This paper shows a traditional Rubidium atomic clock - E10-MRX. The incorporated lamp and ultra-miniature waveguide cavity introduced in the physics package is the main means to reduce the volume of the clock; the electronics consisting of 6.8GHz direct synthesizer and digital servo system is to enhance the automatic functions which ensure large scale production is available with low costs. In all, the clock has the physical dimensions of an oven controlled crystal oscillator (OCXO), measuring 51mm × 51mm (2′ × 2′) and standing at a mere 25mm (1′). The Allan deviation of 3E-11/s; the temperature coefficient is 3E-10 in the range of −20°C ∼ 60°C; the frequency aging is below 5E-12/day. The clock has the same dimension and pin definitions as OCXO's used in telecoms, but higher performance, and will be popular in telecom equipments and base-stations instead of OCXO in future. This paper focuses on the low noise synthesizer development.