21 September
The oscillator is an energy conversion device that converts DC power into AC power with a certain frequency, and the circuit formed by it is called an oscillating circuit. Some electronic devices require an AC signal with a highly stable frequency, but the LC oscillator has poor stability and the frequency is easy to drift. A special component-quartz crystal is used in the oscillator, which can generate a highly stable signal. This kind of oscillator using a quartz crystal is called a crystal oscillator.
Catalog
&#; Characteristics
1 quartz crystal
(1) Shape, structure, and graphic symbols
Cut a thin slice on the quartz crystal in a certain direction, polish the two ends of the slice and coat it with a conductive silver layer, and then connect two electrodes from the silver layer and package them. This component is called a quartz crystal resonator, or quartz crystal for short. The shape, structure, and graphic symbols of the quartz crystal are shown in the figure.
Figure 1. Quartz crystal
(2) Features
Quartz crystals have two resonant frequencies, fs and fp, fp is slightly larger than fs. When the frequency of the signal applied to the two ends of the quartz crystal is different, it will show different characteristics, as shown in the figure, and the specific description is as follows.
Figure 2. Characteristics of Quartz Crystal
&#; When f=fs, the quartz crystal is resistive, which is equivalent to a small resistance.
&#; When fs<f<fp, the quartz crystal is inductive, which is equivalent to inductance.
&#; When f&#;fp, the quartz crystal is capacitive, which is equivalent to capacitance.
2 Circuit symbol
The 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. The graphic symbol of the crystal oscillator is shown in the figure.
Figure 3. The graphic symbol of the crystal oscillator
3 Composition
The crystal oscillator is mainly composed of crystal and peripheral components. The picture shows the physical appearance and internal structure of the crystal oscillator, as well as the circuit graphic symbols and equivalent circuit.
Figure 4. The physical appearance and internal structure of the crystal oscillator, as well as circuit graphic symbols and equivalent circuit
&#; Working principle
The crystal oscillator has a piezoelectric effect, that is, the crystal will deform when a voltage is applied to the two poles of the wafer. Conversely, if an external force deforms the wafer, the metal sheets on the two poles will generate a voltage. If an appropriate alternating voltage is applied to the chip, the chip will resonate (the resonance frequency is related to the tilt angle of the quartz slope, and the frequency is constant). The crystal oscillator uses a crystal that can convert electrical energy and mechanical energy into each other. It can provide stable and accurate single-frequency oscillation when working in a resonance state. Under normal working conditions, the absolute accuracy of ordinary crystal frequency can reach 50 parts per million. Using this feature, the crystal oscillator can provide a more stable pulse, which is widely used in the clock circuit of the microchip. The wafers are mostly quartz semiconductor materials, and the shell is encapsulated by metal.
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The crystal oscillator is often used in connection with the mainboard, southbridge, sound card, and other circuits. The crystal oscillator can be likened to the "heartbeat" generator of each board. If there is a problem with the "heartbeat" of the main card, it will surely cause other circuits to malfunction.
&#; Classification
1. Parallel crystal oscillator
The parallel crystal oscillator is shown in the figure. Transistor VT and R1, R2, R3, R4 form an amplifying circuit; C3 is an AC bypass capacitor, which is equivalent to a short circuit for AC signals; X1 is a quartz crystal, which is equivalent to inductance in the circuit. It can be seen from the AC equivalent diagram that the circuit is a capacitive three-point oscillator, C1, C2, and X1 constitute a frequency selection circuit. The frequency selection frequency is mainly determined by X1, and the frequency is close to fp.
