[time-nuts] Low cost GPS module for < 100ns timestamping error
tnuts at toneh.demon.co.uk
Fri May 2 22:54:25 UTC 2014
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
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,
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