Cesium Atomic Clock

The 5071A Cesium Primary Frequency Standard was developed by Hewlett-Packard (HP) in the 1990’s. In 1999, an HP spinoff company, Agilent Technologies continued manufacturing the 5071A. In August 2005, Agilent sold the 5071A to Symmetricom, Inc., which then became the supplier. Finally, Microsemi Corporation acquired Symmetricom in October 2013. Microsemi continues to offer the 5071A
Caveat: The following information is adapted from the Hewlett- Packard Operating and Programming Manual, HP 5071A Primary Frequency Standard, October 1997, Introduction, p. iv. The basic description is assumed to apply to the Symmetricom 5071A as well, but with possible differences in some operating parameters and features.
What is the 5071A Primary Frequency Standard?
The 5071A is a source of extremely accurate and stable output frequencies. The accuracy of the 5071A is within a few parts in 10E12 of the internationally accepted definition of frequency. This accuracy is made more usable and practical by the 5071A’s excellent environmental stability.
How does the 5071A work?
The 5071A uses a fundamental property of the element cesium to define frequency. Inside the cesium beam tube assembly, an applied microwave signal causes energy-level transitions to occur in the cesium atoms. The microwave signal is synthesized from a 10 MHz Voltage-Controlled Crystal Oscillator (VCXO).
Patented Cesium II circuit and software technology detects departures of the microwave frequency from the cesium energy-level transition frequency and corrects them by tuning the VCXO to run precisely at the proper rational-fraction of the cesium frequency. The microprocessor (an integral part of this servo loop) performs the error determination and correction several times a second to ensure that the VCXO is always closely locked to the cesium transition frequency.
Many functions within the 5071A are under Cesium II software control. The software manages the initial warm-up and alignment, then continuously monitors the appropriate signals using this information to control all key operating parameters for optimum performance. Starting the 5071A merely requires connecting ac or dc power.
5071A major features.
∙ 5 or 10 MHz programmable outputs
∙ Time and date clocks
∙ Easily synchronized 1 pulse per second (1PPS) outputs
∙ 45 minute standby battery
∙ Accurate and precise output frequency steering
∙ Event log
For further information on the 5071A, see:
Brief description of an atomic clock
Electromagnetic waves of very specific and consistent frequencies can induce atoms to fluctuate between two energy states, and by measuring that frequency we can determine the “tick” of an atomic clock. A second in a cesium clock, for example, is defined as 9,192,631,770.0 cycles of the frequency that causes the cesium atom to jump between those states. Different atoms “tick” at different rates – strontium atoms tick about 10,000 times faster than cesium atoms – but all atoms of a given element tick at the same rate, making atomic clocks much more consistent than clocks based on macroscopic objects such as pendulums or quartz crystals.
Steven Jefferts, physicist, National Institute of Standards and Technology.
For additional background information go to:
Symmetricom, Incorporated
Physical Description
plastic (overall material)
gray (overall color)
metal (overall material)
overall: 5 3/4 in x 16 3/4 in x 19 3/4 in; 14.605 cm x 42.545 cm x 50.165 cm
overall: 5 5/8 in x 16 3/4 in x 20 1/2 in; 14.2875 cm x 42.545 cm x 52.07 cm
ID Number
nonaccession number
catalog number
serial number
Credit Line
Symmetricom, Incorporated
Modern Physics
Science & Scientific Instruments
See more items in
Medicine and Science: Modern Physics
Measuring & Mapping
Science & Mathematics
Time and Navigation
Time and Navigation, National Air and Space Museum
Data Source
National Museum of American History, Kenneth E. Behring Center


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