A radio clock is a clock that is synchronized by a time code bit stream transmitted by a radio transmitter connected to a time standard such as an atomic clock. The picture shows a type of radio-controlled digital clock. Radio-controlled analog clocks are also available.
Further there are radio clocks used for setting the time of a PC. Such devices can be built into the PC or an external device, for example in the form of a dongle in the cable running from the parallel port to the printer. External clocks give their synchronization signal after a command or at fixed intervals to the PC.
A radio-controlled clock consists of an antenna for receiving the RF time-code signal, a receiving circuit to convert the signal into the simple (digital) time code, and a controller circuit to decode the time-code bit stream(s) and to drive an output circuit, which could be an LCD in case of digital clocks or stepping motors in the case of analog clocks.