The Design Of A High-frequency And Low Noise Temperature Compensation Crystal Oscillator

Crystal oscillators are widely used in the electronic information industry,and they are of vital importance to the development and innovation of technology in the fields of electronic instruments,defense engineering,communications,aerospace and other fields.It can be said that sophisticated crystal oscillators are the most important part of an electronic system.However,under natural conditions,the output frequency of the crystal oscillator drifts with temperature.In order to improve the temperature frequency characteristics of crystal oscillators,temperature compensation is often used to improve the stability of the frequency in the event of temperature changes.Through the analysis and comparison of various traditional temperature compensation schemes,combined with the analysis of the principle of temperature compensation,a new temperature compensation scheme is proposed,which avoids the use of variable-capacity diodes and the deterioration of phase noise indicators.The traditional temperature compensation methods include analog compensation crystal oscillator(TCXO)using the thermistor network,digital compensation crystal oscillator(DTCXO)and today's commonly used microcomputer compensation crystal oscillator(MCXO).However,the above compensation method in the final analysis can not be separated from the use of variable-capacity diodes,need to be at different temperature nodes,at both ends of the variable-capacity diode loaded a specific voltage,to the crystal oscillator's output frequency to a fixed value,this particular voltage can also be called compensating voltage.Because of this,the inherent AC voltage in the crystal oscillator will certainly interfere with the compensating voltage to some extent,resulting in the deterioration of phase noise.This topic designed 12.8MHz temperature compensation prototype,first measured the frequency temperature curve in the uncompensated condition,while using a specific temperature coefficient of capacitor and crystal resonator series to form temperature compensation,avoidthes the use of variable-capacity diodes,temperature coefficient can be determined by the measured frequency temperature curve,the capacitance can be called compensation capacitor.Real-time temperature is obtained through the temperature sensor(DS18B20)and an analog switch circuit is designed to select different compensatory capacitors at different temperatures(a total of 3 different compensatory capacitors were selected in this topic).The experimental data obtained confirmed that: in the temperature range of-20? to +70?,the frequency stability after temperature compensation is within the range of±1.95 ppm,but also avoids the use of variable-capacity diodes,the deterioration of phase noise indicators is reduced,at the off-carrier 100 Hz,the phase noise is-110dBc/Hz,and at the off-carrier 1KHz,the phase noise is-140 dBc/Hz.