“For crystal oscillators, we can divide them into active crystal oscillators and passive crystal oscillators. But, have you ever heard of constant temperature crystal oscillators and temperature compensated crystal oscillators? Do you know the difference between these two different types of crystal oscillators? In the second half of the article, the editor will introduce why single-chip microcomputers need crystal oscillators to operate normally.
For crystal oscillators, we can divide them into active crystal oscillators and passive crystal oscillators. But, have you ever heard of constant temperature crystal oscillators and temperature compensated crystal oscillators? Do you know the difference between these two different types of crystal oscillators? In the second half of the article, the editor will introduce why single-chip microcomputers need crystal oscillators to operate normally.
1. Constant temperature crystal oscillator and temperature compensated crystal oscillator
Oven Controlled Crystal Oscillator is abbreviated as Oven Controlled Crystal Oscillator, or OCXO (Oven Controlled Crystal Oscillator) in English. It uses a constant temperature bath to keep the temperature of the quartz crystal resonator in the crystal oscillator constant, reducing the amount of change in the oscillator output frequency caused by changes in the surrounding temperature. To the smallest crystal oscillator. OCXO is composed of thermostatic bath control circuit and oscillator circuit. Usually people use a differential series amplifier composed of a thermistor “bridge” to achieve temperature control.
Temperature-compensated crystal oscillator, or temperature-compensated crystal oscillator (TCXO), is a kind of quartz crystal oscillator that reduces the amount of oscillation frequency change caused by ambient temperature changes through an additional temperature compensation circuit.
The term temperature-compensated crystal oscillator comes from a compensation method of quartz crystal oscillators that has reached the accuracy requirements of product applications. The definition of temperature-compensated crystal oscillator is to change the original physical properties of piezoelectric quartz crystal (the frequency changes with the temperature as a cubic curve under the piezoelectric effect) through the reverse change of the peripheral circuit to make the original frequency of the quartz crystal change as small as possible. Quartz crystal oscillator made by a compensation method.
Constant temperature crystal oscillator
Temperature compensated crystal
2. Working principle
Constant temperature crystal oscillator, because the oscillation frequency of the crystal oscillator changes with temperature, in order to maintain the stability of the frequency, the crystal oscillator is controlled to work at a constant temperature to improve the phase frequency characteristics of the crystal oscillator.
Temperature-compensated crystal oscillator, because the oscillation frequency of the crystal oscillator changes with temperature, in order to offset the influence of temperature on the frequency of the crystal oscillator, the resonant capacitance of the crystal oscillator is controlled to change with temperature changes to offset the influence of the temperature crystal to improve frequency stability.
3. Measurement accuracy
The frequency stability of the general constant temperature crystal oscillator is more than two orders of magnitude higher than that of the temperature compensated crystal oscillator. For example, the temperature-compensated crystal oscillator can generally reach the order of -7, while the constant temperature crystal oscillator can reach the order of -9. Therefore, constant temperature crystal oscillators are generally used in high-end measuring instruments, such as frequency meters, signal generators, and network analyzers.
The start-up characteristics of the temperature-compensated crystal oscillator are better. The constant temperature crystal oscillator needs a heating process even if the best heating element is used. It takes about 5 minutes to reach the magnitude of -7, and it even takes a day to reach the magnitude of -9. Therefore, the equipment that needs to work when it is turned on is not suitable. Such as weapons.
The general constant temperature crystal oscillator can do better than the temperature compensated crystal oscillator.
Whether it is a constant temperature crystal oscillator or a temperature-compensated crystal oscillator, it is nothing more than a signal source, providing a time reference for your equipment. As long as you understand its performance indicators, you can substitute for each other.
Second, why does a single-chip microcomputer need a crystal oscillator
First of all, one of the necessary conditions for the normal operation of the single-chip microcomputer is the clock circuit, so the single-chip microcomputer needs a crystal oscillator.
Crystal oscillator, the full name is quartz crystal oscillator, is a high-precision and high-stability oscillator. Through a certain external circuit, a sine wave with stable frequency and peak value can be generated. When the single-chip microcomputer is running, it needs a pulse signal as the trigger signal to execute the instruction. It can be simply imagined as: the single-chip microcomputer executes one or more instructions when it receives a pulse.
When the microcontroller works, it fetches instructions from the RoM one by one, and then executes them step by step. The time for the single-chip microcomputer to access the memory once is called a machine cycle, which is a time reference. ―A machine cycle includes 12 clock cycles. If a single-chip microcomputer selects a 12 MHz crystal oscillator, its clock cycle is 1/12us, and one of its machine cycles is 12×(1/12)us, which is 1us.
Among all the instructions of the MCS-51 single-chip microcomputer, some are completed relatively quickly, and only one machine cycle is sufficient, some are relatively complete, requiring 2 machine cycles, and two instructions require 4 machine cycles. In order to measure the length of instruction execution time, a new concept is introduced: instruction cycle. The so-called instruction cycle refers to the time to execute an instruction. For example, when you need to calculate the time required for the completion of the DJNZ instruction, you must first know the frequency of the crystal oscillator. If the crystal oscillator used is 12 MHz, one machine cycle is 1 us. The DJNZ instruction is a two-cycle instruction, so it takes 2us to execute once. If the instruction needs to be executed 500 times, it is exactly 1000us, which is 1ms.
The machine cycle is not only of great significance for the execution of instructions, but also the time base of the single-chip timer and counter. For example, a single-chip microcomputer selects a 12 MHz crystal oscillator, then when the value of the timer increases by 1, the actual elapsed time is 1us, which is the timing principle of the single-chip microcomputer.
The crystal oscillator provides the working signal pulse to the microcontroller. This pulse is the operating speed of the microcontroller. For example, 12 trillion crystal oscillator. The operating speed of the single-chip microcomputer is 12 megabytes per second. There is also a crystal oscillator inside the microcontroller. Connect an external crystal oscillator to get a more stable frequency.