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
_______________________________________________ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
