Performance tests on an MCXO combining ASIC and hybrid construction

Abstract

A microcomputer-compensated crystal oscillator (MCXO) using hybridized crystal oscillator circuits combined with an application specific integrated circuit (ASIC) and a microcontroller is discussed. The authors describe the performance of a prototype 10 MHz design. The MCXO provides both frequency and time accuracies of 30 parts per billion over the temperature range of -55 to +85 degrees C with negligible warm-up time and low power consumption. The ASIC contains the signal mixers, divider chains, counters, phase comparators, digital control logic, and a direct digital synthesizer (DDS). During calibration, the frequency/temperature (F/T) characteristics of both the fundamental and the third overtone of the SC-cut resonator are measured. Two seventh-order polynomials are fitted to the F/T data collected for each crystal, and the coefficients are stored in the nonvolatile memory of the MCXO. Through the microcomputer, the polynomial controls the correction frequency generated by the DDS which, when summed with the output of the third overtone frequency oscillator, using the phase-locked loop produces the very stable temperature-corrected output frequency. The frequency tolerance and single-sideband phase noise measurements are discussed. >