Sequenced mode TCXO

TCXO's first became available in the early 1960's. The accuracy was limited to about 0.5 ppm by the degree to which the analog thermistor networks could track the crystal temperature curve. The advent of digital compensation eliminated the curve fitting problem, but accuracy was still limited to about 0.1 ppm due to retrace problems in the oscillator. Making the oscillator pullable also made it less stable. The effect was reduced by synthesizing the output frequency in some way rather than pulling the crystal to frequency. This also allowed the use of SC-cut crystals and improved the accuracy to a few parts in 108. Dual mode temperature sensing improved thermal tracking, but hysteresis effects in the crystal still limited the accuracy. Implementation of a dual mode oscillator presents some challenges that, depending on the circuit, may result in additional hysteresis being introduced by circuit elements.