Stanford Research FS725 – Benchtop Rubidium Frequency Standard

The Stanford Research Systems FS725 is a benchtop rubidium frequency standard delivering ultra-stable 10 MHz reference signals for precision metrology, calibration, and research applications. Built around the SRS PRS10 rubidium oscillator and housed in a compact half-width 2U chassis, the FS725 combines exceptional long-term aging performance with demonstrated reliability exceeding 200,000 hours MTBF. The instrument outputs two 10 MHz sine waves, one 5 MHz sine wave, and one 1 pps signal, each with low phase noise and minimal jitter.

Optional internal distribution modules expand output count to 22 total channels while maintaining specification compliance across all ports.

Technical Specifications

• Two 10 MHz sine wave outputs


• One 5 MHz sine wave output


• One pulse-per-second (1 pps) output Output Amplitude & Power


• 10 MHz / 5 MHz: 0.5 V rms ±10% into 50 Ω (1.41 V pp, +7 dBm)


• 1 pps pulse: 2.5 V into 50 Ω; 5 V into high-impedance loads


• 1 pps pulse width: 10 µs Stability & Aging


• One-second Allan variance: <2 × 10⁻¹¹


• 20-year aging rate: <5 × 10⁻⁹ (−130 dBc/Hz at 10 Hz offset (SSB)


• >−140 dBc/Hz at other offsets 1 pps Synchronization


• External 1 pps locking to GPS or similar references


• Lock time constant: 8 minutes to 18 hours (computer-adjustable)


• Output jitter: 200,000 hour MTBF


• Sub-nanosecond 1 pps jitter for timing applications


• Stratum 1 performance via external GPS 1 pps locking


• Low-noise AC power supply integrated


• Computer control and remote monitoring via RS-232


• Modular output expansion without performance degradation

Typical Applications

• Primary frequency reference in calibration laboratories


• Timing reference for test equipment synchronization


• GPS disciplined clock systems and time-keeping networks


• Precision measurement and metrology setups


• Research environments requiring long-term frequency stability

Compatibility & Integration The FS725 operates as a standalone instrument or integrates into larger timing systems through its external 1 pps locking input. RS-232 control enables automation and remote monitoring in networked lab environments. Optional distribution modules allow a single FS725 to serve multiple test instruments and data acquisition systems from one reference source.