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Access Reed

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Access Reed Temperature compensated crystal device The temperature compensated crystal oscillator (TCXO) is a quartz crystal oscillator that reduces the amount of oscillation frequency variation caused by changes in ambient temperature by an additional temperature compensation circuit. In TCXO,...

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Temperature compensated crystal device
The temperature compensated crystal oscillator (TCXO) is a quartz crystal oscillator that reduces the amount of oscillation frequency variation caused by changes in ambient temperature by an additional temperature compensation circuit. In TCXO, there are two types of compensation methods for quartz crystal oscillator frequency temperature drift: 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 RC 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 equivalent capacitance of the crystal series change accordingly, thereby offsetting or reducing the temperature drift of the oscillation frequency. The compensation method has the advantages of simple circuit, low cost, saving printed circuit board (PCB) size and space, and is suitable for small and low voltage and small current applications. However, when the accuracy of the crystal oscillator is required to be less than ±1 pmm, the direct compensation method is not suitable.
(2) Indirect compensation type The indirect compensation type is divided into two types: analog and digital. The analog indirect temperature compensation uses a temperature sensing element such as a thermistor to form a temperature-voltage conversion circuit, and applies the voltage to a varactor diode connected in series with the crystal oscillator, and changes the capacitance of the series through the crystal oscillator. , compensate for the nonlinear frequency drift of the crystal oscillator. This compensation method can achieve high accuracy of ±0.5ppm, but is limited under low voltage conditions of 3V or less. The digital indirect temperature compensation is to add a first-order analog-to-digital (A/D) converter after the temperature-voltage conversion circuit in the analog compensation circuit to convert the analog quantity into a digital quantity. The method can realize automatic temperature compensation, which makes the crystal oscillator frequency stability very high, but the specific compensation circuit is more complicated and the cost is higher, and it is only suitable for the case where the base station and the broadcasting station require high precision.

TCXO development status
TCXO has made great progress in the past ten years, among which Japan is leading and dominant in the research and development and production of precision TCXO. In the late 1970s, the volume of TCXO for car phones reached more than 20, and the current mainstream products fell to 0.4. The ultra-small TCXO devices were only 0.27. In 30 years, the volume of TCXO has shrunk to 1/50 or even 1/100. The surface mount TCXO thickness produced by Japan Jing Ceramics Co., Ltd. by reflow soldering method is reduced from 4mm to 2mm, and can reach 90% of the rated oscillation amplitude after 4ms of oscillation start. The TCXO frequency range produced by Jinshi (KSS) Group is 2~80MHz, the stability when the temperature changes from -10°C to 60°C is ±1ppm or ±2ppm; the frequency coverage of digital TCXO is 0.2~90MHz, frequency stability It is ±0.1ppm (-30°C to +85°C). The TCO-935/937 chip type direct temperature compensation type TCXO produced by Tosoh Corporation of Japan has a frequency-temperature characteristic (point frequency of 15.36MHz) of ±1ppm/-20 to +70°C at a supply voltage of 5V±5%. The frequency and voltage characteristics are ±0.3ppm, the output sine wave waveform (amplitude 1VPP), the current loss is less than 2mA, the volume is 1, and the weight is only 1g. PiezoTechnology's X3080 TCXO is available in both surface mount and perforated packages with sine or logic outputs that are accurate to ±0.25 to ±1ppm from -55°C to 85°C. The domestic product level is also high. For example, the TCXO (32~40MHz) introduced by Beijing Ruihuaxin Technology Development Co., Ltd. has an accuracy of better than ±1ppm at room temperature, and the frequency aging rate of the first year is ±1ppm. The frequency (mechanical) is fine-tuned. ≥±3ppm, power consumption ≤120mw. The current high stability TCXO device has an accuracy of ±0.05ppm.

High precision, low power consumption and miniaturization are still the research topics of TCXO. In terms of miniaturization and chipping, there are many difficulties. The main ones are two points: First, the miniaturization will make the frequency of the quartz crystal vibrator smaller, and the temperature compensation will be more difficult. Second, after the chip package is In the reflow soldering operation, since the soldering temperature is much higher than the maximum allowable temperature of the TCXO, the frequency of the crystal oscillator changes. If the local heat dissipation and temperature reduction measures are not adopted, it is difficult to control the frequency variation of the TCXO to ±0.5×10- 6 or less. However, the improvement of the technical level of TCXO has not reached the limit, and the content and potential of innovation are still large.

TCXO application
The development of quartz crystal oscillator and its application in wireless systems, due to the high frequency stability of TCXO, and its small size, it can be started quickly under small currents, and its application field is extended to mobile communication systems. The TCXO serves as a reference oscillator to provide a frequency reference for the transmission channel and serves as the first-stage local oscillator of the receive channel. The other TCXO acts as the second-level local oscillator and inputs its oscillation signal to the second inverter. Currently, mobile phones require a frequency stability of 0.1 to 2.5 ppm (-30 to +75 ° C), but for cost reasons, the specification is usually 1.5 to 2.5 ppm. One of the representative products of TCXO for mobile phones with 12 to 20 MHz is VC-TCXO-201C1, which is produced by Japan Jinshi (KSS) Co., Ltd. using direct compensation.

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