Electronics > RF, Microwave, Ham Radio
DCF77 transmitter
Miti:
Hi Engineers,
I have a DCF77 clock I brought from Europe when I moved to Canada and it drifts quite significantly. I want to make a transmitter so I can restore its glorious accuracy but I just need it to cover my basement and not going out with any significant power. I found a sketch for ESP32 and it works well for baseband, but on the RF side I'm not that versed. At this moment I've set my FY6600 to 77.5KHz, sine, 3V amplitude and I'm using a ferrite antenna, in the attachment, in a tuned series RLC and I modulate it ASK from the ESP32. It works well at about 4-5 meters or less and I'm ok with that but I want to make something more permanent, put it in a box and forget about it. I want to pick your expert brains for what would be the most efficient antenna/circuit that I can use to have the most efficient transmitter, with the least power that, preferably, stays as much as possible in my basement but is reliable enough to set my clock every time. You get the idea. I'm thinking of using one of the boards in attachment to make a sine wave and modulate it and if the antenna is ideal, the AD8051 that the board comes with should be more than enough. It also has a potentiometer so I can control the level nicely. What do you think?
Cheers,
Miti
berke:
I'd say run a loop of thick wire around your basement, say AWG14 or thicker, make it resonant and drive it with a power MOSFET. Reduce power until Industry Canada vans stop showing up. How big is your basement?
Kleinstein:
A loop around the basement would have a mainly vertical magentic field inside. This would not work well with most clocks - as they have the ferrite rod horizontal.
So it is more a relatively large loop mounted with a horizontal axis.
There is usually no need to run with much power, as the transmission is near field. It is actually a good thing as this way there is not much interference in the far field.
A relatively thick wire and resonance helps.
Ideally there is not need to run the signal all day the clocks usually only synchronize in a relatively short window at night (less interference).
berke:
--- Quote from: Kleinstein on January 04, 2023, 01:19:21 pm ---A loop around the basement would have a mainly vertical magentic field inside. This would not work well with most clocks - as they have the ferrite rod horizontal.
--- End quote ---
True but only exactly in the plane of the loop. I'd imagine the loop would be either on the floor or on the ceiling while the clock would be at an intermediate level; the field lines will circle the coil wire.
--- Quote ---So it is more a relatively large loop mounted with a horizontal axis.
--- End quote ---
But what happens if the rod is orthogonal to the coil?
In the end it all depends on the "link budget", power and distance vs. sensitivity and the fringe fields.
Maybe Miti should start with a small loop and enlarge it until it works?
--- Quote ---There is usually no need to run with much power, as the transmission is near field. It is actually a good thing as this way there is not much interference in the far field.
A relatively thick wire and resonance helps.
--- End quote ---
I find that a bit vague, do you know how many amps would be enough? I'd assume that Miti is not happy with the range he gets from driving a coil directly from the ESP32, even though that is "not much" too but is not enough.
I think the needed field strength can be estimated based on the known power of the DCF77 transmitter and its coverage. From there the required coil current can be deduced, at least as an order of magnitude.
--- Quote ---Ideally there is not need to run the signal all day the clocks usually only synchronize in a relatively short window at night (less interference).
--- End quote ---
That's a good point.
Miti:
--- Quote from: berke on January 03, 2023, 11:58:58 pm ---I'd say run a loop of thick wire around your basement, say AWG14 or thicker, make it resonant and drive it with a power MOSFET.
--- End quote ---
Sorry, that's not acceptable. It has to be as small and discrete as possible.
I ran the setup that I showed in the first post since mid December and it works well. I'm waiting for my small generator board to put everything in a small box, see how it works.
--- Quote from: Kleinstein on January 04, 2023, 01:19:21 pm ---Ideally there is not need to run the signal all day the clocks usually only synchronize in a relatively short window at night (less interference).
--- End quote ---
My clock tries to synchronize at ten to the hour, every hour, and keeps trying for about 5 minutes. The software on the ESP32 can be configured to go to sleep and wake up any time so I set it to wake up at xx:49 and go to sleep at xx:00. I could make it synchronize once a day and I think the drift in one day would be acceptable, the issue is that if it fails to find a signal to sync, the antenna sign that shows the sync status goes away, so I wouldn't know if it is accurate or not. Therefore I decided to transmit the sync signal every hour.
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