Category: Electronics Workbench

Electrical & Electronic gadgets

WWVB Synch Reliability

I have the WWVB clock set to synch after receiving four consecutive valid time frames, which is pretty restrictive. The question is: can it still synch every night?

Here’s six days with the antenna sitting 3 cm above the receiver board, in front of our living room window, aimed more-or-less broadside to Colorado. We’re in the Eastern Time Zone, which is currently UTC-5, so our midnight corresponds to UTC 0500.

Set: 10 015 05:14:59.9 Loc=12 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=15
Set: 10 015 05:25:59.9 Loc=12 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=15
Set: 10 015 05:28:59.9 Loc=12 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=15
Set: 10 015 06:15:59.9 Loc= 1 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=15
Set: 10 015 07:05:59.9 Loc= 2 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=15
Set: 10 015 11:30:59.9 Loc= 6 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=15
Set: 10 015 13:41:59.9 Loc= 8 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=15
Set: 10 015 13:44:59.9 Loc= 8 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=15
Set: 10 015 15:22:59.9 Loc=10 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=15
Set: 10 015 15:29:59.9 Loc=10 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=15
Set: 10 016 07:49:59.9 Loc= 2 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=16
Set: 10 016 09:46:59.9 Loc= 4 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=16
Set: 10 016 14:06:59.9 Loc= 9 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=16
Set: 10 017 04:54:59.9 Loc=11 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=17
Set: 10 017 05:15:59.9 Loc=12 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=17
Set: 10 017 05:20:59.9 Loc=12 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=17
Set: 10 017 09:42:59.9 Loc= 4 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=17
Set: 10 017 10:04:59.9 Loc= 5 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=17
Set: 10 017 10:37:59.9 Loc= 5 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=17
Set: 10 017 10:42:59.9 Loc= 5 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=17
Set: 10 017 10:45:59.9 Loc= 5 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=17
Set: 10 017 11:38:59.9 Loc= 6 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=17
Set: 10 017 11:56:59.9 Loc= 6 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=17
Set: 10 017 20:44:59.9 Loc= 3 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=17
Set: 10 018 01:26:59.9 Loc= 8 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=18
Set: 10 018 03:49:59.9 Loc=10 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=18
Set: 10 018 08:30:59.9 Loc= 3 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=18
Set: 10 018 09:47:59.9 Loc= 4 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=18
Set: 10 018 11:11:59.9 Loc= 6 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=18
Set: 10 018 11:34:59.9 Loc= 6 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=18
Set: 10 018 12:10:59.9 Loc= 7 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=18
Set: 10 018 12:13:59.9 Loc= 7 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=18
Set: 10 018 16:10:59.9 Loc=11 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=18
Set: 10 019 05:52:59.9 Loc=12 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=19
Set: 10 019 05:55:59.9 Loc=12 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=19
Set: 10 019 06:59:59.9 Loc= 1 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=19
Set: 10 019 07:48:59.9 Loc= 2 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=19
Set: 10 019 08:06:59.9 Loc= 3 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=19
Set: 10 019 08:12:59.9 Loc= 3 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=19
Set: 10 019 09:08:59.9 Loc= 4 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=19
Set: 10 019 09:33:59.9 Loc= 4 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=19
Set: 10 019 10:08:59.9 Loc= 5 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=19
Set: 10 019 10:32:59.9 Loc= 5 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=19
Set: 10 019 12:34:59.9 Loc= 7 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=19
Set: 10 019 13:49:59.9 Loc= 8 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=19
Set: 10 020 05:22:59.9 Loc=12 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=20
Set: 10 020 05:37:59.9 Loc=12 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=20
Set: 10 020 07:41:59.9 Loc= 2 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=20
Set: 10 020 09:47:59.9 Loc= 4 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=20
Set: 10 020 10:15:59.9 Loc= 5 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=20
Set: 10 020 10:26:59.9 Loc= 5 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=20
Set: 10 020 10:46:59.9 Loc= 5 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=20
Set: 10 021 07:44:59.9 Loc= 2 Age=0     LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=21

As you’d expect, WWVB synch is an overnight thing, with occasional synchs during the morning hours.

Winter has the absolute best RF propagation, so demanding four good frames probably isn’t going to work during the summer…

2010-02-06

Ceramic Resonator Frequency Compensation

Although this isn’t a real long-term experiment, here’s a week of continuous WWVB clock operation, sitting by the window in our living room, with the circuit board open to ambient conditions.

