Comment by Johnythree | Hacker News Reader

Comment by Johnythree | Hacker News Reader

The O.P. appears to live in Demark and is concerened about interference to the German DCF77 signal on 77.5kHz from a buried-loop lawnmower control signal on approx 77.5KHz.

His Spectrum Analyser pictures are not much help. About all they prove is that his local DCF77 signal is very, very weak.

In order to investigate the problem, he needs to accurately measure the modulation type, bandwidth and the frequency of the Loop TX, and then analyse exactly how the interference is occuring.

Is the Lawnmower system genuinely overlapping in frequency with DCF77? Or does the Clock receiver have insufficient Selectivity to separate the two? Or is the Clock receiver being overloaded? Or is the local DCF77 signal just too weak?

It is critical that he use a resonant and shielded magnetic antenna to measure the DCF77 signal, so as to eliminate the electric-field rubbish his Spec-An is picking up. If he has simply used a random wire antenna, his spectrum plots will be pretty much meaningless.

Note-1. Being a buried loop, the Lawnmower antenna is designed to radiate only a Magnetic field (eg the earth shielding and the loop balance should suppress any EM waves). Either the loop is incorrectly installed, or the transmitter and feed lines are physically close to his DCF77 Clock, and/or the DCF77 signal is very weak.

Note-2. In the USA, the equivalent time signal is WWVB on 60KHz. It is possible that the Lawnmower system he has installed was intended for North America only, and is not authorised for use in Europe.

Note-3. I don't know what the Service Area of DCF77 is, but it is likely that it's signals will be very weak in Denmark. It is possible that he is simply outside the DCF77 service area.

The modulation type for the lawn-mower is digital current pulses with no bandwidth limiting, as is obvious from the scope signal I conveniently included for your and everybody elses information.

As for what exactly goes wrong with DCF77, that is pretty obvious from the two spectrum-analyser traces provided by that signal: It gets overloaded with AM noise which the simple AM-discriminator receivers used in clocks cannot deal with.

The key is that the DCF77 signal is weak, and the lawnmower loop is very close. It's a classic example of poor signal-to-noise ratio, made worse by the DCF77 receiver being within the "near-field" of the lawnmower loop.

You could improve the S/N ratio by:

- separating the wiring for the two systems as much as possible, and fitting a mains noise filter to keep the loop signal out of the mains wiring.

- putting a remote (ferrite rod) antenna on the DCF77 receiver and experimenting to find a quiet location (possibly the interference can be nulled out by rotating the antenna).

- attending to the shielding, earthing and balance of the lawnmower loop (especially shielding and twisting the feed wires).

You could document the problem and write to your local radio licensing people. Unfortunately, I expect that they would consider that neither the DCF77 clock or the Lawnmower Loop come under their jurisdiction.

You could also write to the manufacturer of the loop and ask for their assistance. They may be surprisingly helpful.

Finally (if you haven't already) you could research the methods that Radio Amateurs have developed for receiving VLF signals in noisy domestic environments.

Bottom line: In my experience, correctly earthing the loop and shielding the feed wires should do a lot to eliminate the interference.

But as before, you need to understand the modulation characteristics of the lawnmower loop in detail. What bandwidth does the system actually need to operate? Can the interference be reduced by fitting a better low-pass-filter to the loop transmitter? Can you put a notch filter at 77.5 KHz?

Don't worry about what I can do to mitigate my problem, I have the electronic and RF skills+kit to deal with this.

My point is that John F. Consumer will likely conclude that his "radio controlled clock" is dying, throw it out, waste money on another one, and maybe never make the connection to his (or his neighbors) lawnmower robot.

And more importantly, that if popular enough, such (badly designed) signals will eventually make VLF timekeeping useless for everybody.

You're right of course, but this sort of thing happens whenever there is a poor signal-to-noise ratio.

Like any Broadcast service, VLF timekeeping relies on a saturation signal so it can override domestic electrical noise.

The level of Electrical Interference is steadily increasing world wide. Already services such as AM Broadcast Radio are unusable for many people.

The problem is caused by (1) the laws of physics, and (2) by our compromised EMC regulations.

At least your European EMC laws are much improved over the watered-down American ones.

Bottom line: The issue is much bigger than just VLF Time keeping.

I used to have an alarm clock, purchased in Germany, that would reset itself to German time every day. This was undesirable, since I lived in England (1 hour behind).

The diagram from Wikipedia suggests a very long range, Denmark should be no problem: http://en.wikipedia.org/wiki/DCF77#Reception_area

You should have bought a "Rugby" clock in England. Many of these clocks come in four different versions: Japan, Europe, UK and USA, depending on which signal they use (JFY, DCF77, Rugby, WWVB)

DCF77 works OK in Denmark, provided you don't have too much local noise (switch-modes, old televisions etc. etc.)

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