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

Richard B. Gilbert rgilbert88 at comcast.net
Fri Aug 3 15:49:38 UTC 2007


Pete Stephenson wrote:
> 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 

VCC is a DC Voltage, typically 5V.  PON might be Power ON or something 
else entirely.
<snip>
> 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?
WWVB radio circuits come in all sizes and shapes and may use anything 
from the technology of the 1940s and 50s to "state of the art" solid 
state devices.

> 
> 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? 

Measure it!!!!
>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? 
> 
D cells would probably die of old age before the active ingredients were 
consumed.  I've been running a WWVB wall clock on the same pair of AA 
batteries for 3-1/2 years now!

>snip>
> 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?

In large parts of the world, the WWVB signal is available only during 
the hours of darkness! In some parts of the world the signal is not 
available at all! The clock synchs up whenever it has a usable signal.

> 
> 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.

Do you have a useable signal 24x7x365?  If not, your clock is going to 
be "free running" from 8-14 hours per day.

<snip>




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