[time-nuts] WWVB phase plots

Dennis Ferguson dennis.c.ferguson at gmail.com
Tue Mar 20 02:34:11 UTC 2012


DCF77's AM modulation is a much better fit for what they did, and a
much better design in general.  All the useful phase modulation needs
to be carried by the carrier at full power.  DCF77's AM modulation drops
the carrier power for only 100 ms or 200 ms at the beginning of the second,
which gives them a full 0.8 seconds in every second at full power (if I'm
remembering right the minute marker has no carrier reduction, so the very
longest carrier reduction is only 0.2 seconds).  Their chip sequence is
just under 0.8 seconds long and sits in the full power part of each second.
WWVB is not nearly so convenient.  The carrier reductions for WWVB are
deeper than DCF77, making it even more imperative that the information be
carried in the high power segments only, but WWVB's carrier drops are 0.2,
0.5 or 0.8 seconds long, so in many seconds they only have 0.5 seconds of
high power and in 7 seconds per minute there is only 0.2 high power seconds.
I think there's no good way to make DCF77's silk purse out of the WWVB
sow's ear.

It is also the case the DCF77's phase modulation probably isn't as good
as it could be if the goal is to find it in the noise since it only swings
+/- 15 degrees rather than +/- 90.  Its big advantage might be that it
is high speed, with lots of transitions, so you can probably measure
phase alignment pretty accurately with that.  As a national time service,
however, it only needs to serve a fairly compact country relative to
WWVB's intended coverage area, so that plus WWVB's crappy AM format
probably pushed them to forget about trying to match DCF77 and to
just concentration on doing the best they could to improve coverage.

That would be my guess, anyway.

Dennis Ferguson


On 19 Mar, 2012, at 19:47 , ehydra wrote:

> Hm. I had a quick look at http://en.wikipedia.org/wiki/WWVB
> I cannot see why it won't work with the DCF77 scheme. The carrier is
> always on-air. Do I miss something? To low bandwidth of the transmitting
> antenna?
> 
> Sorry, I didn't followed the thread in whole.
> 
> - Henry
> 
> 
> Brooke Clarke schrieb:
>> Hi Henry:
>> There are millions of WWVB clocks in use and the new signal must be fully compatible with them.
> 
> 
> 
> -- 
> ehydra.dyndns.info
> 
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