What is the function of Temperature Compensate X'tal Oscillator and what is the difference with constant temperature crystal oscillator?
Crystal oscillator is one of the most commonly used electronic components in electronic circuits. Crystal oscillator can provide relatively stable pulses and is widely used in clock circuits of microchips. In order to enhance everyone's understanding of crystal oscillators, this article will introduce the role of temperature-compensated crystal oscillators, the working principle of temperature-compensated crystal oscillators, and the differences between temperature-compensated crystal oscillators and constant temperature crystal oscillators. If you are interested in crystal oscillators, please continue reading.
Temperature Compensated Crystal Oscillator
A temperature-compensated crystal oscillator can measure the temperature and then automatically adjust the external matching capacitor matrix (change the connected capacitor value) to make the frequency more accurate and stable.
Use temperature compensation to reduce frequency distortion, because when the oscillator is working, there will be a temperature rise due to the effect of resistance (transistors or integrated circuits have internal resistance), and the temperature rise has a great impact on the semiconductor, which will cause the operating point of the semiconductor to occur. Drift leads to changes in the oscillation frequency. These changes have a great impact on users. For example, radio communications, local clocks (single-chip microcomputers or computers) require high frequency stability, so developers produce active oscillators with temperature compensation. These The crystal oscillator with temperature compensation has very low frequency variation and can work stably for a long time to provide a high stability frequency reference.
Working principle of temperature compensated crystal oscillator
Temperature Compensated Oscillator (TCXO) is a type of quartz crystal oscillator that reduces the variation of oscillation frequency caused by ambient temperature changes through an additional temperature compensation circuit. In TCXO, there are two main types of compensation methods for frequency temperature drift of quartz crystal oscillators: direct compensation and indirect compensation:
(1) 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 the oscillator. When the temperature changes, the resistance value of the thermistor and the equivalent series capacitance of the crystal change accordingly, thereby offsetting or reducing the temperature drift of the oscillation frequency. The compensation method has a simple circuit, low cost, saves the size and space of a printed circuit board (PCB), and is suitable for small and low-voltage and low-current occasions. But when the accuracy of the crystal oscillator is required to be less than ±1pmm, the direct compensation method is not suitable.
(2) Indirect compensation type Indirect compensation type is divided into two types: analog type and digital type. Analog indirect temperature compensation is to use temperature sensing elements such as thermistors to form a temperature-voltage conversion circuit, and apply the voltage to a varactor diode connected in series with the crystal oscillator, through the change of the capacitance of the crystal oscillator in series , to compensate the nonlinear frequency drift of the crystal oscillator. This compensation method can achieve a high precision of ±0.5ppm, but it is limited in the case of low voltage below 3V. Digital indirect temperature compensation is to add an analog/digital (A/D) converter after the temperature-voltage conversion circuit in the analog compensation circuit to convert the analog quantity into a digital quantity. This method can realize automatic temperature compensation, so that the frequency stability of the crystal oscillator is very high, but the specific compensation circuit is relatively complicated and the cost is high, so it is only suitable for base stations and broadcasting stations that require high precision.
The difference between temperature compensated crystal oscillator and constant temperature crystal oscillator
1. By definition
Oven Controlled Crystal Oscillator is referred to 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, and reduces the amount of change in the output frequency of the oscillator caused by ambient temperature changes. to the smallest crystal oscillator. The OCXO is composed of a constant temperature bath control circuit and an oscillator circuit. Usually people use a differential series amplifier composed of a thermistor "bridge" to achieve temperature control.
The temperature-compensated crystal oscillator is a temperature-compensated crystal oscillator (TCXO), which is a type of quartz crystal oscillator that reduces the oscillation frequency variation caused by ambient temperature changes through an additional temperature compensation circuit.
The term temperature-compensated crystal oscillator comes from a compensation method of a quartz crystal oscillator that has reached the accuracy requirements of product applications. The definition of temperature-compensated crystal oscillator is to change the original physical characteristics of the piezoelectric quartz crystal (the frequency changes with the temperature into a cubic curve under the piezoelectric effect) through the peripheral circuit to reversely change the original frequency of the quartz crystal with the change of temperature as small as possible. A quartz crystal oscillator made by a compensation method.
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 -7 level, while the constant temperature crystal oscillator can reach the -9 level. Therefore, constant temperature crystal oscillators are generally used in high-end measuring instruments, such as frequency counters, signal generators, network analyzers, etc.
The temperature-compensated crystal oscillator has better start-up characteristics. Even if the constant temperature crystal oscillator uses the best heating element now, it still needs a heating process. It takes about 5 minutes to reach the magnitude of -7, and even a day to reach the magnitude of -9 or more. Therefore, it is not suitable for devices that need to work when they are turned on. 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 each other.
If you like this article, may wish to continue to pay attention to our website Oh, later will bring more exciting content. If you have product needs, please contact us
