[time-nuts] Low cost GPS module for < 100ns timestamping error
Edesio Costa e Silva
time-nuts at tardis.net.br
Fri May 2 21:07:36 EDT 2014
Welcome!
Take a look at NavSpark from SkyTraq (http://www.skytraq.com.tw/). They had
an Indiegogo
(https://www.indiegogo.com/projects/navspark-arduino-compatible-with-gps-gnss-receiver)
campaign recently and should deliver real soon now. The NavSpark chip has an
trigger pin for time capture, a feature suggested by a fellow time-nut and a
100 MHz clock.
Edésio
On Fri, May 02, 2014 at 07:59:14PM -0400, EWKehren at aol.com wrote:
> 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,
> tnuts at 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
>
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