The firmware checks the local oscillator drift against the WWVB time signal if more than 3 hours (10800 seconds) has elapsed since the last synch, so you’re not seeing every WWVB synch event.

Drift: TS   5281890 UTC 10015.113059 Elapsed 15900 Offset 0 Corr +0 ICR1 39841
Drift: TS   5283109 UTC 10016.074959 Elapsed 55680 Offset 2 Corr +1 ICR1 39842
Drift: TS   5283486 UTC 10016.140659 Elapsed 15600 Offset -1 Corr -2 ICR1 39840
Drift: TS   5284662 UTC 10017.094259 Elapsed 15720 Offset 0 Corr +0 ICR1 39841
Drift: TS   5285324 UTC 10017.204459 Elapsed 31680 Offset 1 Corr +1 ICR1 39842
Drift: TS   5285606 UTC 10018.012659 Elapsed 16920 Offset 0 Corr +0 ICR1 39842
Drift: TS   5286030 UTC 10018.083059 Elapsed 16860 Offset 0 Corr +0 ICR1 39842
Drift: TS   5286490 UTC 10018.161059 Elapsed 14220 Offset 0 Corr +0 ICR1 39842
Drift: TS   5287312 UTC 10019.055259 Elapsed 49320 Offset 0 Corr +0 ICR1 39842
Drift: TS   5288722 UTC 10020.052259 Elapsed 55980 Offset 0 Corr +0 ICR1 39842
Drift: TS   5290304 UTC 10021.074459 Elapsed 75480 Offset 0 Corr +0 ICR1 39842
Drift: TS   5291588 UTC 10022.050859 Elapsed 77040 Offset 1 Corr +0 ICR1 39842

The frequency offset is on the order of 1 in 75000 seconds: 13 parts per million or about 0.0013%.

The last line shows that the clock went 21.4 hours between synchs while drifting less than two seconds. If the clock didn’t synch for an entire week, it’d be within 15 seconds of the correct time. That’s not wonderful for a clock, but it’s good enough for this application: the display shows just hours and minutes.

Not bad for a cheap ceramic resonator on an Arduino Pro…

2010-02-05

WWVB: 7 dB More Modulation!

I read a whole stack of NIST doc on the WWVB transmitter & time code format last year, figuring out how to build a WWVB simulator and then the Totally Featureless Clock. The Circuit Cellar article on the simulator just appeared in print and a reader gave me a heads-up: the transmitter power now drops 17 dB during the low-power part of the PWM pulse.

The relevant doc is there.

How could I miss it? Well, all the doc is quite old and the change happened in 2006…

Fairly obviously, the C-Max WWVB receiver I’m using doesn’t have the mojo to track the signal during the day, no matter how fancy the modulation. Those pulses, the low-power part of the signal, just aren’t present amid all the other noise!

Also of interest: the WWVB transmitter has been running at half-power during the daylight hours since September 2009 while they do antenna maintenance. That’s supposed to be finished right about now, so the signal should be 3 dB better. I’ve got a nearly continuous record of the last month or so, which means a comparison will be in order after a few weeks.

Search for WWVB to find the other posts I’ve done on this topic…

2010-02-04
Monthly Aphorism: On Buying Test Equipment
- When you’re buying test equipment, buy all the options.
Mad Phil taught me, long ago, to buy everything available in one package, rather than try to figure out what you’ll eventually need and go through the justification process for each piece you forgot.

That applied in a corporate setting, but it’s worth pondering even for your own gear: you’re likely to own it longer than the company producing it will offer parts.

Or, these days, it’s more likely you’ll outlive the company…
2010-01-31
WWVB Antenna: Oops!

Ferrite inductor cores are notoriously fragile: they do not withstand much abuse at all. Given the amount of fiddling I’ve been doing with the Totally Featureless Clock, it was inevitable that I’d manage to drop the antenna…

Broken ferrite bar antenna

Gluing it back together with cyanoacrylate demonstrated that some things just never work the same. The antenna depends on a continuous flux path through the winding and even the minute gap introduced by the adhesive is enough to ruin the antenna.

What they say about hearts and wheels is also true of ferrite bar antennas:

“Once you bend it, you can’t mend it…”

2010-01-30
More WWVB 3D Glitchiness

The next day of WWVB Glitchiness, with the “!” limit characters changed to “|” to move them above the plot where they belong… which really doesn’t make that much difference.

Gnuplot Glitchiness 2

It’s worth mentioning that the WWVB transmitter is running in degraded mode during the day, down 3 dB, while they work on the antenna system. It probably doesn’t make much difference, given the noise around here, but you can see a definite jump as the frame marker pulses pop up off the floor.