Figure 5. Parallel crystal oscillator
Circuit oscillation process: After the power is turned on, the transistor VT is turned on, and a changing Ic current flows through VT, which contains weak signals of various frequencies from 0 to &#;. These signals are added to the frequency selection circuit formed by C1, C2, and X1, and the frequency selection circuit selects the f0 signal from it. There is f0 signal voltage at both ends of X1, C1, and C2, and the f0 signal voltage at both ends of C2 is fed back and amplify between the base and the emitter of VT. After amplification, the output signal is added to the frequency selection circuit. The signal voltage at both ends of C1 and C2 increases, and the voltage at both ends of C2 is sent to the VT base-emitter again. Repeatedly, the more the signal output by VT The larger the value, the magnification of the VT amplifying circuit gradually decreases. When the magnification of the amplifying circuit is equal to the attenuation coefficient of the feedback circuit, the output signal amplitude remains stable and will not increase, and the signal is sent to other circuits.
2. Series crystal oscillator
The series crystal oscillator is shown in the figure. The oscillator uses a two-stage amplifying circuit. In addition to forming a feedback circuit, the quartz crystal X1 also has a frequency selection function. The frequency selection frequency f0=fs and the potentiometer RP1 is used to adjust the amplitude of the feedback signal.
Figure 6. Series crystal oscillator
3. Quartz crystal oscillator classification
Quartz crystal oscillators are divided into non-temperature-compensated crystal oscillators, temperature-compensated crystal oscillators (TCXO), voltage-controlled crystal oscillators (VCXO), oven-controlled crystal oscillators (OCXO) and digital/μp-compensated crystal oscillator (DCXO/MCXO) and so on. Among them, the non-temperature-compensated crystal oscillator is the simplest one, which is called Standard Package Crystal Oscillator (SPXO) in the Japanese Industrial Standards (JIS).
&#; Oven controlled crystal oscillator. An Oven Controlled Crystal Oscillator (OCXO) is a crystal oscillator that uses a constant temperature bath to keep the temperature of the crystal oscillator or quartz crystal oscillator constant and reduces the change in the oscillator output frequency caused by the surrounding temperature change to the minimum, as shown in Figure. In OCXO, some only put the quartz crystal oscillator in a constant temperature bath, some put the quartz crystal oscillator and related important components in the constant temperature bath, and some put the quartz crystal oscillator in the internal constant temperature bath. The oscillation circuit is placed in an external constant temperature bath for temperature compensation, and a dual constant temperature bath control method is implemented. Using a proportionally controlled constant temperature bath can increase the temperature stability of the crystal to more than times and keep the oscillator frequency stability at least 1×10-9. OCXO is mainly used in equipment and instruments such as mobile communication base stations, national defense, navigation, frequency counters, spectrum, and network analyzers. OCXO is composed of a thermostatic bath control circuit and an oscillator circuit. Usually, people use a differential series amplifier composed of a thermistor "bridge" to achieve temperature control. The Clapp oscillation circuit with automatic gain control (AGC) is an ideal technical solution for obtaining high stability of the oscillation frequency. In recent years, the technical level of OCXO has been greatly improved.
Figure 7. The appearance of an oven-controlled crystal oscillator
&#; Temperature compensation crystal oscillator. The temperature-compensated crystal oscillator (TCXO) is a quartz crystal oscillator that reduces the amount of oscillation frequency change caused by the surrounding temperature change through an additional temperature compensation circuit, as shown in the figure. In TCXO, there are mainly two types of compensation methods for the frequency and temperature drift of the quartz crystal oscillator: direct compensation and indirect compensation:
Figure 8. Temperature controlled compensation crystal oscillator
a. Direct compensation type. The direct compensation type TCXO is a temperature compensation circuit composed of a thermistor and a resistance-capacitance element, which is connected in series with a quartz crystal oscillator in an oscillator. When the temperature changes, the resistance of the thermistor and the capacitance of the equivalent series capacitance of the crystal change correspondingly, thereby offsetting or reducing the temperature drift of the oscillation frequency. The compensation circuit is simple, low in cost, saves the size and space of the printed circuit board (PCB), and is suitable for small and low-voltage and small-current occasions. But when the crystal oscillator accuracy is required to be less than ±1×10-6, the direct compensation method is not suitable.
