need to choose a ublox timing module: LEA vs NEO? - Page 1
Forum section: electronics / testgear

author: linux-works
author_rank: Super Contributor
timestamp: November 08, 2017, 11:18:32 pm
content: hi, trying to pick a GPS module for timing, NOT positioning. this is for something like a GPS-DO where we need to get low phase noise and high accuracy on the 10mhz and 1pps outputs.from what I know about the 2 ublox module types, the NEO is preferred for driving around and doing regular old gps things. and the LEA is preferred for fixed-location timing applications.however, there is a NEO-xxx-T module and the -T seems to imply timing.so, at this point, I'm not clear if there is much difference. the NEO is about $8 cheaper than the equiv model number LEA. it may be easier to buy the NEO, as well.anyone have any direct experience with those 2 module types? I don't know if the LEA is 'always better' than the neo-T for timing; everything I have read implies the LEA is the preferred module for timing, but maybe that's old info?

author: texaspyro
author_rank: Super Contributor
timestamp: November 09, 2017, 12:05:55 am
content: The phase noise of the frequency outputs of the Ublox modules is rather poor. The oscillator is not a multiple of 10 MHz. Also, the PPS output has quite a bit of sawtooth error. You really need a decent GPSDO. .. one where the 1PPS is derived from the disciplined oscillator, not just buffering the GPS 1PPS. The Trimble Thunderbolt is one of the best. It's GPS system is designed so that there is no sawtooth error to correct for.The best raw GPS for timing stuff is probably the Venus/Skytraq timing receivers. They have very low sawtooth error due to the 120 MHz oscillator. Navspark sells one. Also Nick Sayer on Tindie sells one (for less) that has pinouts like the Adafruit Ultimate GPS.

author: DaJMasta
author_rank: Super Contributor
timestamp: November 09, 2017, 12:25:45 am
content: The -T is definitely the timing branded devices. While I haven't measured any of the faster frequency outputs for the modules, I have measured the PPS outputs on several of them.... and they're all the same, around about a 20ns adjustment step size for timing of the output pulse, about 50ns of peak to peak variation in 15ish hours of measurement. Timing variants and versions with TCXO instead of normal crystals didn't seem to impact the performance at all, though the short term stability may be improved somewhat (but using a PPS output, that's not nearly as important).I think only some of the newer series have the customizable output frequencies, with most of the available modules, there is no frequency output aside from the PPS.

author: texaspyro
author_rank: Super Contributor
timestamp: November 09, 2017, 12:30:49 am
content: Quote from: DaJMasta on November 09, 2017, 12:25:45 amMost of the lower end modules do let you set the 1PPS freq. It is limited to 1-10 KHz max. The newer ones have one or two outputs that can go to at least 10 MHz. Some have a 1-10 KHz limit on one of the outputs and the other can go higher.

author: linux-works
author_rank: Super Contributor
timestamp: November 09, 2017, 12:43:23 am
content: thanks for the replies.I'm still not clear if the neo-xxx-T and the lea-xxx-T are really different or not?clearly I don't want the non-T version. but if I select -T as the suffix, is there really any diff between the 2 modules?

author: texaspyro
author_rank: Super Contributor
timestamp: November 09, 2017, 01:08:57 am
content: The hardware is the same... the difference is in the firmware. Timing receivers can operate with your fixed location set (needs to be very accurate) and can then produce time outputs with a little as one satellite visible. Timing receivers have a command for doing a location survey. The default survey can take an hour to a day or so.Also the timing firmware optimizes the GPS hardware for highest timing performance rather than location performance. Ublox seems to "kink" their modules so that you can't load timing firmware on plain modules... they apparently do this using one of "one-time-programmable" fuses. Normal modules have the fuse blown and it cannot be reset to allow running timing firmware... bastards!

author: linux-works
author_rank: Super Contributor
timestamp: November 09, 2017, 01:28:16 am
content: the specific modules I am trying to decide between:neo-6tlea-6tthe spec sheet shows identical features and performance, with the lea being a larger module and having a reset pin (not sure I need that).

author: DaJMasta
author_rank: Super Contributor
timestamp: November 09, 2017, 03:42:02 am
content: Don't know if they still have one of these for the 6 series, but for the 8 series, there's a little comparison chart at the bottom:https://www.u-blox.com/en/product/neolea-m8t-seriesVery little, the NEO has an extra LNA and the LEA is part of a larger footprint. I know in the 7 series the letter (7M vs. 7N) was differences in terms of oscillator type and stuff, but I think there's some sort of feature comparison chart for them, too. I think the "T" part of it is more important as that's the part that gives you the timing specific featureset.Ah, a quick google shows the 3 6T variants:https://www.u-blox.com/en/product/neolea-6t, though it may be incomplete, I've got an LEA-6T-2 marked module that doesn't seem to be listed as its own thing.

author: Gyro
author_rank: Super Contributor
timestamp: November 09, 2017, 10:02:24 am
content: Quote from: linux-works on November 09, 2017, 01:28:16 amThe fundamental difference is...NEO-6T: 24 Pin package, ONE Programmable timepulse outputLEA-6T: 28 Pin package, TWO Programmable timepulse outputsGo for the LEA.

author: texaspyro
author_rank: Super Contributor
timestamp: November 09, 2017, 05:34:21 pm
content: Rule of thumb: whenever they make something smaller, something gets compromised. Go for the bigger one.