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In fact, it is necessary to understand the characteristics of temperature compensated crystal oscillator. The temperature compensated crystal oscillator is a quartz crystal oscillator that reduces changes in oscillation frequency caused by changes in ambient temperature by adding a temperature compensation circuit. Quartz crystal oscillators with temperature compensation function can be divided into three categories: direct compensation, indirect compensation and digital compensation.
Of course, for the temperature compensated crystal oscillator circuit, an additional temperature compensation network is used to make the capacitance of the crystal series circuit reversely change after the ambient temperature changes. The temperature compensated crystal oscillator is divided into a direct compensated crystal oscillator and an indirect compensated crystal oscillator. Temperature compensation crystal oscillators are divided into three categories: direct compensation, indirect compensation and digital compensation. These are the characteristics of temperature compensated crystal oscillators.
Feature 1: Temperature compensated crystal oscillator
A temperature compensated crystal oscillator (TCXO) is a quartz crystal oscillator with an additional temperature compensation circuit to reduce changes in oscillation frequency caused by changes in ambient temperature. The quartz crystal oscillator has its own temperature compensation function. The temperature compensation crystal oscillator circuit uses an additional temperature compensation network to make the capacitance of the crystal's series circuit reversely change after the ambient temperature changes to offset the frequency-temperature drift generated by the crystal.
Feature 2: Direct compensation
The thermistor, resistor and capacitor form a temperature compensation network and are directly connected in series in the crystal oscillator circuit. The indirect compensation reference voltage generates a voltage that changes with temperature through a compensation network composed of resistors and thermistors, changes the load capacitance of the time-sensitive crystal oscillator, and reversely compensates the frequency and temperature characteristics of the crystal. It can be divided into analog and digital.
Feature 3: Analog indirect compensation
The output voltage of the compensation network directly drives the varactor diode, and a T-type filter is added between the compensation network and the varactor diode. This compensation is linear, third order, fifth order, seventh order. It can be well compensated in a wide temperature range of -40 ~ 85. According to the compensation network and connection position, temperature compensated crystal oscillators are divided into direct compensation and indirect compensation crystal oscillators, which are currently widely used.
Feature 4: Digital indirect compensation
The signal sent by the temperature sensor enters the ADC and becomes a digital signal. The control/PC works normally, the DAC becomes an analog signal, and the varactor is driven by the matching circuit. Because the compensation circuit is relatively complex and the cost is relatively high. The general curve, due to the complexity and high cost of the compensation circuit, generally reaches the advanced level abroad and can be used as a reference. Temperature compensation is performed on the frequency-temperature characteristics of the crystal to make the frequency-temperature characteristic curve of the crystal oscillator as close to a straight line as possible. According to the frequency-temperature characteristics of the crystal, if the high-temperature turning point of the crystal can be maintained, the frequency-temperature stability of the crystal oscillator will naturally be very high.
