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March 10th 04, 06:02 AM
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WWVB decoder circuit
In article , tommyknocker
writes: Dennis wrote: In article , says... Dennis wrote: HI, Does anybody have a circuit that will decode the WWVB signal,and output to a clock led circuit? TIA, Dennis Hi, Unless you want to have fun building a clock, there are many such clocks or watches out there. I wear a watch. Very accurate in time keeping. Tony, VE6CGX Yes, I have one of these clocks also. The thing is, these clocks are set 6 times in a 24hr perod by the WWVB signal. The rest of the time the time base is an ordinary crystal. I want my clock to be set on a more continous basis. This could only happen with the continous decoding of the signal. This is how the "professional" clocks, such as those made by Spectracom work. -Dennis You'll find that most of the schematics on the net for atomic clocks are of the crystal variety-they set once or twice a day with WWVB and the rest of the time use a crystal. You'll have to do some digging to come up with something that does continuous decoding, but I'm sure it's out there. The equipment is "out there" but the closest you will come to any sort of detailed description/circuitry is the NIST site itself. An example is the IRIG range timing, usually derived from a rubidium or cesium standard, they may do an update via the NIST time code but more likely it will phase-lock to the WWVB carrier. "Ordinary" quartz crystals in the radio clocks can have all sorts of tolerances on their frequency. There are 86,400 seconds in one day. A common 50 ppm (parts per million) tolerance (5 x 10^-5) would show a 4.32 second per day maximum error. By stabilizing, or perhaps trimming, the oscillator circuit in a radio clock, one can get it down to 5 ppm or 0.432 seconds per day error. However, an update circuit might exhibit a quantization error of 1 second which would be additive to the radio clock's crystal oscillator tolerance and possibly defeat the "accurate setting." As a practical matter, I used to spot-check the Oregon Scientific radio clock against the WWV ticks and found it to be better than 6 ppm, the worst error being about a half-second. I just checked again and found it to be bang-on the WWV ticks. :-) The big radio clock is in the same sync on seconds with the little one. To avoid quantization errors of setting the internal calendar clock, the "update" could advance or retard the crystal through a VCXO circuit driven by a D-to-A whose digital correction is derived from a comparison of the NIST time code to the internal calendar clock. Kind of a very slow phase-lock done the hard way. Might take days to get in phase. :-) I'm very satisfied with better than +/- 1 second error from these two inexpensive things and won't change them. Ain't in the TV BC biz like CBS affiliate stations back in '56 that needed +/- 1 second maximum errors to "take network" when scheduled or lose money. Len Anderson retired (from regular hours) electronic engineer person |
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