Application of NTC Thermistor in TCXO
Temperature compensated crystal Oscillator is abbreviated as TCXO. TCXO is a kind of crystal Oscillator that reduces the frequency of oscillations produced by ambient Temperature changes through additional Temperature compensation circuits.
TCXO is characterized by small volume and high frequency stability, which is mainly used in mobile communication system. As a reference oscillator, TCXO can be started quickly in small current, providing a frequency baseline for sending channels. At present, mobile phones usually choose the specification with high cost performance: 1.5 ~ 2.5 PPM, one of the representative products is vc-tcxo-201c1, which adopts direct compensation method.
In TCXO, there are two kinds of compensation methods for the frequency temperature drift of quartz crystal oscillator: direct compensation and indirect compensation.
The direct compensated TCXO is a temperature-compensated circuit composed of thermistors and resistors, which are in series with quartz crystal oscillator in the oscillator. When temperature changes, the resistance value of thermistor and the capacitance of the crystal equivalent series capacitance are correspondingly changed to offset or reduce the temperature drift of oscillation frequency. Compared with ordinary fixed resistance, NTC thermistor is more effective in suppressing the shock current. Direct compensation circuit is simple, low cost, suitable for small and low voltage small current occasions. However, it is only suitable for the crystal oscillator to be used when the accuracy is greater than or minus 1ppm.
Indirect compensation model is divided into two types: analog and digital.
Analog indirect temperature compensation is to use the thermistor temperature sensing element composition and temperature - voltage conversion circuit, and the voltage applied to a concatenated with the crystal oscillator varactor, through the change of the crystal oscillator series capacitance, the compensation for the nonlinear frequency drift of the crystal oscillator. This compensation method can achieve high accuracy of 0.5 PPM, but is limited under low voltage under 3V. Digital indirect temperature compensation is the temperature - voltage conversion circuit in analog compensation circuit, and then the analog quantity is converted to digital quantity by adding the first order mode/number (A/D) converter. The method can realize automatic temperature compensation, make the crystal oscillator frequency stability is very high, but the specific compensation circuit is more complex, the cost is higher also, applies only to base stations and radio stations and other requirements of high precision.
Our CT series NTC thermistor is suitable for the production of TCXO, with high accuracy, high reliability and high stability. It can be used in bonding or welding. The main function of NTC thermistor is temperature detection and temperature compensation, which can still be stable in high pressure environment. To know more about the NTC thermistor, please refer: http://www.exsense.cn/ct/show/77.html