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www.changpuak.ch\ electronics\DCF77.PHP - April 01 2011 19:09:00


Homebrew DCF-77 Signal Generator


Time machine without the (difficult to obtain) flux capacitor



Warning  WARNING - TRANSMITTING YOUR OWN TIME-SIGNAL IS EVIL.



DCF-77 Signal Generator


Well, our meetings take place on wednesdays at 10:30 (sharp). A radio controlled clock is used to determine whether you are late (and must bring a cake next time) or not. Unfortunately the identical radio controlled clock in my office always shows a different time :-(

After baking a lot of cakes, I thought about synchronising these disreputable clocks ...




Approach



The straight approach would be to have a crystal oscillator, amplitude modulated by a microprocessor.

77.5 KHz crystal crystal oscillator

As we did promise (2859!) not to abuse the donated crystals for that reason, another approach is used here - which makes the system even more flexible.

To generate a stable frequency, usually a PLL is used (or DDS). In order to gather some insight in the nature of a PLL, a small PLL-Demoboard (including a small PLL-Seminar) was designed.


Callsign

Frequency

Modulation

Location

MSF

60 kHz

-

Anthorn (United Kingdom)

HBG

75 kHz

ASK

Prangins (Switzerland)

DCF77

77.5 kHz

ASK

Frankfurt (Germany)

TDF

162 kHz

-

Allouis (France)

HBCZ77

77.5 kHz

ASK

unknown :-)


Table above : some frequencies, generated easily with jumpers on the PLL-Demoboard ...


The project uses three PCB's, one for generating the carrier (PLL-Demoboard) and one for the correct (!) modulation of the carrier and one for the Power Supply. As radio controlled clocks use some kind of checking the received codeword
( 30th of February will never be accepted :-) there must be a microprocessor, assuring that the sent generated codeword is valid.


• PCB#1 may be any clock source of the carrier of your choice. We use the PLL-Demoboard.

• PCB#2 is the Frontpanel, containing the Microprocessor and ASK Modulator. (Picture below)

• PCB#3 is some kind of Power Supply, delivering +12V...+30V/200 mA and + 5V/100 mA.


DCF-77 Signal Generator Front PCB

Picture above : The Mainboard, assembled, behind the Frontpanel.


Modulation



The Amplitude Modulation is done with 4 AND-Gates and a R-2R Network. It is therefore possible to modulate the TTL Signal with 16 Steps.

This is somehow cracking a nut with a sledgehammer, but we wanted to present something nifty to comply with the 'educational aspect'.

Note: As we wanted to have a single sided PCB, we reduced this later to 3 Bit.

AM Modulator


... and what you see on a Scope



Where a '0' is represented by a 100 ms drop to 25% Amplitude and a '1' drop lasts 200 ms.


DCF-77 Signal



DCF-77 Signal

Picture above : Horizontal: 200 ms/DIV, Vertical: 1 V/DIV




DCF77 Time Code (similiar to HBCZ77 Time Code :-)



Bit/Sec.

Meaning

Remarks

0

Start of a Minute

always = 0

1-14

reserved / for future use

15

0=Normal Antenna, 1=Backup Antenna

16

Summer Time Announcement

Set 1 hour before

17

Summer Time 0=No, 1=Yes

18

Winter Time 0=No, 1=Yes

19

Leap Second announcement

Set 1 hour before

20

Startbit encoded Time

always = 1

21

Minutes, weight = 1

22

Minutes, weight = 2

23

Minutes, weight = 4

24

Minutes, weight = 8

25

Minutes, weight = 10

26

Minutes, weight = 20

27

Minutes, weight = 40

28

Parity Bit Minutes (even)

Bits 21 - 28

29

Hours, weight = 1

30

Hours, weight = 2

31

Hours, weight = 4

32

Hours, weight = 8

33

Hours, weight = 10

34

Hours, weight = 20

35

Parity Bit Hours (even)

Bits 29 - 34

36

Day of the Month, weight = 1

37

Day of the Month, weight = 2

38

Day of the Month, weight = 4

39

Day of the Month, weight = 8

40

Day of the Month, weight = 10

41

Day of the Month, weight = 20

42

Day of the Week, weight = 1

Monday = 1

43

Day of the Week, weight = 2

Tuesday = 2

44

Day of the Week, weight = 4

Wednesday = 3

45

Number of the Month, weight = 1

46

Number of the Month, weight = 2

47

Number of the Month, weight = 4

48

Number of the Month, weight = 8

49

Number of the Month, weight = 10

50

Year, weight = 1

( 00 - 99 )

51

Year, weight = 2

52

Year, weight = 4

53

Year, weight = 8

54

Year, weight = 10

55

Year, weight = 20

56

Year, weight = 40

57

Year, weight = 80

58

Parity Bit Date (even)

Bits 36 - 57

59

Minute Sync

Carrier unmodulated




Putting it all together ...


A radio controlled clock is modified therefore, that the generated Signal is fed directly into the Amplifier following the Antenna. (Decoupling with an 1 kΩ Resistor).

It is assumed, that the portion radiated through the Antenna is very small, as we use an
Attenuator of 30 dB. (and of course operate it in a fully shielded room :-)



DCF-77 Signal Generator Coupling to a Clock


Connecting the Signal Generator to the Clock - using a DC-Block and a 30 dB Attenuator from
Mini Circuits - the Situation is as follows :


DCF-77 Signal Generator Coupling to a Clock



And this is how it würgs works :


DCF77 Signal Generator on You Tube - Coming Soon !!!


niet



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