Re: [time-nuts] WWVB phase plots
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.
Could somebody please say a bit more about that area. My Shannon level
theory is weak.
Why does more transitions help anything? Or what does it help?
I can see how it might make it easier/faster to get synced up, but if the
goal is to accurately measure frequency or phase, I'd expect that you are
already locked and looking for the next layer of detail. In that case, I'd
expect better results with fewer transitions. Fewer transitions means lower
bandwidth so you can use a narrower bandwidth filter and get rid of more
noise.
--
These are my opinions, not necessarily my employer's. I hate spam.
In message 20120320031431.BF564800037@ip-64-139-1-69.sjc.megapath.net, Hal Mu
rray writes:
Could somebody please say a bit more about that area. My Shannon level
theory is weak.
Why does more transitions help anything? Or what does it help?
The transitions (where the phase change!) are what you correlate,
the more, the better S/N you get.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
The transitions (where the phase change!) are what you correlate,
the more, the better S/N you get.
Yes---it's too bad that the proposed WWVB changes don't increase the
number of transitions at all. Could they not do the
low-modulation-index DCF77-like signal on top of the BPSK? That is,
put some small, fast phase wiggles on top of the slow 180-degree
transitions (or 120-degree transitions if NIST can be convinced to
change to that)?
But maybe some Loran-like tricks could be tried with an ordinary WWVB
signal and a receiver with a few kHz bandwidth. The first part of the
exponential decay of the carrier amplitude (at the on-second marks)
might be relatively uncorrupted by sky wave, just as with Loran.
Considerable averaging would be needed I guess. If the
characteristics of the transmitting antenna are known, a model of the
pulse decay could be used to estimate the transmit time.
The phase transitions happen during the low-power intervals (-17 dB),
so they would seem to be less useful than the amplitude transitions:
an 11 dB penalty, counting the gain from the antipodal signaling.
What is the inherent bandwidth of the DCF77 system, by the way?
John, if you're reading this, would your receiver be capable of
recording with wider RF bandwidth? Your recordings made during the
test period have a bandwidth of about 30 Hz; can it go any wider? I
think your web page says you're using an active whip antenna, which is
good because the resonant loops would impose their own bandwidth
limit. If you could eliminate the narrowband receiver and record the
antenna signal directly with the 192 ksa/s ADC, that would be ideal.
(I should really cobble up a system of my own, but I'm a fair distance
from Colorado.)
Cheers,
Peter
Jeeze the answers simple DCF is metric and wwvb in english.
They never work correctly together. ;-)
Regards
Paul
WB8TSL
On Tue, Mar 20, 2012 at 6:18 AM, Peter Monta pmonta@gmail.com wrote:
The transitions (where the phase change!) are what you correlate,
the more, the better S/N you get.
Yes---it's too bad that the proposed WWVB changes don't increase the
number of transitions at all. Could they not do the
low-modulation-index DCF77-like signal on top of the BPSK? That is,
put some small, fast phase wiggles on top of the slow 180-degree
transitions (or 120-degree transitions if NIST can be convinced to
change to that)?
But maybe some Loran-like tricks could be tried with an ordinary WWVB
signal and a receiver with a few kHz bandwidth. The first part of the
exponential decay of the carrier amplitude (at the on-second marks)
might be relatively uncorrupted by sky wave, just as with Loran.
Considerable averaging would be needed I guess. If the
characteristics of the transmitting antenna are known, a model of the
pulse decay could be used to estimate the transmit time.
The phase transitions happen during the low-power intervals (-17 dB),
so they would seem to be less useful than the amplitude transitions:
an 11 dB penalty, counting the gain from the antipodal signaling.
What is the inherent bandwidth of the DCF77 system, by the way?
John, if you're reading this, would your receiver be capable of
recording with wider RF bandwidth? Your recordings made during the
test period have a bandwidth of about 30 Hz; can it go any wider? I
think your web page says you're using an active whip antenna, which is
good because the resonant loops would impose their own bandwidth
limit. If you could eliminate the narrowband receiver and record the
antenna signal directly with the 192 ksa/s ADC, that would be ideal.
(I should really cobble up a system of my own, but I'm a fair distance
from Colorado.)
Cheers,
Peter
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
On Mar 20, 2012, at 4:18 AM, Peter Monta wrote:
John, if you're reading this, would your receiver be capable of
recording with wider RF bandwidth? Your recordings made during the
test period have a bandwidth of about 30 Hz; can it go any wider? I
think your web page says you're using an active whip antenna, which is
good because the resonant loops would impose their own bandwidth
limit. If you could eliminate the narrowband receiver and record the
antenna signal directly with the 192 ksa/s ADC, that would be ideal.
So the goal here is to measure the bandwidth of their antenna system? (or what they limit the transmitted bandwidth to be to make sure no power is wasted by the bandwidth limitations of the antenna).
The first IF of the 3586B is 50 MHz and is filtered to 10 kHz BW before mixing to 15.625 kHz. So recording the second IF is one possibility. The other as you say is to connect the active antenna directly to the sound card, crank up the input gain and hope for the best. No reason I couldn't playback the recording and run it through the SA and 3586 again to see what it looks like. Let me work on that.
On Mar 20, 2012, at 4:18 AM, Peter Monta wrote:
John, if you're reading this, would your receiver be capable of
recording with wider RF bandwidth?
Okay. A little 3586B hacking was required, but here are some wide-band results: http://www.jks.com/wwvb/wwvb.html#wideband
Okay. A little 3586B hacking was required, but here are some wide-band results: http://www.jks.com/wwvb/wwvb.html#wideband
Thanks very much. This data shows the full-bandwidth WWVB signal very
well. Attached are some plots and an octave script.
The first plot shows the demodulated WWVB waveform over one second,
averaged across the full 300-second recording, so it's the sum of 300
successive one-second periods. The sharp drop in power at about 45
milliseconds is the main on-the-second marker. Also visible is the
mixture of carrier-power increases at 200 ms, 500 ms, and 800 ms after
the on-the-second marker.
The second plot is a closeup of the on-second marker. The falling
edge is quite fast, with a time constant of about 350 microseconds,
corresponding to a 3 dB one-sided bandwidth of about 450 Hz. I would
guess that this edge might be estimated to within 5% of the time
constant, or 20 microseconds (about one carrier cycle), which would be
well below other sources of systematic error from propagation.
The SNR is just huge, and this is for only five minutes of
averaging---an hour, or a day, would be even better. Granted, though,
these are good reception conditions. I should pick up one of those
wideband USB audio sticks and try it from here in California.
I wonder whether the WWVB receiver chips could save power by sampling
only near these fast edges ("narrow correlator" in GPS-speak), going
to sleep for the remaining 99% of the time. Unless the local clock is
disastrously bad, one would think the device would only need to read
the full time code once per month, say, and in between just do
occasional trims using the WWVB edges.
They seem to be having some difficulty holding the carrier power
steady during the low-power intervals. Is that 10-Hz tremolo at the
start of the second a power-supply thing? some limitation of the PA?
There's some undershoot and overshoot too.
I've found a few documents describing the WWVB antenna bandwidth:
Page 136 of NIST Special Publication 250-67, showing a scope photo of
the waveform:
http://tf.nist.gov/general/pdf/1969.pdf
Page 5 of this technical report:
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA299080
and another scope photo on page 2 of this magazine article:
http://tf.nist.gov/general/pdf/2429.pdf
Cheers,
Peter
