At 23:11 28/10/98 -0600, you wrote: >John Payson wrote: >> >> >Hmm... for people in North America, how about a 10.00Mhz radio >> >receiver? The WWV broadcasts are amplitude modulated on a >> >*VERY* accurate 10MHz carrier. Provided you're not too close >> >to some other source of 10MHz'ish noise, this should give you >> >a good 10MHz reference frequency (if you use an analog PLL to >> >fill any brief gaps caused by nearby random EMI, it should be >> >accurate to within a part per trillion or so. >> >> | I doubt that it would end up being that accurate. Your PLL would begin to >> | drift during EMI spikes, as you suggest. Also, this signal is primary >> | reaching most of N.A. through skip propagation, which involves reflection >> | from the ionosphere, which is a somewhat non-linear system and I would >> | think would tend to vary the frequency somewhat. Also, the signal will be >> | modulated due to propagation changes, which ALONE will cause its frequency >> | to differ slightly, by a few Hz, much more than a ppT. >> >> Hmm... actually I meant to suggest a combination analog/digital PLL (since >> both types have advantages); essentially my idea would be to use the WWV >> signals to trim a crystal oscillator to maintain frequency and phase with the >> incoming signal. Since the loop gain wouldn't need to be very high, and it >> should be possible to detect incoming signal loss (and hold the current >> frequency in such an event) it should be possible to keep accurate time >> during brief interruptions. As for problems from changing signal paths, each >> time you lock onto a new reference signal, you are going to gain or lose at >> most half a cycle. If your design is balanced so that gains and losses >> will more or less match, 1ppT accuracy should be obtainable I would think >> (gaining or losing one cycle, on average, per 100 seconds). To be sure, >> there would be some short term variations (e.g. if a plane flies overhead >> and your signal is reflected off the plane you may have some doppler shift) >> but I would expect that over any significant time those effects would be >> more or less balanced. >> >> | I have often thought that one could get a VERY accurate frequency standard >> | by taking several xtal oscilators and putting them in one oven. The >> | capacitors in half of them would have a positive temp coefficient and those >> | the the other half would have a negative temp coefficient(we could even >> | weight these coefficients to compensate for the xtals' coefficients). We >> | could then average the outputs of all of them together, and use it to >> | create error signals for each one, which could be fed back to maybe a >> | varactor in each one. Of course, I don't think anyone will be using this >> | for a PIC any time soon >> >> A somewhat simpler approach is to use a crystal oscillator and a temperature >> sensor. When the unit is assembled, you measure the frequency of the crys- >> tal at different temperatures within the device's operating range. From what >> I've been told, this technique can be a good way to eke out a little big of extra >> accuracy provided the circuit's characteristics don't change too much over >> time (e.g. from contamination on the PCB altering the trace capacitance, etc.) >> This sort of thing is well within a PIC's abilities, though it would probably be >> better to use an external crystal oscillator. >> >> Name: WINMAIL.DAT >> Part 1.2 Type: unspecified type (application/octet-stream) >> Encoding: x-uuencode > > >Another route you can go if you want good accuracy would be to us the >1PPS >output of a GPS receiver. An artical in the ARRL Ham radio magazine >QST showed >using a PLL to lock on to the 1 pulse per second signal put out by >a GPS receiver. I don't remember the issue it was in, but it seems that >it would >be a lot less trouble than having a lot of xtal oscillators and ovens. > >Another place to look would be the TAPR web pages on the TAC-1 kit. >The TAC-1 [totaly accurate clock] info can be found at >http://www.tapr.org/tapr/html/tac2.html >And the kit is for sale for less than $140 I beleive. > >Hope that might help you out! > >AG > > And that's not all. Yes Dennis is back from Japan. Ok there are also many other methods to get an accurate frequency. Your telecommunications carrier will have two numbers that you can ring up, the first is a 3kHz tone and the second is a 2kHz tone, mix and 1kHz! Very accurate. Here in Australia, one can tap into the re sync signal in the ABC TV B/CST. This will provide an accurate 16625Hz ref that is based on a 1-12 accurate clock. Until recently there was also the Omega ground stations, these where also very accurate, and where in the 9 to 14kHz range (If I remember correctly) Also we can wait for Digital TV, there should also be data in there that we can use. There are also pre aged crystals (And TCXOs) that have known characteristics over time, and 1ppm is very possible. I think that there is a Phillips system that uses this (Among others) Dennis