NEW DIGITAL COMPENSATION TECHNIQUE FOR THE DESIGN OF A MICROCOMPUTER COMPENSATED CRYSTAL-OSCILLATOR

The study of the stability of frequency sources is a matter of major i nterest due to the evolution of communication and instrumentation syst ems resulting in increasing the number of channels in a limited spectr um, and reducing size and power consumption. The paper relates laborat ory experiences that explain the behavior versus temperature of thickn ess-mode quartz crystal resonators (AT- and SC-cut crystals) that are applied to the control of frequency sources, and the performance of di gital compensation techniques. Prototypes of Microcomputer-Compensated Crystal Oscillators (MCXO's) have been developed to compare the compe nsation performance using the resonator as the temperature sensor agai nst the use of an external sensor and verify the reduction of compensa tion errors due to thermal lags and hysteresis. The design of a CMOS i ntegrated circuit for the MCXO is also included, A frequency correctio n method that does not modify the crystal resonance has been implement ed in the circuit, This allows sensing the temperature by means of the crystal and improving its long-term stability (aging). A new frequenc y comparator is also introduced, Its aim is to obtain the difference b etween two very close frequencies at its output, without being affecte d by the phase variations that the new frequency correction method and the digital circuit introduce, This detector has been implemented to get a high-resolution thermometric frequency and to realize a frequenc y-locked loop that includes a crystal controlled local oscillator, all owing the use of the MCXO as good short-term stability source.