Stanford Research Systems / SRS SR625

• Rubidium atomic timebase
• 2 GHz prescaler input
• 11-digit frequency resolution (1 s)
• 10 MHz Rb timebase output
• Statistical analysis & Allan variance
• GPIB and RS-232 interfaces

The SR625 Frequency Counter is a NIST traceable frequency counting standard for calibrating base stations, transmitters and many other types of communication systems. It combines the high resolution and wide variety of features found in the SR620 counter with the atomic accuracy of a rubidium timebase.

Low Drift, High Accuracy

The SR625 Frequency Counter consists of a frequency counter (SR620), a high-accuracy rubidium timebase (PRS10), and a 2 GHz input prescaler. The combination of the SR620 and the prescaler allows direct frequency measurements up to 2 GHz with twelve digits of resolution in a 100 s measurement.

The rubidium timebase ensures excellent short-term stability (<2 × 10^-11 Allan variance (1 s)) and long-term drift (<5 × 10^-11/month).

Simple, Portable Operation

The SR625's warmup time is less than ten minutes making it ideal for field applications. An additional back-panel output provides a rubidium stabilized 10 MHz signal which can be used to drive other test equipment (e.g., synthesizers or spectrum analyzers). The standard GPIB and RS-232 interfaces allow for complete control and data acquisition from any laboratory computer. The SR625's performance makes it the standard for remote applications or laboratory calibration.

The following specifications relate to the 2.2 GHz prescaler and the rubidium timebase of the SR625. Please see the section on the SR620 for general specifications relating to the counter.