DCF77 signal recieving - Page 1
Forum section: electronics / projects

author: Dajgoro
author_rank: Frequent Contributor
timestamp: January 13, 2014, 05:55:45 pm
content: Hi.I just got from ebay one of those cheap DCF77 radio time receiver modules and it sorta works.I am at ~750 km from the transmitter, and when I place the receiver on the balcony I do get a signal, but only in certain orientations. Indoors it is impossible to get a signal, due to emi, and the thick concrete walls of the building.So I was wondering how should I set up that ferrite loop antenna to get an optimum signal, and can I run the signal from the antenna through a coaxial cable without attenuation, so that I can have the antenna somewhere outside the clock, where the signal can be received in a decent manner.

author: deephaven
author_rank: Frequent Contributor
timestamp: January 13, 2014, 06:32:06 pm
content: You could remote the whole receiver and power it through the COAX. You would need a bit of extra circuitry to separate the power from the data driver at the remote end.

author: AG6QR
author_rank: Frequent Contributor
timestamp: January 13, 2014, 07:02:43 pm
content: Quote from: Dajgoro on January 13, 2014, 05:55:45 pmUnder ideal conditions, you should set the bar perpendicular to the path from your location to the transmitter. Local reflections, interference sources, and other things may mean you get slightly better reception with a slightly different angle, so you may need to experiment. Don't point the long axis of the ferrite bar at the transmitter -- a ferrite bar antenna has a null along the long axis, so you should get very poor reception that way.Sometimes pointing the null (long axis) directly at a source of interference can give best reception, because the reduction in noise improves the signal to noise ratio. That may work even if the signal is somewhat reduced as well -- you can sometimes get a large reduction in noise, which is more important than a small reduction in the signal.Regarding using coax to locate the antenna separate from the receiver: There will always be some attenuation in coax, but at the low DCF frequency (77.5kHz), the attenuation will be very small for a few meters, or even tens of meters, of reasonable quality coax. Go ahead and separate your antenna from the receiver if that makes thing easy. The type of coax probably isn't critical.If you place any antenna outdoors, even a small ferrite bar, you should consider lightning protection issues. The antenna itself isn't such a big lightning attractor, but the cable may be enough to provide a dangerous path for lightning to enter a building. Check local electrical codes.

author: Zbig
author_rank: Frequent Contributor
timestamp: January 13, 2014, 11:13:35 pm
content: IMO there's no point in remote-connecting the antenna only and deal with the RF over coax. If your receiver module is anything like mine, it's a tiny, next to no power-consuming PCB. You should leave it near the antenna, connected with a short wire to it and just connect the power, GND and already decoded and conditioned digital signal remotely with thin, 3-wire cable.Also, keep in mind you have to be extra careful with your firmware decoding and validating the DCF-77, especially in poor reception conditions. I personally had two DCF-77 alarm clocks which actually failed to wake me up for some important things on more than one occasion because they went out of whack overnight and set some random time because of some lazy firmware programmers. DCF-77 frame has a single parity bit so in case of two data bit flips you're SOL. If I were to code the DCF-77 clock, I would follow these general rules:Always verify parity, of course, but:If time is currently undefined (just powered up, after reset, etc.), keep a running window of few last DCF77 frames received. Only trust the time if you have at least two frames n-apart differing by n minutes. Having a circular buffer of few frames (instead of just two subsequent ones) will increase your odds of setting the clock in reasonable time as the chances of getting every other DCF frame invalid during considerable timeframe are pretty high.If "locked" (previous condition is met), while during regular scheduled correction, receiving a frame differing from current time by more than is "reasonable" (more than the expected crystal drift), switch into extra-paranoid mode (previous point) again. Keep in mind the daylight saving changes, though.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: January 14, 2014, 05:25:21 am
content: Thanks for the tips, I'll try experimenting with the receiver.I do remember last time I visited my aunt, she had such a clock, and most of the time it was showing some random time, or it was stuck at noon...So indeed, a lot of software poking will be needed to read the correct time from such poor signal conditions.

author: Co6aka
author_rank: Unknown
timestamp: January 14, 2014, 06:49:20 pm
content: Quote from: Dajgoro on January 13, 2014, 05:55:45 pmThe antenna is likely a loop or a tuned circuit (typically a "ferrite rod" and cap in parallel-resonance) which are typically bi-directional. Being anywhere near ferrous objects detunes a "ferrite rod" antenna, especially at VLF, and the Q-factor of that type of antenna typically sucks-in-general, SO indoor use of these is typically disappointing. Loops can be relocated away from conductors and noise sources, so that's what you want. Also, VLF propagation makes for some fascinating physics lessons -- do some 'net searching and reading. And while you're at it, search for and read about loops and active loops, and specifically search for articles on VLF loops.Many moons ago I built an untuned VLF-HF active loop with a matched pair of (2N3866?) RF power transistors in common-base configuration. Worked quite the charm. (Untuned loops exhibit a very low impedance, so the amp is basically a transimpedance amp; under 1 Ohm to 50 Ohms.) Tuned loops at VLF, at least high-Q loops, are touchy to tune and tuning will be temperature-unstable -- you'll need to include remote-fine-tuning in your design.You can also use an "E-field probe" type of antenna ("active whip") but my experience with those is that they fantastically pick up noise in all directions, in addition to signals. And, you've gotta be very concerned with grounding and static voltage accumulation (unless you like changing/fixing your amp regularly) which is headscratcher because you want high capacitance and low inductance in your antenna (the whip) and very high impedance and very low capacitance (input) in your amp.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: January 15, 2014, 10:10:17 am
content: I was thinking about placing the receiver circuit and antenna as a separate device. It should consist of a small solar panel, rechargeable battery, DCF77 receiver module with the antenna, and a 433MHz transmitter. So I could just leave that device somewhere outdoors, and then have a 433MHz receiver in the clock. I'll try experimenting with that once I get the stuff from ebay, and I hope that transmitter won't affect the DCF77...

author: Zbig
author_rank: Frequent Contributor
timestamp: January 15, 2014, 10:26:12 am
content: I was going to suggest the very similar thing initially, only I would go as far as implementing the whole "master clock" logic in the outdoors receiver part as well. This way you could make as many "thin" receiver clock clients as you like, using simplified logic, relying on the single outdoors master. This way you could implement the DCF logic I described in my previous post and then protect the packet being sent over 433 MHz with some proper strong checksum protection on the sender (master) side and just check it on the clients, not bothering with implementing the verification logic on each of the "clients". Also, I'd design the master as a modular device, with possibility of switching it from DCF-77 to GPS whenever you feel like it.

author: kripton2035
author_rank: Super Contributor
timestamp: January 15, 2014, 11:42:19 am
content: you can also put a longer ferrite antenna it works.I had a dcf77 watch that refused more and more often to set the time during the nightI also have a dcf77 clock that has a signal level rssi in it and I found that at the place where I left the watch at night the signal was very very poor. and that at 50cm after it was nice - problem solved.dcf77 has very different strongness around the house, specially if you have an old house with big rock walls.

author: kripton2035
author_rank: Super Contributor
timestamp: January 15, 2014, 11:45:44 am
content: some active antenna I did not test but may be usefull to any ?

author: Dajgoro
author_rank: Frequent Contributor
timestamp: January 15, 2014, 12:26:40 pm
content: Quote from: Zbig on January 15, 2014, 10:26:12 amWith nowadays moder mcu-s it should be doable to implement such a system in a low power mcu, with a tiny solar cell...About the antenna, I did some poking, and the antenna seems to be on a sorta differential line, since I can't find any direct connection to ground or vcc from any of the antenna poles, I just get a constant resistance to both of them.The only thing that worries me is the debugging phase, I'll have to find some usb extension cables to run the entire board to the balcony, so I can get some sort of reception to work with...Or I could build a emulator of some sort, but then I am doing double work, and buying a existing one doesn't sound cheap.

author: sigxcpu
author_rank: Regular Contributor
timestamp: January 16, 2014, 05:52:43 pm
content: For some time I'm trying to set-up a low power transmitter. I have some DCF-77 clocks brought from Germany, but the signal is very very low here, in Romania and you are very lucky if you sync.I dream about a house where all the clocks (including the stupid ones from ovens) are synchronized.The module above seems to do the thing. Sadly, I still need to do the RF part myself