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 ex tra > 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