There's some wrong information being propagated here. Somehow the
failure of frequency comparators and/or frequency standards that were
based on doing a PLL with the CARRIER of the WWVB signal is being
extrapolated to the failure of all Radio Controlled Clocks which do not
track the carrier but instead read the time code that is imposed on that
carrier. This is completely wrong.
The new Phase Modulation signal is imposed on top of the legacy PWM
signal that has been broadcast pretty much since the beginning. Radio
Controlled Clocks that just read the PWM encoding should not be affected
by the phase changes of the PM signal, and will still work as before.
The reason that some clocks may only get a lock at night is purely an
issue of propagation, and will almost certainly be true for clocks
located in the eastern part of the US. In fact, that is the primary
reason for adding the PM signal, since it can be decoded at lower signal
strengths than the PWM signal can be decoded.
However, devices like the HP-117 and Spectracom 8170 performed a PLL on
the carrier of the signal, since the carrier frequency itself is very
accurate. These are broken by the addition of the PM signal, since it is
flipping the phase of the signal by 180 degrees periodically. None of
the cheap consumer "Atomic Clocks" do this (phase lock the carrier).
People have developed external circuits that "reclock" the WWVB signal
and allow these type of devices to continue to work.
Also, WWVB did shut off the PM part of the signal for a few hours every
night during a transition period that ended in May 2013. That is no
longer happening, but again this only affects phase locking receivers.
Specifically LaCrosse states clearly on their website that their clocks
are not affected by the addition of the PM signal.
Finally, I don't think there any commercially available clocks, at least
at the consumer level, that support the new PM signal at this time. The
decoder is patented, and I believe chips that support it are either
still in development or only recently released. I'll try to do some more
research regarding this.
Regards,
John
AC0ZG
On 11/28/2014 1:18 PM, Ken G Kopp wrote:
I have a long-time close friend who's retired from
the NBS in Boulder and was the project engineer
on the NBS #7 cesium standard. After the recent
reflector postings about WWV / WWVB I thought
I would get first-hand recent information from him.
The GPS satellites all carry on-board cesium
standards that are synchronized with NTIS, and
because of changes (below) to WWVB, cell phones
remain the most accurate source of time for most of us.
All the HF transmitters at Ft. Collins are the same TMC
units that were put in service when the facility was built.
The 2.5 and 20 MHz transmitters run at lower power due
to propagation considerations.
The time and other station-related voice info is sourced
on site in Ft. Collins, and the various propagation and
weather info comes from various "dial in" land-line
sources. Hence the widely varying quality of these
announcements. I forgot to ask about the individual
who made the voice recordings ...
Here's the most important info .... as of about a year
ago the modulation scheme on WWVB (60 kHz) was
changed (phase reversal each minute) and this has
rendered most of the end-user equipment inoperative.
Most (all ?) tracking receivers like the HP-117's are
now useless without extensive modification.
Most of the "atomic" clocks now in use ->do not<- synch
to the current modulation scheme on the 60 kHz signal.
This will explain the differences in displayed time on
supposedly identical clocks and how some appear to
not be getting sufficient enough signal to synchronize.
He offered no comment on how to locate "consumer"
clocks that -do- respond to the "new" modulation scheme.
The 60 kHz transmitter is indeed an ex-LORAN C unit,
and because of the higher power of the "new" transmitter
the antenna system was rebuilt using material from the
LORAN C site.
The 20 kHz transmitter was "home made" by NBS staff
at the old Beltsville, MD facility and moved to Ft. Collins.
There is no longer an antenna for this transmitter and it
will not return to the air. Trivia: The antenna was of
such high-Q that a near-by thunderstorm system would
often detune the system and cause the overload protection
to trip the transmitter off.
He suggests that a visit to the NBS website would be
"informative".
73
Ken Kopp - K0PP
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