Welcome to the nuts Tony You are not specifying exactly how accurate time has to be but in my book and based on tests the most reasonable priced GPS with 1 pps is a Ublox 6M that you can get with antenna for less than $ 22 antenna included from _www.DX.com_ (http://www.DX.com) . They have volume discount. Shipping is very slow but included. They seem to be presently out of the 1 pps version but all ublox units have a 1 pps output and I use with and without and all I do is solder a wire to pin 3. Bert Kehren In a message dated 5/2/2014 7:02:57 P.M. Eastern Daylight Time, tn...@toneh.demon.co.uk writes:
Hi, I'm new here so please be gentle! I'm considering designing and building some dataloggers, probably ARM Cortex based (eg. STM32F4xx), which record the time of infrequent events, preferably to better than 100ns and if possible better than 50nS. The data loggers will be continuously powered, in fixed locations and should have reasonably good views of the sky so the use of a low cost GPS module should be feasible. I believe it shouldn't be too difficult to resolve the PPS timing to +/- 5ns or better with a 100MHz+ microcontroller clock, but obviously jitter would add to the error requiring the GPS to be better than perhaps 90ns or so worst case. Inevitably cost and power constraints apply - ideally the GPS would cost less than $20 (in quantities of 100), and < $15 would be good, but it doesn't seem easy to find very lost cost receivers with timing outputs that are properly specified, presumably because of the relative market volumes. The power consumption of most timing receivers also seem to be higher than navigation units - eg. the Trimble SMT-x spec is 100mA compared to the ADAfruit MTK3339-based module which draws 20mA (but they are a bit too expensive at $24 apiece). There are several cheap modules that have PPS outputs but no accuracy specification; it's possible that these could be used with sufficient averaging/filtering of the PPS output. Actually repeatability is the important requirement rather than accuracy as they could be calibrated. Perhaps even a PPS o/p is not absolutely necessary - could the NEMA output timing be used given enough averaging and a sufficiently stable oscillator? Compromising the timing accuracy requirement a bit to say 150ns may be acceptable if the GPS device is cheap enough. I understand that the PPS outputs of some cheap modules sometimes become ill-behaved, but in this application the time stamp can be adjusted (or anomalous clocks ignored) post-event if necessary to correct for temporary disturbances. This also raises questions about the short term stability of the microcontroller oscillator required to maintain sufficient accuracy when GPS timing is temporarily lost for some reason - but how long would that need to be? 30s? 5 minutes? 30 minutes? An OCXO or a Stratum-3 TXCO would be too expensive, but oscillator manufacturers don't seem to publish short term frequency stability specifications for low cost/low power oscillators, and finding such information isn't easy. Can anyone point to figures for a typical non-TXCO low cost oscillator, 10 or 16MHz? I did find this study, http://tf.nist.gov/general/pdf/2276.pdf, measuring the stability of some low cost quartz wristwatches which gives some interesting data of 20 to 65ppb Allan deviation over 100s. That, but a 32kHz oscillator might give rise to jitter problems when multiplied up to a suitable frequency. Some oscillator datasheets specify Allan deviation values, but I guess what I need for estimating worst case timestamp error during holdover periods are actually MTIE values. Is there any way to estimate the latter from Allan deviations specs? Would an ADev of 65 x 10^-9 over 100s imply up to 6.5us of error after 100s? Any thoughts? Thanks, Tony H _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