b. Indirect compensation type. There are two types of indirect compensation: analog and digital. Analog indirect temperature compensation uses temperature sensing elements such as thermistors to form a temperature-voltage conversion circuit, and applies the voltage to a varactor diode connected in series with a crystal oscillator, and changes the capacitance in series through the crystal oscillator to compensate the nonlinear frequency drift of the crystal oscillator. This compensation method can achieve high accuracy of ±0.5×10-6, but it is limited under low voltage conditions below 3V. Digital indirect temperature compensation is to add an analog/digital (A/D) converter after the temperature-voltage conversion circuit in the analog indirect temperature compensation circuit to convert the analog quantity into a digital quantity. This method can realize automatic temperature compensation so that the crystal oscillator frequency stability is very high, but the specific compensation circuit is more complicated, and the cost is also high. It is only suitable for base stations and broadcast stations that require high precision.
&#; Simple packaged crystal oscillator. Simple packaged crystal oscillator. (SPXO) is a simple crystal oscillator, usually called a clock oscillator. It is a crystal oscillator whose work is done entirely by crystal-free oscillation. This type of crystal is mainly used on occasions where stability is not required. The figure shows an ordinary crystal oscillator.
&#; Voltage controlled crystal oscillator. Voltage-controlled crystal oscillator (VCXO) is a quartz crystal oscillator whose oscillation frequency can be changed or modulated by applying an external control voltage. In a typical VCXO, the frequency of the quartz crystal oscillator is usually "pulled" by changing the capacitance of the varactor diode by tuning the voltage. VCXO allows a wide frequency control range, and the actual traction range is about ±200×10-6 or even larger. If the output frequency of the VCXO is required to be higher than the frequency that the quartz crystal oscillator can achieve, a frequency doubling scheme can be used. Another way to extend the tuning range is to mix the output signal of the crystal oscillator with the output signal of the VCXO. Compared with a single oscillator, this heterodyne two-oscillator signal tuning range has significantly expanded.
To begin oscillation, the circuit must satisfy the following Barkhausen criteria: 1) the loop gain exceeds unity at the resonant frequency, and 2) the phase shift around the loop is n2π radians (where n is an integer. Intuitively, it can be seen that the amplifier provides the gain for the first criteria. The amplifier is inverting, causing a π rad (180-deg.) phase shift to meet the requirements of the second criterion. The filter block provides an additional π rad phase shift for a total of 2-π rad (360 deg.) around the entire loop. By design, the filter block inherently provides the phase shift in addition to providing a coupling network to and from the amplifier. The filter block also sets the frequency of oscillation, using a tuned circuit (inductor and capacitor) or crystal.
Operation of an oscillator is generally broken up into two phases: startup and steady-state operation. An oscillator must start itself with no external stimulus. When the power is first applied, voltage changes in the bias network result in voltage changes in the filter network. These voltage changes excite the natural frequency of the filter network and signal buildup begins and the signal developed in the filter network is small. Positive feedback and excess gain in the amplifier continuously increases the signal until the nonlinearity of the amplifier limits the loop gain to unity. At this point, the oscillator enters steady-state operation; the time from power on to steady-state operation is the oscillator start-up time.
An oscillator's steady-state operation is governed by the amplifier and the tuned circuit of the filter block. Loop gain steadies at unity due to the non-linearity of the amplifier. The tuned circuit reactance will adjust itself to match the Barkhausen phase requirement of 2-π rads. During steady-state operation, the main concerns are the power output and loading of the tuned circuit. The amplifier circuit is typically implemented with a bipolar junction transistor (BJT) or metal-oxide-semiconductor field-effect transistor (MOSFET). The linear characteristics of the transistor determine the starting conditions of the oscillation while the nonlinear characteristics determine an oscillator's operating point.
The filter block sets the frequency that the oscillator will operate, which is accomplished by using an inductive-capacitive (LC) tuned circuit or crystal.
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