Rubidium Standards vs. Quartz Reference Oscillators
Filed under Calibrators, Standards
In high-precision test, measurement, and calibration environments, selecting the appropriate frequency reference is critical. Engineers typically choose between rubidium frequency standards and quartz crystal reference oscillators based on their stability, aging, and signal purity requirements. Rubidium standards utilize atomic transitions to achieve superior long-term stability and resistance to temperature fluctuations. Quartz reference oscillators, particularly oven-controlled crystal oscillators (OCXOs), rely on the physical resonance of quartz crystals. While quartz references provide low phase noise and excellent short-term stability, they are subject to higher drift rates over time. This guide evaluates these performance trade-offs using technical parameters from representative laboratory reference standards.
- Rubidium Temperature Stability
- < 1 x 10^-10 (0 to 50 °C)
- Quartz Temperature Stability
- < 7 x 10^-9 (0 to 55 °C)
- Quartz Daily Aging Rate
- < 5 x 10^-10 / day
- Quartz Yearly Aging Rate
- < 1 x 10^-7 / year
Long-Term Stability and Aging Profiles
The most significant differentiator between atomic rubidium standards and quartz crystal references is long-term stability and aging. Quartz crystal resonators experience physical aging over time due to stress relaxation and contamination of the crystal element. For example, the quartz-based Keysight Technologies (Agilent HP) 70310A has a specified daily aging rate of less than 5 x 10^-10 after 24 hours of continuous operation, and a yearly aging rate of less than 1 x 10^-7.
In contrast, rubidium standards lock an internal quartz oscillator to the ultra-stable hyperfine transition of rubidium-87 atoms. This atomic reference drastically reduces long-term drift. The temperature stability of the Stanford Research Systems FS725 rubidium standard is rated at less than 1 x 10^-10 over its 0 to 50 °C operating range, demonstrating a significant stability advantage over the quartz-based Keysight Technologies (Agilent HP) 70310A, which exhibits a temperature stability of less than 7 x 10^-9 over a similar 0 to 55 °C range.
Warm-Up Dynamics and Power Consumption
Quartz and rubidium references have different operational behaviors during startup. Oven-controlled quartz references must heat their internal crystal oven to a stable operating temperature. The Keysight Technologies (Agilent HP) 70310A reaches within 5 x 10^-9 of its final frequency within 20 minutes at an ambient temperature of 25 °C, utilizing an auxiliary power pack to maintain standby oven power when the main system is shut down.
Rubidium standards require substantial energy to vaporize the rubidium metal and light the physics package lamp during startup. The Stanford Research Systems FS725 draws 100 W during its initial warm-up phase, which drops down to 30 W once it reaches a steady-state locked condition. This higher power envelope is the trade-off for achieving atomic-level stability.
Output Configurations and System Integration
Depending on the application, the frequency reference must distribute specific signal types. Laboratory standards like the Stanford Research Systems FS725 distribute standard 10 MHz, 5 MHz, and 1 PPS (pulse per second) outputs. It features dual 10 MHz sine wave outputs, a 5 MHz sine wave output, and a 10 µs wide 1 PPS TTL output, with channel-to-channel isolation exceeding 100 dB and low harmonics of less than -40 dBc.
Modular references, such as the Keysight Technologies (Agilent HP) 70310A, are optimized for integration into larger instrumentation frames. The 1-slot Modular Measurement System (MMS) module provides 10 MHz and high-frequency 100 MHz reference signals directly to nearby test modules, and it can accept external reference input frequencies of 1 MHz, 2 MHz, 5 MHz, or 10 MHz to synchronize with an external primary standard.
Example instruments
Frequently asked questions
- How does the temperature stability of a rubidium standard compare to a quartz reference?
- A rubidium standard generally offers much tighter temperature stability. The Stanford Research Systems FS725 rubidium standard has a temperature stability of less than 1 x 10^-10 from 0 to 50 °C, whereas a high-stability quartz reference like the Keysight Technologies (Agilent HP) 70310A has a temperature stability of less than 7 x 10^-9 over 0 to 55 °C.
- What are the power requirements of a rubidium frequency standard?
- Rubidium standards require a significant amount of power to warm up the physics package. For example, the Stanford Research Systems FS725 requires 100 W during warm-up and 30 W once it reaches its steady-state operating temperature.
- How long does a precision quartz reference take to stabilize after power-on?
- A precision quartz reference such as the Keysight Technologies (Agilent HP) 70310A can stabilize to within 5 x 10^-9 of its final frequency in 20 minutes when starting at an ambient temperature of 25 °C.