# DCF77 Time and Standard Frequency Station

located in Central Europe

This is just a local copy of a document translation carried out by Peter Lamb, July 1993.
It has been slightly modified and shorted to provide a compact document.
To view the original document, please have a look at http://www.eecis.udel.edu/~mills/ntp/dcf77.html
Another convenient source of DCF77 information is http://www.hw-server.com/docs/dcf/dcf.html

## Abstract

DCF77 is a long-wave radio transmitter at 77.5 kHz for precise time information. It is located near Frankfurt, Germany in Central Europe and available across most of Europe. It serves as exact time source for a lot of applications, starting from simple radio clocks and numerous public clocks (including churchs) up to demanding scientific experiments.

## Translation

Original in German available from the address below.
Translation errors courtesy of Peter Lamb,
Swiss Federal Institute of Technology, Zurich, Switzerland

July 1993

Physikalisch-Technische Bundesanstalt (PTB) Braunschweig
Lab 1.21
Bundesallee 100
D-3300 Braunschweig
Germany
February 1984

Legal basis and responsibility for the transmissions of DCF77:
--------------------------------------------------------------

The 1978 law on time standards defines legal time in Germany on the
basis of Coordinated World Time (UTC) and gives the PTB responsibility
for the keeping and broadcasting of legal time. As well as this, the
time standards law empowers the Federal government to issue regulations
for the introduction of Summer Time.

Legal time in the FRG is either Middle European Time (MEZ - German
abbreviation) or, in case of its introduction Middle European Summer
Time (MESZ). The following relationships hold between UTC and MEZ and
MESZ.

MEZ(D)  = UTC(PTB) + 1h
MESZ(D) = UTC(PTB) + 2h

Legal time is generated in the PTB Atomic Clock Building in Braunschweig
and it is broadcast mainly through the LF transmitter DCF77 which the
PTB rents from the German Post Office (DBP). The PTB has sole
responsibility for the control of DCF77, while the DBP has
responsibility for the transmitter and antennas. Queries should be
directed to the above address or by telephone to 0531/592 1212 or
0531/592 1210, or by telex to 952822 ptb d.

DCF77 Specifications:
---------------------

Location:      Mainflingen transmitter complex, (50:01 N, 09:00 E),
about 25km south-east of Frankfurt a. Main, Germany.

Carrier Frequency: Standard frequency 77.5kHZ, derived from the PTB
atomic clocks. Relative deviation of the carrier from
specifications:

averaged over 1d:   <1e-12
averaged over 100d: <2e-13

The carrier phase is controlled so that deviations
relative to UTC(PTB) are never greater than +-0.3us.
Larger phase and frequency variation observed at the
receiver are due to summation of ground and space waves.

Power output:  Transmitter power 50kw, estimated emitted power approx.
25kW.

Antenna:       150m high (backup antenna 200m high) vertical
omnidirectional antenna with top capacitance.

Transmission times: 24-hour continuous service. Short interruptions (of
a few minutes) are possible if, because of technical
problems or servicing, the service must be switched to a
backup transmitter or antenna. Thunderstorms can cause
longer interruptions to the service.

Time signal:   The carrier is amplitude-modulated with second marks. At
the beginning of each second (with the exception of the
59th second of each minute), the carrier amplitude is
reduced to 25% for the duration of either 0.1 or 0.2
seconds. The start of the carrier reduction marks the
precise beginning of the second. The minute is marked by
the absence of the previous second mark.

The second marks are phase-synchronous with the carrier.
There is a relatively large uncertainty possible in the
time of the second mark which depends on the receiver
position. The causes are the relatively low bandwidth of
the antenna, space wave and other interference sources.
Despite this, it is possible to achieve accuracy better
than 1ms at distances of several hundred kilometers.

Time code:     The transmission of the numerical values for minute,
hour, day, weekday, month and year are BCD-encoded
through the pulse duration modulation of the second
marks. A second mark with duration 0.1s encodes a binary
0 and a duration of 0.2s encodes 1. The order of encoding
is shown in the following diagram [replaced by a table in
this translation]. The three test bits P1, P2 and P3
extend the 3 major sections of the time code (7 bits for
minutes, 6 bits for the hour and 22 bits for the date,
including the week day number) to maintain an even count
of 1's.
The second marks No. 17 and 18 indicate the time system
for the transmitted time codes. In the case of
transmission of MEZ, mark 18 has a duration of 0.2s and
mark 17 a duration of 0.1s. If MESZ is being transmitted,
this is reversed. Furthermore, an approaching transition
from MEZ to MESZ or back is announced by extending mark
16 from 0.1s to 0.2s for one hour prior to the changeover.

