the operating frequency of the crystal. This frequency is called the crystal series frequency, ƒs. In addition to this series frequency, a parallel resonance occurs when Ls and Cs resonate with a parallel capacitor Cp,
As the frequency increases across its terminals, at a particular frequency, the interaction between the series capacitor Cs and the inductor Ls creates a series resonant circuit that minimizes the crystal impedance and equals Rs, this frequency point is called crystal series Below the resonant frequency ƒs, the crystal is capacitive.
As the frequency increases above this series resonance point, the crystal behaves like an inductor until the frequency reaches its parallel resonance frequency, ƒp.
At this frequency point, the interaction between the series inductance Ls and the parallel capacitor Cp creates a parallel tuned LC resonant circuit, so the impedance across the crystal reaches a maximum.
6. Crystal reactance frequency
The slope of reactance versus frequency above shows that the series reactance at frequency ƒs is inversely proportional to Cs because below ƒs and above ƒp the crystal is capacitive.
Between frequencies ƒs and ƒp, the crystal is inductive due to the cancellation of the two parallel capacitors.
7. Main Factors Affecting Crystal Oscillation Frequency
Change of working point
We have learned about transistors before and know the importance of the operating point. For crystal oscillators, the stability of this operating point requires higher consideration.
The operation of the active devices used is tuned to the linear part of their characteristics, this point shifts due to temperature changes, so stability suffers.
temperature change
Oscillation circuits in oscillation circuits contain various components such as resistors, capacitors, and inductors. All their parameters are dependent on temperature, and their values are affected due to changes in temperature, which in turn affects the frequency of the vibrating circuit.
power impact
Variations in supply power affect the frequency, and variations in the supply cause changes in V cc, which in turn affects the frequency of the resulting oscillation.
To avoid this, a regulated power supply system, or RPS for short, is implemented.
Output load change
Variations in output resistance or output loading can affect the frequency of the oscillator. When a load is connected, the effective resistance of the tank circuit changes.
The Q factor of the LC tuned circuit changes, which causes the output frequency of the oscillator to change.
Variation in Capacitance Between Elements
Inter-element capacitance is the capacitance that develops in PN junction materials such as diodes and transistors due to the charge present during their operation.
Capacitance between elements changes due to various factors such as temperature and voltage. However, this problem can be solved by connecting the capacitor across the problematic inter-component capacitor.
Q value
The Q (quality factor) value in the oscillator must be high. The Q value in a tuned oscillator determines the selectivity. Since this Q is directly proportional to the frequency stability of the tuned circuit, the value of Q should be kept high.
If the Q value changes, it will affect the frequency stability.