What is TCXO

  • 2020-09-27 17:58:19

What is TCXO

TCXO(Temperature Compensate X'tal (crystal) Oscillator)

TCXO is a type that reduces the amount of oscillation frequency change caused by ambient temperature changes through an additional temperature compensation circuit, and has the characteristics of high accuracy.

In TCXO, there are two main methods for compensating the frequency and temperature drift of the quartz crystal oscillator:

Direct compensation

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 an oscillator. When the temperature changes, the resistance of the thermistor and the capacitance of the equivalent series capacitance of the crystal change accordingly, thereby offsetting or reducing the temperature drift of the oscillation frequency. The compensation method is simple in circuit, low in cost, saves the size and space of the printed circuit board (PCB), and is suitable for small and low-voltage and small-current occasions. But when the crystal oscillator accuracy is required to be less than±1ppm, the direct compensation method is not suitable.

Indirect compensation

There are two types of indirect compensation: analog and digital. Analog indirect temperature compensation uses temperature sensing elements such as thermistors to form a temperature-voltage conversion circuit, and applies the voltage to a varactor diode connected in series with a crystal oscillator, and changes the capacitance in series through the crystal oscillator , To compensate the nonlinear frequency drift of the crystal oscillator. This compensation method can achieve a high accuracy of±0.5ppm, but it is limited under low voltages 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 crystal oscillator frequency stability is very high, but the specific compensation circuit is more complicated, and the cost is also high. It is only suitable for base stations and broadcast stations that require high precision.

Common parameters, TCXO common frequency points are as follows:

10MHz, 12.8MHz, 13MHz, 14.4MHz, 14.7456MHz, 15.36MHz, 16MHz, 16.368MHz, 16.367667MHz, 16.369MHz, 16.8MHz, 18.432MHz, 19.2MHz, 19.44MHz, 19.68MHz, 19.8MHz, 20MHz, 21.250MHZ, 24.5535MHZ, 26MHZ, 30MHz, 32MHz, 38.4MHz,

Dimensions are: 5.0*3.2mm 3.2*2.5mm 2.5*2mm

Voltage: 3.3V 3V 2.8V 1.8V

The accuracy is: 0.5PPM 2.5PPM

Because TCXO has high frequency stability, and is small in size, it can be started quickly under a small current, and its application field is mainly extended to mobile communication systems. TCXO serves as a reference oscillator to provide a frequency reference for the sending channel, and at the same time as the first-level local oscillator of the receiving channel; the other TCXO serves as a second-level local oscillator and inputs its oscillation signal to the second inverter. The frequency stability required by mobile phones is 0.1 to 2.5 ppm (-30 to +75°C),but due to cost considerations, the usually selected specification is 1.5 to 2.5 ppm.

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