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 ...
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.
Anthorn (United Kingdom)
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.
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.