Techno Line WT 768 Instruction Manual page 14

Signal reception mode
The time base for the radio controlled time is a Cesium Atomic Clock
operated by the Physikalisch Technische Bundesanstalt Braunschweig
which has a time deviation of less than one second in one million years.
The time is coded and transmitted from Mainflingen near Frankfurt via
frequency signal DCF-77 (77.5 kHz) and has a transmitting range of
approximately 1,500 km. The radio controlled alarm clock receives this
signal and converts it to show the precise time in summer or wintertime.
EN2