Design Of A Wide Tuning Temperature Compensated Oscillator

As the "heart" of the circuit,the crystal oscillator acts as frequency reference for many systems,and is widely used in almost all military and civilian industries such as positioning,navigation systems,wireless communication,and transportation.Many kinds of crystal oscillators derived in different application scenarios,such as the Oven controlled crystal oscillator(OCXO)with the highest frequency stability,the temperature compensated crystal oscillator(TCXO)with high frequency stability and the most widely used,and the voltage controlled crystal oscillator(VCXO)with the widest frequency tuning range.Crystal oscillators with both wide frequency tuning range and high frequency stability are still lacking in the market.Chinese research in the field of crystal oscillators is not good enough.The highprecision crystal oscillator technology is almost in the control of several large crystal giants abroad.With the rise of domestic independent chips and wireless communication industry,it is necessary to have independently developed high-precision crystal oscillators ourselves.This paper presents a temperature-compensated crystal oscillator with a wide frequency tuning range and high frequency stability.The relative frequency stability is ±1ppm in the range of-70ppm~+90ppm relative frequency offset,and the ambient temperature is-30°C ~ +70°C.Moreover,the temperature compensated crystal oscillator can overcome the Trim effect.The temperature compensated crystal oscillator consists of a microcomputer and a voltage-controlled crystal oscillator.After the initial data is collected,a compensating function will be fitted to compensate the frequency deviation caused by temperature change.In order to explore the optimal circuit structure and the best compensation method,two schemes are used to realize the wide frequency tuning temperature compensation crystal oscillator.The first one: two varactor diodes are used to form oscillating circuit,the voltagecontrolled crystal oscillator can be more stable because of the circuit structure theoretically,and then the two varactors are controlled by two voltage-control terminals respectively,one variable diode is used for frequency modulation and the other varactor is used for frequency stabilization.The second one: only one varactor is used in the oscillating circuit,the voltage controlled crystal oscillator can have a wider frequency tuning range theoretically.The frequency modulation voltage and the frequency stabilization voltage are considered as a whole.We fitting a binary function to compensate the frequency deviation,the control voltage is determined by the relative frequency offset and the ambient temperature according to the function.Making hardware boards according to two schemes respectively,completing software communication,and then performing temperature experiments.The first experiment obtained the initial data,and then function was fitted for temperature compensation.The second experiment verifies the compensation effect.The experimental results show that the second scheme is obviously superior to the first one,and the latter has a simpler circuit structure,a wider frequency tuning range and higher frequency stability.As the biggest factor affecting the accuracy of crystal oscillator temperature compensation,trim effect is the difficulties that all researchers have to overcome.This paper verifies the trim effect and proves that the three-dimensional compensation method can overcome the trim effect of the crystal oscillator,thus achieving high frequency stability.