The graph depics the long term stability over time for each of the different frequency standards: Quartz, Rubidium, Cesium, and Hydrogen Maser. Three of these standards are atomic based;  Cesium, Hydrogen and Rubidium.

Rubidium Oscillator Manufacturers

Oscillator Acronyms

Rbxo - rubidium-crystal oscillator
XO - Clock Oscillator
CXO - Crystal Oscillator
DTCXO - Digitally Temperature Controlled Crystal Oscillator
EMXO - Evacuated Miniature Crystal Oscillator
FCXO - Frequency Controlled Crystal Oscillator
HPXO - High Precision Crystal Oscillator
MCXO - Microprocessor Compensated Crystal Oscillator
OCXO - Oven Controlled Oscillator
OCVCXO - Oven Controlled Voltage Controlled Oscillator
PXO - Precision Oscillator
PXO - Programmable Crystal Oscillator
SPXO - Simple Packaged Crystal Oscillators
TCXO - Temperature Compensated Oscillator
TCVCXO - Temperature Compensated Voltage Controlled Oscillator
VCXO - Voltage Controlled Oscillator
VCSO - Voltage Controlled Sinewave Oscillator