VCXOs

VCXOs, or voltage-controlled crystal oscillators, are quartz crystal oscillators whose output frequency can be tuned by a control voltage. The tuning is achieved by changing the load capacitance seen by the crystal, typically with a varactor diode or other tunable capacitor network, allowing small frequency adjustments while preserving the crystal’s stability.

Most VCXOs operate in the fundamental or overtone modes and are designed for use in phase-locked loops and frequency synthesizers where agile frequency control is required. The control voltage, often 0 to 5 V or 0 to 3.3 V, is applied to the tuning element; some devices use 0 to 1 V or ±voltage ranges. The total tuning range is device dependent, usually from a few tens of ppm to a few hundred ppm, with sensitivity expressed in ppm per volt. The relation between control voltage and frequency is typically monotonic.

Frequency ranges for commercially available VCXOs generally span tens of megahertz up to a few hundred megahertz, with higher-frequency units achieving greater values via overtone crystals. Performance depends on the crystal and circuit; VCXOs offer better short-term stability and lower phase noise than many tunable active oscillators, but typically do not reach the long-term stability of TCXOs or OCXOs unless temperature compensation or external disciplining is used.

Applications include radio communications, wireless infrastructure, test and measurement equipment, and any system requiring rapid, precise frequency adjustment without a PLL. VCXOs are often supplied in surface-mount packages with either sine-wave or clipped square-wave outputs and standard supply voltages such as 3.3 V or 5 V.