l oscillators (TCXO) are crystal oscillators designed to provide improved frequency stability despite wide variations in temperature. TCXOs have a temperature compensation circuit together with the crystal, which measures the ambient temper- ature and compensates for any change by altering the voltage applied to the crystal. By aligning the voltage to values within the possible temperature range, the compensation circuit stabilizes the output clock frequency at different temperatures. Oven-controlled crystal oscillators (OCXO) are crystal oscillators where the crystal itself is placed in an oven that attempts to maintain a specific temperature inside the crystal housing, independent of the temperature changes occurring outside. This reduces the temperature vari- ation on the oscillator and thereby increases the stability of the frequency. As you can imagine, oscillators with additional heating components end up bulkier and costlier than TCXOs. The basic approach with the TCXO is to compensate for measured changes in temperature by ap- plying appropriate changes in voltage, whereas for the OCXO, the temperature is controlled (by be- ing elevated above the expected operating temperature range). Figure 5-2 shows a typical OCXO. Figure 5-2 Typical OCXO An oscillator is the core component of a clock, which alone can significantly impact the quality of the clock. An approximate comparison of stability between these different oscillators suggests that the stability of an OCXO might be 10 to 100 times higher than a TCXO class device. Table 3-2 in Chapter 3 outlines the characteristics of the common types of oscillator. The stability of an oscillator type also gets reflected in the cost. As a very rough estimate, cesium- based oscillators cost about $50,000 and rubidium-based oscillators around $100, whereas an OCXO costs around $30 and a TCXO would be less than $10. 5.1.1 PLLs A phase-locked loop (PLL) is an electronic device or circuit that generates an output clock signal that is phase-aligned as well as frequency-alig