Temperature-Compensated Xtal (crystal) Oscillator (hereinafter referred as TCXO) is a kind of crystal oscillator which can work in a wide temperature range and keep the output frequency of the crystal oscillator within a certain accuracy range (106˜10−7 orders of magnitude) through a certain compensation. It has a characteristic of low power consumption, working upon power-up, high stability and so on. Therefore it has been widely used in various communications, navigation, radar, satellite positioning system, mobile communication, program-controlled telephone switch and various electronic measuring instruments.
The temperature-compensated crystal oscillator in prior art is essentially a Voltage-Controlled Xtal (crystal) Oscillator (hereinafter referred as VCXO) with a temperature compensated network which produces a temperature-dependent compensation voltage. The key component in the uncompensated voltage-controlled crystal oscillator is a AT-cut quartz crystal, which temperature characteristic curve is approximately a cubic curve, and the cubic curve can be expressed as:f(T)=a3(T−T0)3+a1(T−T0)+a0  (1)
Where a3 is the coefficient of cubic term, a1 is the coefficient of linear term, a0 is the oscillating frequency at a reference temperature T0, T is the temperature of the location the AT-cut quartz crystal is at.
The frequency linear gain characteristics of the VCXO in prior art can be approximately expressed as:f(VC)=−G(VC−VC0)+f0  (2)
where G is the gain of the VCXO, VC is the control voltage of the VCXO, VC0 is the input voltage of the voltage control terminal of the VCXO, f0 is the oscillating frequency when the input voltage is VC0.
Then, the equation of the compensation voltage of the crystal temperature characteristic can be expressed as:VC(T)=A3(T−T0)3+A1(T−T0)+A0  (3)
Where A3=a3/G, A1=a1/G, A0 is a compensation voltage when the temperature T is T0.
In order to implement equation (3), it is necessary to generate a temperature compensation voltage applied to the VCXO for temperature compensation to counteract the frequency temperature characteristic, thereby obtaining a stable frequency output within a wide temperature range, and realizing the purpose of temperature compensation.
FIG. 1 is a diagram of a TCXO based on digital circuit in prior art. The TCXO uses digital temperature compensation with an open-loop architecture. As shown in FIG. 1, it comprises a temperature sensor and conditioning circuit 101, a microprocessor 102, a compensation network 103, and a VCXO 104. The temperature T is acquired by the temperature sensor and conditioning circuit 101, and then input into the microprocessor 102 to find the compensation voltage value according to the temperature in the temperature-compensated voltmeter. Then the compensation network 103 converts the compensation voltage value into a compensation voltage, and inputs the compensation voltage to the voltage control terminal of VCXO 104 to make it output a signal with stable frequency. It can be seen that the temperature-dependent compensation voltage is directly applied to the voltage control terminal of the VCXO 104 (to be compensated) to realize temperature compensation.
Where the temperature-compensated voltmeter is constructed in advance by collecting the voltage of the VCXO 104 at different temperatures while maintaining the frequency stabilization. A more detailed description of the construction can be found in following references:    1. Huang X, Liu D, Wang Y, et al. 100-MHz Low-Phase-Noise Microprocessor Temperature-Compensated Crystal Oscillator[J]. Circuits & Systems II Express Briefs IEEE Transactions on, 2015, 62(7):636-640;    2. “Temperature compensation for an oscillator crystal”, Inventor: Markus Hammes etc, US Patent Publication Number: US 20170085271A1, Date of Publication: Mar. 23, 2017;    3. “Digitally compensated phase locked oscillator”, Inventor: Nicholls Charles William tremlett, etc, US Patent Publication Number: US 20160365865A1, Date of publication: Dec. 15, 2016.
In sum, the TCXO in prior art uses an open-loop compensation architecture, a temperature sensor is needed, and the temperature sensor should be as closer as possible to the crystal resonator on a circuit. However, the resonant wafer of the crystal oscillator is individually enclosed in a confined space, which inevitably produces a temperature hysteresis between the temperature sensor and the resonant wafer, leading that there is no significant breakthrough in the TCXO frequency temperature characteristics. Especially for the crystal oscillator with high frequency output, the temperature hysteresis is more obvious.