Pll-based Programmable Crystal Oscillator

The crystal oscillator is famous for its high stability. Different kinds of crystal oscillator fulfill the different engineering. Generally, it works at a fixed frequency. And the major characteristic of the programmable crystal oscillator is to set a series of frequencies of high stability at short notice. So, several hundreds kinds of fixed crystal oscillators in stock are substituted by a multipurpose programmable crystal oscillator. Designers, factories and companies will benefit from its flexibility and save a lot for its convenience of integration.Based on the phase-locked loop (PLL), the crystal oscillator is programmed in this design. Although it brings certain negative influence on the high stability of the crystal oscillator, it works well enough for general industry requirements. Moreover, it is convenient for the frequency control.First, the basic principle of the phase-locked loop circuit is introduced in this paper. Second, the discussion and improvement of the design of the low passed filter (LPF) is presented. Third, an important component of the PLL, Voltage Controlled Oscillator (VCO), is designed. Finally, a total solution and the software design of the MCU are proposed at the end of this paper. The design has excellent performances of wide rang frequencies, small comparison frequency, easy to program, simple structure and low cost. Together with its strong practicability and economy, it is easy to make it popular.We use low cost 12MHz AT-Cut Crystal Oscillator and the low power consumption PLL chip LMX2306 of National Semiconductor Corporation in this design. In its peripheral circuits, we use Matlab to simulate the LPF and design a new Shelle common collector VCO in which a stage is added for driving LMX2306. Meanwhile, the emluator MINI-51 programmed by assemble language is used to generate the dividing frequency sequence for LMX2306 so as to get the output serial frequencies (25M -150M) whose step achieves 2.5 kHz.