The Efratom Model M-100 Rubidium Frequency Standard (RFS), Part Number (P/N) 70502-1, is a subcompact, light weight, atomic resonance-controlled oscillator. The unit provides a pure and stable 10 MHz sinusoidal signal from a 10 MHz voltage-controlled crystal oscillator (VCXO), which is referenced and locked to the hyperfine transition of Rubidium 87 (Rb). The reference element is an optically-pumped integrated rubidium vapor cell, contained within the physics package.