DCF77 Abstract
The DCF77 radio clocks made by Meinberg receive the signal from the
low frequency transmitter DCF77 which is located in Mainflingen near
Frankfurt/Germany. The LF transmitter
disseminates the Legal Time
of the Federal Republic of Germany which is either the Central European
Time, CET (in German: Mitteleuropäische Zeit, MEZ) or the Central
European Summer Time, CEST (in German: Mitteleuropäische Sommerzeit,
MESZ). The signal from DCF77 can be received in wide parts of
Europe.
The transmitted frequencies and signals are derived from the atomic
clocks of the
Physikalisch-Technische Bundesanstalt (PTB)
in Braunschweig/Germany, the national institute for science and
technology and the highest technical authority of the Federal Republic
of Germany for the field of metrology and physical safety engineering in
Germany. Transmission is controlled by the
PTB's Department Length and Time.
The coded information includes the current time of day, date of month
and day of week in coded second pulses. Once every minute the complete
time information is transmitted.
At the beginning of every second the amplitude of the high precision
77.5 kHz carrier frequency is reduced by 75% for a period of 0.1 or 0.2
sec. The length of these time marks represent a binary coding scheme
using the short time mark for logical zeroes and the long time mark for
logical ones. The information on the current date and time as well as
some parity and status bits can be decoded from the time marks of the
15th up to the 58th second of every minute. The absence of any time mark
at the 59th second of a minute signals that a new minute will begin with
the next time mark.
In order to increase the accuracy of the demodulated time marks, the
carrier of DCF77 is additionally modulated with a pseudo-random phase
noise. The pseudo-random sequence has a length of 512 bits which are
transmitted in the time interval between the AM marks. Due to the
pseudo-random characteristics of the sequence, the mean deviation of the
carrier phase is still zero. The phase modulated carrier can be received
with a larger bandwidth. Correlation algorithms known from satellite
techniques let PZF receivers determine the correct time with an accuracy
of some microseconds which is much superior to the accuracy achieved by
standard AM receivers.
DCF77 Coding Scheme
 |
| M |
Start of Minute (0.1sec) |
| R |
RF Transmission via secondary antenna |
| A1 |
Announcement of a change in daylight saving |
| Z1, Z2 |
Time zone identification
Z1, Z2 = 0, 1: Daylight saving disabled
Z1, Z2 = 1, 0: Daylight saving enabled
|
| A2 |
Announcement of a leap second |
| S |
Start of time code information (0.2sec) |
| P1, P2, P3 |
Even parity bits |
|
|
Difference Atomic Clock - Radio Clock
|
|
