Quartz crystal resonators have been used for many years as the frequency reference for radios and other communication devices. When exceptional precision was needed, the crystal was placed in an oven that held the crystal at a constant temperature – hence the term Oven Controlled Crystal (XTAL) Oscillator or OCXO. The OCXO has long represented the greatest precision that can be obtained from a quartz crystal based frequency reference.

Although OCXO’s offer the greatest precision, there are several considerations that limit how they can be applied to many system requirements. These include:
• Demand power to heat the oven, with internal temperature in the 75°C to 95°C range;
• Significant time from turn-on for the OCXO to warm-up and stabilize in frequency;
• Relatively long time to reach the precision frequency; and
• Larger package size, needed to have insulation around the ovenized portion.

To avoid many of these OCXO limitations, the Temperature Compensated Crystal Oscillator or TCXO was developed. In this case, circuitry was added around the crystal resonant element to correct or pull the crystal to a stable frequency versus temperature. The TCXO capabilities were typically not quite as precise or accurate as the OCXO, but since TCXO’s operated at much lower power and could be smaller in size, they satisfied much of the market’s demand.