Block diagram of the clock. LD: laser diode; LDC: laser diode controller; BS: beam splitter; SAS: saturated absorption spectrum; OTC: oven temperature controller; CFC: C-field controller; LO: local oscillator; MFS: microwave frequency synthesizer; CVC: current-voltage converter; DSS: digital servo system. Note that C-field coils and magnetic shield are not shown explicitly in the diagram.