On Sat, 27 Jun 2015 14:38:33 -0700 (PDT), Jeroni Paul
wrote:
I live in a fringe reception area for the german time
signal at 77.5kHz
Spain? According the coverage map at:
http://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44/ag-442/dissemination-of-legal-time/dcf77/reach-of-dcf77.html
Yes, the signal is probably not very strong and you're in the skywave
only region.
and have many clocks that use this signal but because
a marginal reception they do not always sync.
That sounds like the older amplitude modulated system. DCF77 also
transmits a phase modulated signal,
https://en.wikipedia.org/wiki/DCF77#Phase_modulation
which works better for weak signal due to better processing gain. I'm
in the USA and have not tried it with over-the-air DCF77, but have
played with simulators and with WWVB, which also has a phase modulated
BPSK signal. The problem is that the technology seems to be patented
and chips are not forthcoming. Also, the addition of BPSK modulation
caused some old WWVB receiver chips to fail. No clue what the
situation was with DCF77 but it might be worth checking.
The curious thing about this is every night I have a different set
of clocks sync, they seem to decide at random when the signal was
good enough.
Kinda sounds like you're getting some local interference. It doesn't
take much computah or switcher noise to trash the signal, especially
when it's weak in the first place. If your devices have a loopstick
for an antenna, they are directional, with the strongest signal
perpendicular to the loopstick. I was having random updates until I
moved my WWVB clock away from several switching power supplies.
The oldest one is 13 years old and is an alarm clock I check daily,
in these years I have had it three times take an incorrect time/date
probably from interference. It either takes the right time or does
not sync, three times in 13 years seems not bad for the simple parity
bit protection the protocol uses.
It's a bit more than just the parity bit. Some chips require that the
correct time be received successfully more than once before it will
sync. This is a function of the chip design.
I think, however, it is somewhat dangerous to have the clocks sync
at night *after* you have checked they are set correctly, so if
bad reception sets wrong data they will fail to wake you up at
the right time.
Sorry, but you don't have a choice as to what time to sync.
Propagation at VLF frequencies works best after midnight in the USA. I
think it's the same for a north-south path, but I'm not sure. Note
the increases in coverage area after midnight for WWVB:
http://tf.nist.gov/stations/wwvbcoverage.htm
http://www.febo.com/time-freq/wwvb/sig-strength/
I couldn't find something similar for DCF77.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558