The clock synched with WWVB nine times during the Valley of the Shadow of Night. Each synch requires four consecutive glitch-free minutes, which obviously doesn’t happen during daylight hours.

That’s with the antenna perched 3 cm over the top of the clock, aligned with the circuit board: the hardware seems quiet enough.

2010-01-29
WWVB Glitchiness Histogram in 3D
The character based Glitchiness histograms described there work pretty well for short time scales, but more than a screen full is too much. It turns out that Gnuplot can chew up the histograms and spit out a perfectly serviceable 3D map plot.

The trick is to extract the histogram characters into a file, then persuade Gnuplot to regard the file as a binary array, with the ASCII character values giving the Z height of the dot for each XY cell.

Click for bigger picture:

Gnuplot Glitchiness

The axes:
- Front edge = 51 pulse durations, 0 – 1 second, 20 ms resolution
- Right edge = 1363 histograms = 22.7 hours of WWVB reception
- Z axis = histogram counts
The flat plane has the vast majority of points having zero (or just a few) counts.

The three front-to-back hillocks show the durations of the binary-zero, binary-one, and frame markers within each second; the resolution is 20 ms per sample perpendicular to those lines.

The fuzzy mountain peaks along the left edge represent intense noise; you’re looking for the very few intervals of zero noise when the WWVB signal is readable. Those would be flat lines from the left to right edges, with just three bumps at the proper durations.

The valley between the mountain peaks is the nighttime reception, when the noise drops to bearable intensity and RF propagation brings in enough WWVB signal to make a difference. The fact that you can see the proper pulse widths through much of the day suggests the signal is in there, but it’s so noisy you (well, I) can’t make make much use of it.

How to get the graph…

The clock produces three lines of output every minute that look like this:
```
UTC: 10 013 16:36:00.0 Loc=11 Age=367   LY=0 LS=0 DST=0 Chg=0 UT1=1 Mon=1 DOM=13
Glitchiness:  268 Histogram: W!ieTHG3A35412132.11...............................
Light: 02CA Min=0005 Max=038B
```
Extract just the lines with histograms:
```
grep Histo 2010-01-12\ LR\ Window\ 80\ cm\ V\ on\ shelf\ -\ shield\ box.log > 1.txt
```
Chop out the histogram data, which has a leading space:
```
cut -d ':' -f 3 1.txt > 2.txt
```
Discard the leading space and put the histogram text in the final file:
```
cut -d ' ' -f 2 2.txt > histo.txt
```
The last few lines of that file look like this:
```
Q!njLDG896D6341...1................................
BpgcSHD7B35531311.21..1..2....2....................
L!jPQECA856231.221.....1.1................1........
W!ieTHG3A35412132.11...............................
```
You could do that all in one gargantuan Bash line, piping the results from one filter to the next, but that’s hard to explain.

Now, fire up Gnuplot and have at it:
```
gnuplot
set xyplane at 0
set zrange [0:128]
splot 'histo.txt' binary format="%uint8" record=52x1363 using 1 with points lt 3 pt 0
```
The doc suggests record=52xInf should work, but that draws a useless picture. If the record value is bigger than the number of actual records (found with wc -l histo.txt, the plot ends at the end of file; if it’s smaller, then you get only that many records. I suppose you could just use 99999; it’d work well enough.

The 52 comes from the number of characters in the line: 51 histogram bytes per line, plus a newline character at the end. The newline produces the distinct line below everything else along the right edge of the plot. You could get rid of the newline characters and turn it into a binary file before plotting, but that’s sort of cheating, I think.

You’ll recall the counting sequence in each histogram character:
- “.” = 0
- 1 through 9 = obvious
- A through Z = 10 – 35
- a through z = 36 – 61
- ! = more than 61
Unfortunately, the “!” has a lower ASCII value than the other characters, so those are the dots below the plane on the left side; they should be along the top surface. I’ll change that to “|” and make the answer come out right.

From here on, it’s a matter of the usual Gnuplot futzing to get a decent-looking plot.

Rotating the view may be useful. For example, set view 60,80 produces this:

Gnuplot Glitchiness – rotated

Now you’re looking more-or-less parallel to the samples for each minute. If you twiddled with the ranges, you could probably see the few valleys where it’d be possible to extract a valid time code.

The alert reader will note that I used record=52×4344 to generate those plots. Homework: why?
2010-01-28