A TCXO (Temperature Control Crystal Oscillator) is a voltage controlled crystal oscillator with a improvement voltage apply to the voltage control pin. This voltage changes with temperature to bring the frequency back to nominal.
Temperature compensated crystal oscillators (TCXOs) offer excellent temperature characteristics with low power consumption and fast warmup. Most models of TCXOs also offer a facility to adjust the frequency by means of an external control voltage. They do not offer as wide a pulling range as a VCXO.
Any crystal related problems such as coupled modes cannot be improved for and are superimposed on the resulting frequency-temperature curve. This makes the design and manufacturing of the crystal a difficult and critical part of the TCXO. The majority of TCXO’s require a voltage control function. This allows for exact setting of frequency, adjustments for long-term aging and the ability to phase lock the device to other sources. This function VCTCXO, works by adjusting the load capacitance presented to the crystal. The changes made by these adjustments should be taken into account for fixed tolerance TCXO’s.
The capacitors used to adjust frequency have a temperature coefficient, and this temperature coefficient changes the nominal value of the capacitors over temperature. This change has an effect on the compensation of the crystal. In lower precision TCXO’s this is usually ignored, but as TCXO precision meets and exceeds 0.5 ppm levels, these effects can no longer be ignored. In a typical application the units will need to be adjusted from 0.5-2 ppm for exact calibration. The device will then need adjustment for long term stability.
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