[ntp:questions] Building/cannabalizing a WWVB radioclock...

Pete Stephenson pete+usenet at heypete.com
Fri Aug 3 08:25:37 UTC 2007


First off, I apologize for the length of this post.

This evening, I was perusing 
http://www.buzzard.me.uk/jonathan/radioclock.html as I'm considering 
building a WWVB reference clock as GPS isn't viable here to due 
environmental conditions (buildings) and CDMA receivers being too 
expensive.

I considered the cost of purchasing the various items needed on that 
list and the various electronic work needed. Me, being lazy, thought 
that there must be something cheaper, easier, and more practical.

Thus, I present my RadioShack Radio Controlled Clock With Thermometer 
(Cat. No. 63-973). I partially disassembled it, and the interior can be 
seen in picture [1]. The ferrite core antenna is in the lower right, and 
is connected to the small green circuit board[2] in the top right. Four 
wires then run to the main circuit board[3] and then connect to the 
lower left.

Fortunately, the nice folks at the clock factory in China have labeled 
all their connection points. There are four solder spots: VCC, PON, 
WWVB, and GND.

GND is obviously ground, and WWVB is (I presume) the modulated clock 
signal. Does anyone know what VCC or PON are? I'm not familiar with the 
terms, but I would imagine they had something to do with power (the 
whole clock runs on 3V DC provided by two AA batteries in series).

My intuition tells me that the VCC wire in my clock corresponds to the 
VSS wire in the website's schematic, and that PON is also ground. It 
also tells me that my WWVB wire corresponds to the MSF wire in the 
schematic. Does this sound reasonable? 

My intentions are as follows:
- Remove the antenna, small board, and wires relating to the radio 
receiver from the clock housing.
- Connect the wiring to a serial connector.
- Connect the serial connector to the computer.
- Use the software at the above site to allow the clock to communicate 
with NTP via the shared memory reference clock driver.

As always, destruction is easier than construction. Removing the stuff 
from the clock housing is the work of a few minutes and a pair of 
scissors. The only thing I'm worried about is remembering which wire is 
which (another reason why I took the pictures).

I have a few questions (and no doubt will have more), so please bear 
with me.

1. Does the radio receiver draw power from the batteries, or is there 
any current provided by the antenna? There doesn't seem to be any power 
converting apparatus on the circuit board, so if power is provided by 
the batteries, it would seem to be 3V. Is this common for WWVB radio 
circuits?

2. The schematics say that it is necessary to connect the VSS wire to 
the DTR (pin 4) of the serial cable for power. What voltage does the 
port supply? Would it cause any problems to use this voltage with the 
radio circuitry?

3. If the serial line voltage would be excessive, I could easily supply 
power from a set of batteries (probably a pair of D cells for long life) 
or a 3V AC-to-DC adaptor. If this is necessary, how would go about 
modifying the schematics for this? 

4. The website and schematics show various capacitors, resistors, and 
other such components wired in with the circuits. I presume that the 
necessary components are already wired into my circuits by RadioShack 
and that the addition of these components would not be necessary. Can 
anyone confirm (or speculate, if you have anything more than a wild-ass 
guess) if this is the case? The green box on my circuit says the 
following:
WiMA
0.01
63-
LO

I'm not sure of that's LO (ell-oh) or L0 (ell-zero).

The two black cylinders (capacitors?) both say:
LRM
105°C(M)
LRM

Do these markings mean anything to anyone?

5. When working as a wall clock, the clock periodically syncs the time 
with the WWVB signal. I would imagine that the clock circuitry is what 
initiates the sync -- there doesn't appear to be any switching mechanism 
to turn on and off the radio receiver, so I presume the radio is 
receiving continuously. Obviously, for use as a reference clock, the 
radio would need to be receiving continuously and I presume the radio 
does run receive on a continuous basis and the clock only listens to the 
clock at the periodic times. Does this sound reasonable?

6. Assuming I can successfully make a working reference clock in this 
manner and fudge it to take into account transmission distances, 
processing by the receiver circuitry, and OS handling of interrupts, how 
accurate a time signal could one reasonably expect to get? I can get +/- 
10ms accuracy from internet time servers, and would hope to get ~1ms 
accuracy from a radio clock. This would be suitably accurate for my 
purposes, and hopefully accurate enough for internet users of my time 
server.

I realize that WWVB is not quite as precise as GPS, but according to 
NIST WWVB can provide an uncertainty of less than 100 microseconds. I 
would be satisfied to get within 1ms, as it'd be a 10x improvement over 
my current setup, and I wouldn't have to pester internet time servers 
anymore. Tucson is 1006km from Boulder, so according to my back of the 
envelope calculations, there's about a 3.353ms delay for the radio 
signals to travel this distance. Does this math seem right? Assuming I 
can get consistent results from the radio, I can fudge it relative to 
better time standards (the local university has an NTP server with PPS 
input from a Trimble Palisade GPS receiver, so if I lug it over to 
campus I can sync it with almost no network delay) to get it more 
precise if necessary.

Anyway, thanks in advance for all the answers folks. These last few 
weeks have been a huge learning experience for me -- I've tinkered a bit 
with ntpd over the years, but nothing really serious. In the last 
several weeks I've managed to successfully get a Linux system up and 
running in a real (not virtual) machine, learned more than I ever 
thought possible about precision timekeeping (and there's still tons 
left to learn), and now have a disassembled radio-controlled clock on my 
desk and am hoping to turn it into a reference clock. That's a lot to do 
in a few weeks. Thanks again!

Cheers!
-Pete

[1] http://www.heypete.com/pete/time/wwvb/radioshack/IMG_2262.jpg
[2] http://www.heypete.com/pete/time/wwvb/radioshack/IMG_2266.jpg
[3] http://www.heypete.com/pete/time/wwvb/radioshack/IMG_2265.jpg

-- 
Pete Stephenson
HeyPete.com




More information about the questions mailing list