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 counter with the atomic accuracy of a rubidium timebase.