Blog: Breaking the power barrier - Designing ultra-low-power OCXOs for the edge

In precision timing, the Oven-Controlled Crystal Oscillator (OCXO) has traditionally set the standard for stability. By enclosing a quartz crystal within a compact oven to protect it from ambient temperature fluctuations, an OCXO achieves Stratum 3E-level precision, with variations of less than 10 ppb across temperature ranges and just 1 ppb/day due to ageing. However, this stability is associated with significant power consumption. Conventional OCXOs typically draw between 1.5 and 2 watts of power, which is acceptable for grid-connected systems but problematic for the increasing number of battery-powered edge devices. At the 2026 Workshop on Timing and Synchronization (WSTS), Rakon's trusted authority on telecom systems synchronisation, Ullas Kumar, outlined the core engineering challenges as well as solutions involved in reducing an OCXO's power consumption to as little as 150 milliwatts.

The rise of precision timing at the edge

Why is there an emerging need for ultra-low-power, highly precise clocks? The reason is that the "edge" is extending into areas where GNSS signals are unavailable, and where replacing batteries is either exceedingly challenging or impossible.

• Autonomous Underwater Vehicles (AUVs): These operate at depths of thousands of feet below the surface for exploration and mapping purposes, in environments where satellite signals are absent.
• Military Manpack Radios: Military personnel in the field require hours or even days of high-precision timing without dependence on vulnerable GNSS networks.
• Industrial Automated Guided Vehicles (AGVs) and Robotics: These wireless-operated vehicles on factory floors necessitate extremely accurate synchronis to ensure safe navigation.
• Mapping and Survey Drones: Aerial vehicles engaged in high-resolution mapping depend on precise timing to synchronise telemetry data accurately during their operations.

Inside the OCXO: Where is the power going?

To address the power management issue, it is essential to understand the architecture of the Oven-Controlled Crystal Oscillator (OCXO). An OCXO operates through two primary feedback loops: the Oscillator Loop, which maintains the crystal’s vibration to produce the desired frequency, and the Thermal Loop, which regulates the temperature by controlling a heater to sustain a stable ambient environment within the crystal chamber. According to Leeson’s Equation (the fundamental mathematical model describing oscillator phase noise), phase noise level is inversely related to the supplied power. Achieving optimal phase noise performance necessitates providing the oscillator with a consistent baseline power input. However, the total power consumption of an OCXO predominantly derives from the thermal management system—the heater and oven—rather than the oscillator circuit itself. Consequently, power efficiency improvements require a focus on thermodynamic processes and temperature regulation, rather than solely electronic circuitry.

The three critical elements for ultra-low power

Engineers are approaching the thermal budget from three distinct angles: crystal physics, application-specific scoping, and advanced packaging materials.

Advantage of SC-Cut Crystals

Quartz crystals are sliced from raw quartz at specific angles, which dictates how they react to temperature. Traditional low-cost oscillators use AT-cut crystals. However, when the internal oven is shrunk down to fit small, modern devices (reducing its thermal mass), AT-cut crystals become incredibly sensitive to mechanical stress caused by internal temperature fluctuations and external environment shifts. To combat this, low-power precision OCXOs should use SC-cut (Stress-Compensated) crystals. SC-cut crystals are structurally insulated against rapid thermal and mechanical shocks, ensuring stability even inside a tiny, lightweight oven.

Redefining the Operating Microclimate

Traditionally, an OCXO is built to handle the full industrial temperature range of -40°C to 85°C. To ensure the oven is always hotter than the outside air, the internal temperature set point is usually pinned way up at 92°C or 93°C. Maintaining a 93°C chamber when it's freezing outside requires massive amounts of power. But what if the target application doesn't need to go from desert heat to arctic cold? Consider subsea applications. Deep ocean water hovers consistently around 4°C. By limiting the operating range of the device to a narrower band (e.g., -10°C to 50°C), engineers can safely drop the internal oven set point to around 62°C.

This narrowed delta radically reduces thermal leakage, allowing the device to hit that coveted 120 mW to 150 mW sweet spot when operating in cold subsea temperatures.

Advanced Thermo-mechanical Packaging

To prevent heat loss from the chamber, the primary objective is to reduce thermal leakage from a typical 5 mW/°C level to below 2 mW/°C. This is accomplished through advanced manufacturing methods:

Vacuum Sealing: Replacing conventional large "Hoder-Crystal" crystal enclosures, typically measuring 25×22 mm, with modern surface-mount device (SMD) strip crystals, sized at 14×9 mm or 9×7 mm. These are hermetically sealed under high vacuum conditions, eliminating air gaps that facilitate heat transfer.

• Low-Emissivity Coatings: Applying low-emissivity coatings to internal metallic components to reduce radiative heat transfer.

• Structural Isolation: Redesigning internal mounting brackets to physically isolate the heating elements, thereby minimising conductive heat loss through external pins.

The performance updates

Testing on 20 MHz devices in ultra-compact form factors (14×9 mm and 9×7 mm) demonstrates that these ultra-low-power units reliably maintain Stratum 3E stability within a narrow 10 ppb range while keeping current consumption well below strict battery-powered limits.

The table below compares the performance of TCXOs, UL-OCXOs and CSAC on power, stability and applications. 

While Chip-Scale Atomic Clocks (CSACs) maintain the highest levels of frequency stability in the market, their significant cost poses challenges for widespread commercial deployment. In contrast, high-performance TCXOs are cost-effective and consume low power but do not deliver the long-term frequency stability necessary for advanced holdover applications. The ultra-low-power OCXO offers a balanced solution, providing high precision suitable for battery-powered edge devices while remaining economically and energetically efficient.

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