Design of a Temperature-Compensated Crystal Oscillator Using the New Digital Trimming Method

Abstract

Crystal oscillators play an important role in modern electronic systems. Since a precise oscillation frequency, low phase noise, and low power are required for all paging applications, a crystal oscillator is the best candidate. The frequency stability of a crystal is limited by its temperature-frequency characteristics. The untrimmed accuracy of crystal resonators is about ±30 ppm, mostly due to temperature variations and aging. If a high accuracy of the oscillation frequency is to be obtained, the temperature compensation is indispensable. The frequency shift due to the temperature variation can be compensated for by controlling the value of the load capacitance connected to the crystal. The frequency stability of analog temperature-compensated crystal oscillators (TCXO) has been limited to a range of a few ppms. Recently, more stable TCXOs have come to be required, and digital TCXOs (DTCXO) have been studied extensively because of their higher frequency accuracy and one-chip implementation possibility. Since implementing varactor diodes and digital-to-analog converters (DAC) used in conventional digital compensation systems is very difficult for VLSI TCXOs, DTCXOs using capacitor arrays which are directly controlled by a digital code from the memory, as shown in Fig. 1, have been proposed [1]. How to organize the capacitor array for DTCXOs was