Frequency correction and frequency locked loop for a microcomputer compensated crystal oscillator

This paper presents a new circuit to be applied to correct frequency deviations digitally in MCXO´s (Microcomputer Compensated Crystal Oscillators). The frequency correction method does not modify the crystal resonance to allow sensing the temperature by the crystal and improving its very long term behaviour (ageing). A new frequency comparator is also introduced. Its aim is to obtain the difference between two very close frequencies at its output, without being affected by the phase variations that the correction method and the digital circuit (jitter) introduce. This detector has been accomplished to implement a frequency locked loop that leads a crystal controlled local oscillator, allowing the use of the MCXO as good a short term stability frequency source. The local oscillator control voltage is the output of a very high resolution digital-to-analog converter specifically designed for the circuit