> 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
<<attachment: wwvb-averaged-second.png>>
<<attachment: wwvb-transient.png>>
#
# plot WWVB's transient response
#
#
# convert from .au to 16-bit .wav:
# sox ant.wide.5min.15.625.kHz.44.1k.24b.au -b 16 ant.wide.5min.15.625.kHz.44.1k.24b.wav
#
[y,fs,bps] = wavread("ant.wide.5min.15.625.kHz.44.1k.24b.wav");
n = length(y);
t = 0:(n-1);
freq0 = (15625-0.455)/fs;
c = exp(2*pi*i*t*freq0)';
d = y .* c;
d = filter([1 1 1],[1],d);
# residual carrier phase, one per second (in lieu of a proper PLL)
pp = zeros(1,300);
for r=0:299,
w = d(r*fs+1:(r+1)*fs+1);
pp(r+1) = sum(w);
endfor
ppc = unwrap(arg(pp));
# coherently sum over 300 seconds
p = 8*5512.662;
a = zeros(1,fs)';
for k=0:299,
s = round(8*5100+p*k);
a = a + real(d(s+1:s+fs)*exp(-i*ppc(k+1)));
endfor
# normalize
ampl = 15.9;
a = a / ampl;
# plot
plot((1:fs)/fs,a);
grid on;
pause;
r=1900;s=2200;plot((0:(s-r))/fs,a(r:s));
grid on;
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