OC3SC25

The OC3SC25 is a high-precision Oven-Controlled Crystal Oscillator (OCXO) from CTI, designed for 3.3V supply systems requiring ultra-stable frequency references. Based on SC-cut crystal technology housed within a precision temperature-controlled oven, this device achieves frequency stability of ±0.02 ppm at room temperature and maintains exceptional stability across the full industrial temperature range. The ‘OC3’ in the part number indicates a 3.3V supply voltage, while ‘SC25’ denotes the standard SC-25 package (25x25mm footprint). Compared to its 5V counterpart (OC5SC25), the OC3SC25 is optimized for lower supply voltage operation, making it ideal for modern 3.3V-based digital systems and battery-powered applications where power efficiency matters. The device uses an SC-cut quartz crystal operating at its turnover temperature (approximately 85°C), maintained by a precision oven control loop. This active temperature control virtually eliminates frequency drift caused by ambient temperature changes, providing stability levels that are orders of magnitude better than TCXOs and standard crystal oscillators. Output options include square wave (CMOS compatible) or sine wave versions. The 3.3V CMOS output is directly compatible with modern FPGAs, microcontrollers, and digital ICs without level shifting. Many OC3SC25 variants also include a voltage control (VC) pin for fine frequency adjustment, enabling applications such as GPS-disciplined oscillators (GPSDO) and phase-locked loop systems. The SC-25 hermetically sealed package provides robust protection against environmental factors including humidity, vibration, and mechanical shock, ensuring reliable operation in demanding industrial and outdoor environments.

The OC3SC25 operates as an oven-controlled crystal oscillator using active thermal management to achieve extreme frequency stability: 1) SC-Cut Crystal: The heart of the oscillator is an SC-cut (Stress-Compensated, double-rotated cut) quartz crystal. SC-cut crystals exhibit an inflection point (turnover temperature) where the frequency-temperature coefficient crosses zero. Operating at this temperature minimizes frequency sensitivity to temperature variations. 2) Oscillator Circuit: A sustaining amplifier with carefully selected components drives the crystal at its series or parallel resonant frequency. The circuit design optimizes for low phase noise and high frequency stability. 3) Oven Chamber: The crystal and temperature-sensitive oscillator components are mounted inside a thermally insulated miniature oven. The oven provides a stable thermal environment, isolating the crystal from ambient temperature changes. 4) Temperature Control Loop: A precision temperature sensor (thermistor or RTD) inside the oven continuously measures the temperature and feeds back to a control amplifier. The amplifier drives a resistive heating element, adjusting power to maintain the oven at the crystal’s turnover temperature. This closed-loop control achieves temperature stabilities in the millidegree range. 5) 3.3V Power Management: The OC3SC25 is designed to operate efficiently from a 3.3V supply. Internal voltage regulation may be used for the oven control and oscillator circuits to ensure consistent performance across the supply voltage tolerance range. 6) Output Buffer Stage: A high-speed buffer amplifies and shapes the oscillator signal. For square wave versions, this produces a clean digital output with 3.3V CMOS logic levels and fast edge rates. 7) Optional VCXO Function: Versions with a voltage control pin include a varactor diode in the oscillator tank circuit. Applying a DC control voltage changes the effective capacitance seen by the crystal, allowing the output frequency to be pulled over a narrow range (typically ±2 to ±10 ppm) for calibration or synchronization purposes. 8) Warm-up and Steady State: At power-up, the oven is at ambient temperature and draws maximum current to heat up. As it approaches the setpoint, power decreases. After 3-5 minutes, the oven reaches operating temperature and the frequency stabilizes to within specification.

Note: Exact pin configuration varies by manufacturer and product version. SC-25 packages typically have through-hole or surface-mount pins on the bottom. Refer to the specific manufacturer datasheet for detailed pinout.

• 3.3V-based precision clock sources for FPGA and SoC systems
• Portable and handheld test instruments
• GPS/GNSS disciplined oscillators (GPSDO)
• Software Defined Radio (SDR) reference clocks
• Network timing cards and PTP/IEEE 1588 grandmasters
• Battery-operated precision equipment
• Telecommunications and optical networking
• High-speed data converter clocking (ADC/DAC)