Encoding Scheme:
----------------

Encoding as follows: Pulse of 0.1s = 0, 0.2s = 1
Numbers are encoded in BCD (binary coded decimal)

Mark number(s)

0              Minute, always 0 (0.1s)

1-14           Reserved, always 0

15             Antenna (0b normal antenna, 1b backup antenna)

16             Time zone change announcement, 1 hour ahead
(0b nothing, 1b change)

17,18          Time zone, difference in hours against UTC
(00b = +0h, 01b = +1h = CET, 10b = +2h = CEST, 11b = +3h)

19             Leap second announcement. The leap second is encoded in
this bit one hour prior to occurrence. (0b nothing, 1b change)

20             Start bit for encoded time, always 1

21-27          1, 2, 4, 8, 10, 20, 40 minutes (mark 21 = 1 minute)

28             P1 maintains even parity for marks 21-28

29-34          1, 2, 4, 8, 10, 20 hours (mark 29 = 1 hour)

35             P2 maintains even parity for marks 29-35

36-41          Day in month (1, 2, 4, 8, 10, 20)

42-44          Day in week (1, 2, 4)

45-49          Month number (1, 2, 4, 8, 10)

50-57          Year (1, 2, 4, 8, 10, 20, 40, 80)

58             P3 maintains even parity for marks 36-58. There is no mark
transmitted for the 59th second.

59             Normally, this bit is not transmitted (space until bit 0,
minute mark). It is transmitted only when there is a leap
second (inserted twice per 3 years, last minute of June
or December).

Announcement bit for a leap second:
-----------------------------------

The DCF77 control unit is currently being modified so that in future an
announcement bit for a leap second can be sent. It is expected that for
the first time on 1st July 1985 the second mark No. 19 will be extended
to a length of 0.2s for one hour prior to the introduction of a leap
second. Intelligent receivers will then be able to recognize the
discontinuity and maintain correct indicated time in spite of a 61s
minute.

-----------------------

Since July 1983, the DCF77 carrier has been phase modulated in a test
configuration. The phase modulation is a pseudorandom binary sequence
sent twice each second. The clock frequency of the binary sequence is
645.833...Hz and the phase shift \Delta\tau about 3% of the period
(\^{=} 10\deg). Equal numbers of shifts of +\Delta\tau and -\Delta\tau
are always sent, so that the mean frequency remains unchanged, and the
use of DCF77 as a frequency standard is unaffected. The timecode is
encoded in the sequence by inverting the sequence or not. Not inverted
sequence corresponds to a 0 bit. The sequence is alleged to be generated
by a 9 bit shift register which is coupled back on positions 5 and 9.
The polynomial might be:  x^9 + x^4 + 1.

Because the pseudo-random bitstring has a strictly deterministic nature,
function with triangular form, and thereby to timing information. Early
test results show that the time information received with the help of
pseudo-random phase modulation is more resistant to interference and
more accurate (standard deviation approx. 10 mu s during the day and
approx. 25 mu s at night) than the conventional method using amplitude
modulated second marks. Since this new modulation method is compatible
with previous usage of DCF77, and that the users have made no
difficulties known to us, the tests have been extended. The transmission
of the pseudo-random phase distortion still has experimental status, and
should not be seen as a permanent commitment. Further information will
be made available in the future.

Literature:
-----------

P. Hetzel, "Die Zeitsignal- und Normalfrequenzaussendungen der PTB ueber
den Sender DCF77: Stand 1982" [The PTB time signal and standard
frequency transmissions from DCF77: Status 1982] in "Funkuhren" [Radio
clocks], W. Hilberg, Oldenburg Publishers, Munich & Vienna 1983, pp 42-
57.

G Becker & P. Hetzel, "Vortraege ueber DCF77" [Lectures: DCF77], PTB
Reports, PTB-Me-23 (1979), pp 185-253. Braunschweig 1984


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