The functional principle of radio controlled clocks
The most accurate clock of the world is the Cäsium-timebasic from the Physikalisch-Technische Bundesanstalt (Federal Standards Institute) in Braunschweig, Germany. It is working so precise, that the deviation to be expected is only one second within one million years. Radio controlled clocks regulated by this time base are working absolutely exact.
The radio controlled clock receives time telegrams on longwave (77,5 kHz) from the official German standard frequency and time signal transmitter DCF77, located in Mainflingen (24 km south-east of Frankfurt/Main). This transmitter has an average range of 1500 km. Until now, this transmitter has been used predominantly for scientific purposes navigation and space travel. Now its precise time indication may be also received privately and free of charge with radio controlled clocks.
The radio controlled clock receives the transmitter DCF77 signal via its built antenna system anywhere where longwave reception is also possible with a good radio broadcast receiver. In many cases, the radio controlled clock operates at even considerably greater distances from the transmitter, nevertheless local conditions play an important role in this respect.
The chip integrated in the radio controlled clock processes the received time telegrams fully automatically and controls the hands accordingly. Once it has set itself correctly by the radio signals, the radio controlled clock continues running on this own, internal, highly accurate 32kHz time base, comparing the indicated time with the time information of the transmitter DCF77 once every hour.
Due to the very high accuracy of the built-in time base, even one time comparision a day would already by sufficient. Thanks to 24 time comparisions a day, however, the precession of the radio controlled clock is no way affected if not all time comparision are successful due to various possible sources of interference (thunderstorms, temporary transmitter shutdowns, household appliances which are not radiosuppressed, television sets, etc.)
Another major advantage of radio controlled clocks is the immediate, fully automatic changeover from normal to summertime and vice versa.