In Germany, the national institute for metrology (Physikalisch-Technische Bundesanstalt, PTB) operates 'Cesium clocks' as time references.

The PTB broadcasts time synchronization signals derived from the cesium clock signal via the transmitter DCF77 operated by the German Corporation (Media Broadcast GmbH). Other countries operate similar services.

The Transmitter is located in Mainflingen near Frankfurt/Main. The LF time signals are receivable in a radius of 2000 kilometers, i.e. in great parts of Europe.

By this technology it is possible to synchronize a wide variety of devices (wristwatches, measuring instruments, computers) to legal time on nearly every location and at low cost.

To synchronize a computer clock to UTC, special external devices are needed. There are expensive ones and cheap ones.

The former are (potentially) high-precision devices, deviating from normal time only in the range of microseconds. They are independent from any computer in having their own processor and firmware. The time signals are processed and formatted into ASCII records of time data. These records are sent via serial port periodically or on request. One character of the record is the synchronization character sent exactly on the second. There may be an additional 'pulse per second' (PPS) output line.

In times of poor signal reception a built-in crystal clock supplies sufficiently accurate time. Actual time and receiver status are shown in a display and by LEDs on the front panel. All components are assembled in an own housing and powered by batteries or a power supply. The antenna may have a separate, weather-proof housing mounted outdoors.

The cheap devices often are passive, lacking even a power supply. They are extremely simple, but the precision is yet in the range of milliseconds.

These devices are mere receivers with a RS232 interface, powered by positive DTR and negative RTS line voltage. The secondly time pulse is demodulated from the AM signal and fed to the RxD line. The pulses are read by a serial line driver (tty) of the operating system and parsed by software like the generic parse driver of the NTP daemon.

Each pulse is one bit of the DCF77 time code. Pulse duration is 100 ms for '0' and 200 ms for '1'. The tty driver is configured for 50 bits per second, so one bit will last 20 ms. Regarding the start bit, a short pulse should result in a byte received on the serial port with the first (low order) 4 bits 0 and the rest 1. A long pulse would generate a byte with all 8 data bits and one stop bit set to 0, but the pulses may be shortened by the receiver to avoid such problems.

The software receives one byte per second as a time pulse. The bytes are interpreted as '0' or '1' bits depending on their bit pattern. A sequence of 59 pulses / bytes / bits in every minute makes up the DCF77 time code which is parsed for date and time, daylight saving time (DST), leap seconds etc.

So the computer does the whole job and no logic hardware is needed in the DCF77 receiver. That's why they are so cheap.

Primary source of information about DCF77 is the PTB, but you will find some (though older) information on the Time Synchronization Server too. Moreover, there is a private website about DCF77 in Germany.

Software and information special to DCF77 and Linux you will find listed on a page of Harald Milz who will link you to some sites in Germany and around.