Inexpensive GPS modules with PPS output with low jitter between modules? - Page 1
Forum section: electronics / general

author: AlienRelics
author_rank: Unknown
timestamp: May 10, 2016, 09:22:07 pm
content: I researched enough to find out the answer to my initial question - apparently the PPS output of a GPS should happen at the start of each second, so from one GPS receiver to a nearby receiver, the PPS outputs should be very close to exactly in phase.But apparently some GPS modules have rather short term jitter.I have a project that requires close synchronization between devices. It is only for sound travel, so nanosecond jitter is perfectly fine. But I don't want to find out that I've bought four GPS modules with massive short term jitter.Has anyone bought any cheap GPS modules, say from an eBay seller or some website, and checked the jitter in the PPS output?Thanks!

author: awallin
author_rank: Frequent Contributor
timestamp: May 11, 2016, 05:04:56 pm
content: 1ns sounds very tough, 10ns between PPSes of different receivers located reasonably close to eachother sounds doable. calibrated or identical antennas and cables required.GPS by itself is jittery, so the better receivers use a TCXO or OCXO that provides short-term stability.tvb has comparisons of OCXO based GPSDOs (I think these are in the 1000$/eur range)http://www.leapsecond.com/pages/gpsdo/here's a measurement of about 6ns RMS jitter on a ublox lea-6thttp://www.imeko.org/publications/tc10-2013/IMEKO-TC10-2013-028.pdf

author: AlienRelics
author_rank: Unknown
timestamp: July 18, 2016, 10:49:24 pm
content: Hm. Well, then perhaps my best solution is just to use a TCXO in each unit and sync them.

author: edpalmer42
author_rank: Super Contributor
timestamp: July 18, 2016, 11:07:56 pm
content: In general, GPS receivers optimized for timing rather than navigation will give you better 1 PPS performance. Here's the results of some measurements that I've made.Code:GPS Devices -- Measure & analyze the period of the 1 PPS OutputDevice ............... Std Dev (ns).... Range (max-min)(ns) ... Device ...... NotesNavsync CW12 ......... 4 - 5 .......... 20 - 25 ............... GPS Rcvr .... 1,7Motorola UT+ ......... 40 - 55 ........ 95 - 110 .............. GPS Rcvr .... 2,7Rockwell Jupiter ..... 10 ............ 50 ................... GPS Rcvr .... 3,7Motorola M12M ........ 10 - 15 ........ 40 - 60 ............... GPS Rcvr .... 7Trimble Thunderbolt .. 0.4 - 0.5 ...... 2 - 4 ................. GPSDO ....... 6,8HP Z3801A ............ 0.1 - 0.2 ...... < 1 ................... GPSDO ....... 6HP Z3817A / CW12 ..... < 0.1 .......... < 1 ................... GPSDO ....... 4,6Jackson Labs GPSTCXO . 0.3 - 0.4 ...... 2 - 3 ................. GPSDO ....... 6NEC NWM-034241-201 ... 0.1 - 0.2 ...... < 2 ................... GPSDO ....... 5Results are based on multiple runs of ~ 1000 measurements each.Sawtooth correction has not been used for any of the GPS receivers. Where supported, it would reduce the numbers substantially.All units were connected to the same antenna system.Notes1. Sawtooth correction not supported.2. Most 'range' results were in this group, but there were a few at 20 - 30.3. Only one test.4. Requires external 1 PPS input. Equipped with E1938 oscillator.5. Measurement made with Fluke PM6681.6. Measurement made with HP 5370B.7. Measurement made with HP 5372A.8. Standard parameters.Ed

author: uncle_bob
author_rank: Unknown
timestamp: July 18, 2016, 11:59:22 pm
content: HiUnless you are lucky, your TCXO probably has a nanosecond of jitter on a second to second basis....Without sawtooth correction, even a timing GPS is going to be a +/- 10 ns sort of thing due to the sawtooth process. With sawtooth correction messages utilized and compensated for in your software, you can get into the low nanosecond range. Doing that between independent receivers will depend a lot on how will surveyed your locations are and how clear a view of the sky your antennas all have.Bob