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Homebrew DCF-77 Signal Generator

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Time machine without the (difficult to obtain) flux capacitor

  WARNING - TRANSMITTING YOUR OWN TIME-SIGNAL IS EVIL.

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 ...

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Approach

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

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.

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.

... 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.

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 :-)

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

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