Factors influencing stability in the microcomputer-compensated crystal oscillator

The stability of a microcomputer-compensated crystal oscillator (MCXO) which uses a direct digital synthesizer combined with phase-locked loop summing is discussed in detail. The characteristics of each subsystem of the MCXO are analyzed and their impact on the stability of the output frequency is discussed. Test results in support of the analyses are given. The stability is required to be better than 20 p.p.b. over the range of temperatures from -55°C to +85°C. A block diagram of the MCXO system is shown. Several areas of interest are examined which may help clarify some of the attributes unique to this MCXO. Examples of calibration runs showing both low and high levels of residuals and hysteresis are shown. It is demonstrated that the MCXO system is capable of correcting for predictable frequency-temperature variations smaller than 2 p.p.b