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DCF77 - Accurate time

DCF77 is a radio station that continuously broadcasts accurate time information, which you can receive and decode in your application.

The DCF77 broadcaster is located in Mainflingen, Germany (about 24 km southeast from Frankfurt a.M., coordinates 50°01' north, 09°00' east. The time information is transmitted in the LW band, at 77.5 kHz. The transmitter's power is 50 kW, estimated radiated output power is about 25 kW. The antenna is vertical, omnidirectional, 150m high (and a 200m backup one), with capacitive extension pole. The DCF77 range is about 1500-2000km (see a picture).

The average value of carrier frequency is 77.5 kHz, and does not deviate from the nominal value by more than 10-12 a week. Relative error for over 100 days is just 2*10-13. It is meaningless to measure errors for a shorter time since the transmitter broadcasts only 77500*3600*24*7 = 4,6872*1010 sine oscillations per week. In order for the measurement error to stay under 10-12, it has to recognize 1/20 of a period per week. Long-time accuracy of 2*10-13 at 77.5 kHz corresponds to one period per two years.

Time information is encoded using pulse-width modulation, by dropping the carrier amplitude to 25% at the beginning of each second. Keying is phase synchronized with the carrier, and differs no more than 10 us from the official time scale of the "Physikalisch-Technischen Bundesanstalt" (PTB - Physics & Technics Institute) in Braunschweig. This drop, however, does not occur at the 59th second of each minute - minute mark. Amplitude drop lasts 100 ms - logical 0 (low), or 200ms - logical 1 (high). However, due to the antenna oscillations, the drop at the beginning of the modulation pulse is a little flattened. 150 us after the keying begins, the amplitude is still at 80% of the nominal value. The reference point that marks the exact beginning of a second is the start of the amplitude drop (it used to be 70% of full amplitude). The time it takes the signal to get from the transmitter to the receiver varies, too. When measured at a 300km distance, maximum fluctuations measured were 37 us during the day and 50 us at night. Another delay is caused by time constants in the receiver itself. High selectivity has a drawback of a strong and unprecisely defined delays. The answer-back code of the broadcaster is transmitted three times per hour in 19th, 39th and 59th minute of each hour via 250 Hz Morse modulation of the carrier, without interrupting time signals.

Encoding of the full time information

Date and time valid for the next minute is transmitted, so that the information is complete at the beginning of the minute. All digits are transmitted in BCD code. This is what the DCFMON program has received:

0        10        20        30        40        50      58

|    |    |    |    |    |    |    |    |    |    |    |  |



 |  Resserved ||1  2||   000||   00||   00|D||   0|   0000|

 |----bits----|Cntrol|Minute||Hour ||Day-||a|Month|-Year--|

                bits                      |y of week

Meanings of individual bits:

Bit No Name Meaning
0 M Minute mark (always 0b)
1-14 Reserved bits
15 R Antenna (0b normal antenna, 1b backup antenna)
16 A1 Time zone change announcement, 1 hour ahead (0b nothing, 1b change)
17-18 Z1,Z2 Time zone (difference in hours against UTC, 00b = +0h, 01b = +1h = CET, 10b = +2h=CEST, 11b = +3h)
19 A2 Leap second announcement, one hour ahead (0b nothing, 1b change)
20 S START, beginning of the time information transmission (always 1b)
21-27 Minute
28 P1 Even parity of the minute (bits 21-27)
29-34 Hour
35 P2 Even parity of the hour (bits 29-34)
36-41 Day
42-44 Day of week (100b Monday, 010b Tuesday, 110b Wednesday, 001b Thursday, 101b Friday, 011b Saturday, 111b Sunday).
45-49 Month
50-57 Year (ones and tens only)
58 P3 Even parity: date + day of week + year (bits 36-57)
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). 

Summer/Winter Time (Daylight Savings Time)

For a better understanding of the "summer" time, standard Central European Time, and their transitions, there is an example of a transmission as it was received by DCFMON.

Before the change, the time zone was 10b = UTC + 2h = CEST and bit A1 (no. 16) has indicated 1 hour ahead, that a time zone change will occur.


 |   bits     ||1  2||   000||   00||   00| ||   0|   0000| * on hook


00000000000000001100111101011010000111100111100001011010010 * 02:57 27.10.96

00000000000000001100100011011010000111100111100001011010010 * 02:58 27.10.96

00000000000000001100110011010010000111100111100001011010010 * 02:59 27.10.96

00000000000000001010100000000010000111100111100001011010010   02:00 27.10.96

00000000000000000010110000001010000111100111100001011010010 * 02:01 27.10.96

00000000000000000010101000001010000111100111100001011010010 * 02:02 27.10.96

00000000000000000010111000000010000111100111100001011010010 * 02:03 27.10.96
In the minute right before 2:00, the time zone info 01b (UTC+1h = CET) has already been transmitted, however, the A1 bit remains high. Since 2:01, everything is back to normal. The DCF77 driver did not believe the change by one hour, and so waited for another minute before synchronizing the time. (Some alarm clocks receiving DCF77 don't "believe" this change and insist on the old time for sometimes 15 minutes or even longer.)

Abbreviations and terms

  • CET - Central European Time (UTC + 1h)
  • CEST - Central European Summer Time (UTC + 2h)
  • UTC - Universal Time Coordinated (international time scale)
  • TAI - international atomic time (defined via the atomic second)
  • UT 1 - world astronomic time (non-homogenous, depends on the angle of Earth's rotation around its axis)
  • UT 2 - is actually UT 1 without yearly fluctuations (homogenous time, defined according to Earth's rotation)
  • Leap second - inserted once in two or three years as the last second in June or December. This time correction is necessary since in UTC the duration of one second is the same as in TAI; however, the average duration of one year in UT 2 does not conform with it.

Download & Links

Software to decode and monitor DCF77

DCF_612.ZIP - Resident driver that decodes DCF77 and controls RTC by Roland Elmiger HB9GAA and Eric Vandenbroucque F5HBN. Doc in German language
DCFOMON.ZIP - My program for monitoring and storage of information broadcasted by DCF77. It utilises the resident driver by HB9GAA and F5HBN. Doc in Czech language
Both programs work under both DOS and Windows 3.11 (I didn't have the courage to try Win95).


  • Schematics and connections of a DCF77 receiver by HB9GAA using UE2125.
  • Schemtics according to [2] using A244D (TCA440)

DCF77 Broadcaster Range

DCF77 range


Czech language only
[1] Ing. Jan Hájek, Vysílání normálových frekvencí a prenos casové informace, Sdelovací technika 7/1974 str.254-258
[2] Ing. Josef Pokorný, Digitální hodiny s prijímacem DCF77, AR 8/1994 str. 11 a? 15; AR 9/1994 str. 16-17

(c) 1998 Michal Poupa OK1XPM

Translation by: Joe Hlavac

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