Hi Sean, I bought this from ebay... Lucent Rubidium Frequency Standard 10MHz RFG-RB 160284303432 about 2 years ago and have done nothing with it, a brief search didn't=20 yield much about the 25? pin d-sub connector on the back. Does this=20 happen to be the same or similar to what you bought? If so I'd love to=20 get any tips from you on firing it up. Cheers, J Sean Breheny wrote: > Is this a one-off or something you need to produce many of? > > As others have said, I don't see why GPS could not be used > continuously rather than just for calibration. > > If not GPS, then an off-the-shelf TCXO would work very well. > > I doubt that a normal clock would be accurate enough because they are > intended for a temperature controlled environment (e.g., a house or a > person's wrist). I'm assuming that this device you are making will be > outdoors. > > For the crystal and caps - the capacitors DO alter the frequency of > the crystal but only over a few tenths of a percent for the full range > of workable capacitance values (say a 1000% variation). So, using > standard, good quality NP0 (also called C0G) ceramic capacitors should > make their temperature variation a negligible factor. However, you > would want to do an initial calibration. > > When doing your accuracy calcs, be sure to include crystal aging. I > believe that crystals age with the square root of time, so an initial > burn-in time of even a week or so may help with this. > > I agree with others that this kind of accuracy probably cannot be > achieved using the internal osc, even with temperature compensation. > The most challenging thing about temperature compensation is that many > types of oscillators exhibit temperature-driven hysteresis. In other > words, the frequency variation is not a simple function of the present > temperature, but depends on the time history of temperature. Aging > would also probably be significant with the internal osc. > > Finally, as an "out-there" option if this is a one-off, good used > Rubidium frequency standards can be had on eBay for about US $100. I > bought two of them and they are amazing. Biggest down side is that > they have a limited life (about 10 years of continuous operation). > However, they achieve better than part per billion accuracy. I took my > two models, purchased from different eBay vendors, and when I power > both on and connect each to a different channel of my scope, once they > achieve their own internal "lock", they have no visible drift between > each other as observed over minutes (in other words, less than about > 10 deg phase change at 10MHz over 5 minutes, which works out to 10 > parts per trillion difference) > > Sean > > > On Wed, Jan 12, 2011 at 2:56 PM, Gordon Williams wrote= : >> Hi, >> >> I'm designing a count down timer to start nordic ski racers at set inter= vals >> over the period of an 8 hour day. I wondering what is the best way to g= et >> an accurate time over this period with little drift. What I want to do = is >> synchronize the device to the rest of the timing system at the beginning= of >> the day and let it run without having to resync it later. The accuracy = that >> I would like to achieve is 0.5 sec over 8 hours or roughly 17 ppm over a >> temperature range of -20 C to +40 C. >> >> The PIC device will be battery powered, have a 4 by 7 segment multiplexe= d >> LED, voltage regulator, buzzer, a button or two and a few leds. Not tha= t >> complicated. It is a one off device for my own use. I'm going to use a= 1 >> sec pulse from a GPS to check calibration. >> >> The question is the best way to get an accurate clock over this temperat= ure >> range. I'm looking for something in the 4 to 10 Mhz range. In past PIC >> projects I've used in internal oscillator and don't have any experience = with >> crystals. >> >> This is what I've considered so far. >> >> My initial though was that I would have to use a temperature compensated >> canned oscillator to get the accuracy over the range because a standard >> crystal has a tolerance of +- 30 ppm and a stability of +- 50 ppm. >> >> Then I thought that I could use a temperature sensor costing $0.30 or so >> http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=3DMCP97= 01T-E/TTCT-ND >> and then use a standard crystal and do the temperature correction. >> >> Next I found some better crystals with +-10 ppm tolerance and +-10 ppm >> stability for about 80 cents >> http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=3D887-1= 265-1-ND >> The problem I see here is what caps should I use? My understanding is t= hat >> if I'm off on the cap value, I will also be off on the frequency. As we= ll, >> the caps will have a temperature coefficient so the load capacitance wit= h >> change with temperature. Any thoughts about the dos and don'ts about th= is. >> >> Lastly, if I have a temperature sensor, could I just use the internal >> oscillator to correct for temperature changes? I would like this as it = is >> easy. What I'm wondering is how repeatable it would be with temperature= .. >> The internal osc will have a range of about +-5% over this temperature. = It >> seems like a bit of a stretch to bring the accuracy from 50,000 ppm down= to >> 17 ppm even with knowing the temperature. Any thoughts? >> >> Thanks, >> >> Gordon Williams >> >> >> -- >> http://www.piclist.com PIC/SX FAQ& list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist >> > --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .