What Makes a Good LP-OCXO?
Low Power Oven Controlled Crystal Oscillators (LP-OCXO) play a pivotal role in many electronics and telecommunications applications. By maintaining a constant temperature, OCXOs ensure that the crystal oscillator operates consistently, regardless of external temperature variations. But what are the key attributes of a good LP-OCXO? Here, we’ll delve deep into the factors that make an LP-OCXO stand out.
1. High Performance Quartz
High Q Factor for Low Phase Noise and High Spectral Purity
The Q factor, or quality factor, of a quartz crystal measures its resonant frequency’s sharpness. A high Q factor indicates low energy losses and therefore results in lower phase noise and greater spectral purity. This is particularly crucial for applications that demand precise timing, like satellite communication or Global Positioning System is a navigation satellite system. See also systems.
Crystal Cut (AT, SC, IT) for Thermal Aging, Drift, Stability at 1s
Different crystal cuts offer varying attributes that impact the oscillator’s performance:
- AT-Cut: This is the most common cut for OCXOs. It offers good temperature stability and a low temperature coefficient, making it ideal for a wide range of applications.
- SC-Cut: Stress Compensated (SC) cut crystals offer better temperature stability and reduced aging rates than AT-cut crystals. They’re particularly suited for applications that need ultra-stable frequencies.
- IT-Cut: This cut provides an intermediate performance between the AT and SC cuts and is less commonly used.
Quartz for Super Turnover Point
The turnover point of a quartz crystal refers to the temperature at which the crystal’s frequency variation reaches minimum (changing slope, derived equals to zero). Quartzes designed for a super turnover point are engineered to have this point at an easily controllable temperature 10 to 15 degrees above the maximum operating temperature, thereby ensuring better stability and performance n all operating temperature conditions.
2. Thermal Insulation
Conducted Loss: Mechanical Design
One of the primary ways heat can escape from an OCXO is through conduction. Proper mechanical design minimizes contact with materials that conduct heat away from the crystal, ensuring that the oven’s heat remains consistent.
Convection Loss: Hermeticity and Vacuum Level
Convection refers to the loss of heat from the OCXO via the movement of air,gas or atmosphere inside the OCXO cavity. To prevent this, OCXOs are typically hermetically sealed and using neutral gas or vacuum inside their cavities. A high-performance LP-OCXO will also maintain a low vacuum level inside, preventing gas molecules from moving around and leading to convective heat loss.
Radiated Loss: Reflective Surfaces
Radiated heat loss can occur when the OCXO emits infrared radiation. By using materials with reflective surfaces, the radiation is reflected back, minimizing energy loss and maintaining the oven’s temperature.
3. Tune OCXO Frequency
Frequency Accuracy: 25-50 ppm Before and .±1 ppm After
The frequency accuracy of an OCXO is a measure of its deviation from the desired absolute frequency needed. Before tuning, an OCXO might have an accuracy of 25-50 ppm (parts per million), but a good OCXO can be tuned to achieve an accuracy as tight as .±1 ppm, making its nominal frequency very accurate.
Internal Oven Temperature Setting: +- .5°C Across -40°C/+85°C
A good LP-OCXO should be able to maintain its oven temperature consistently, even when faced with external temperature variations. The ability to hold the oven temperature within ±0.5°C across a range from -40°C to +85°C ensures that the crystal frequency remains stable around its Turnover point, delivering reliable and consistent ultra-stable frequency whatever the external temperature seen by the OCXO.
A high-performance LP-OCXO is the result of the careful consideration of multiple physical factors. From the choice of quartz and its cut to the design of the oven that controls its temperature, every aspect plays a crucial role in determining the performance of the oscillator. When selecting an LP-OCXO for any application, understanding these key attributes can guide users to make an informed decision, ensuring optimal performance and reliability.