The working principle of a Temperature Compensated Crystal Oscillator (TCXO)

The working principle of a Temperature Compensated Crystal Oscillator (TCXO) is a process that intricately combines temperature sensing, intelligent compensation, and crystal oscillation. Its core objective is to counteract the impact of environmental temperature changes on the resonant frequency of the quartz crystal, providing a highly stable frequency output.

Temperature Characteristics of Quartz Crystals: The resonant frequency of a quartz crystal drifts with changes in environmental temperature. Its temperature stability follows a function resembling a cubic curve.
For ordinary oscillators, this drift can reach ±10–25 ppm or even higher, which is entirely unacceptable in applications such as communications, navigation, and precision measurement, where frequency stability requirements are as stringent as ±1 ppm or even ±0.1 ppm.
This led to the development of the higher-precision TCXO.

The working principle of a TCXO can be summarized as a closed-loop process of “sensing–calculating–compensating.” Its core components and workflow are illustrated in the figure below.
A temperature sensor continuously monitors the ambient temperature around the TCXO and converts the temperature value into a corresponding electrical signal, which serves as the basis for compensation.

The compensation network, based on the received temperature signal, generates a specific compensation voltage to correct the frequency.
This network internally stores the frequency-temperature characteristic curve of the quartz crystal in use. When the temperature sensor provides a temperature value T1, the compensation network determines how much the crystal’s frequency will deviate at this temperature. It then calculates a voltage value Vc, whose magnitude and direction are precisely designed to offset this frequency drift.
The relationship curve between the compensation voltage Vc and temperature T is designed to be perfectly “mirror-symmetric” to the crystal’s frequency-temperature curve.

Subsequently, a varactor diode adjusts its capacitance according to the voltage Vc supplied by the compensation network, thereby fine-tuning the oscillation frequency of the crystal. The compensation voltage Vc is applied to the varactor diode, causing its capacitance to change accordingly.
This varactor diode is connected to the oscillation circuit of the quartz crystal as part of its load capacitance.
According to the principles of crystal oscillators, the oscillation frequency undergoes slight changes with variations in load capacitance, and the magnitude of this change is referred to as the “pulling amount.”

Through this sophisticated closed-loop negative feedback system, frequency deviations caused by temperature changes are effectively canceled out.